JGOFS REPORT No. 27

PARAMETERS OF PHOTOSYNTHESIS: DEFINITIONS, THEORY ANDINTERPRETATION OF RESULTS

August 1998

SCIENTIFIC COMMITTEE ON OCEANIC RESEARCHINTERNATIONAL COUNCIL OF SCIENTIFIC UNIONS

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The JGOFS Report Series is published by SCOR and includes the following:

No. 1 Report of the Second Session of the SCOR Committee for JGOFS. The Hague, September 1988No. 2 Report of the Third Session of the SCOR Committee for JGOFS. Honolulu, September 1989No. 3 Report of the JGOFS Pacific Planning Workshop. Honolulu, September 1989No. 4 JGOFS North Atlantic Bloom Experiment: Report of the First Data Workshop. Kiel, March 1990No. 5 Science Plan. August 1990No. 6 JGOFS Core Measurement Protocols: Reports of the Core Measurement Working GroupsNo. 7 JGOFS North Atlantic Bloom Experiment, International Scientific Symposium Abstracts. Washington,

November 1990No. 8 Report of the International Workshop on Equatorial Pacific Process Studies. Tokyo, April, 1990.No. 9 JGOFS Implementation Plan. (also published as IGBP Report No. 23) September 1992.No. 10 The JGOFS Southern Ocean Study.No. 11 The Reports of JGOFS meetings held in Taipei, October 1992: Seventh Meeting of the JGOFS Scientific

Steering Committee; Global Synthesis in JGOFS - A Round Table Discussion; JGOFS Scientific andOrganizational Issues in the Asian Region - Report of a Workshop; JGOFS/LOICZ Continental Margins TaskTeam - Report of the First Meeting. March 1993.

No. 12 Report of the Second Meeting of the JGOFS North Atlantic Planning Group.No. 13 The Reports of JGOFS meetings held in Carqueiranne, France, September 1993:Eighth Meeting of the JGOFS

Scientific Steering Committee; JGOFS Southern Ocean Planning Group - Report for 1992/93; Measurementof the Parameters of Photosynthesis - A Report from the JGOFS Photosynthesis Measurement Task Team.March 1994.

No. 14 Biogeochemical Ocean-Atmosphere Transfers. A paper for JGOFS and IGAC by Ronald Prinn, Peter Lissand Patrick Buat-Ménard. March 1994.

No. 15 Report of the JGOFS/LOICZ Task Team on Continental Margin Studies. April 1994.No. 16 Report of the Ninth Meeting of the JGOFS Scientific Steering Committee, Victoria, B.C. Canada, October

1994 and The Report of the JGOFS Southern Ocean Planning Group for 1993/94.No. 17 JGOFS Arabian Sea Process Study. March 1995No. 18 Joint Global Ocean Flux Study: Publications, 1988-1995. April 1995No. 19 Protocols for the Joint Global Ocean Flux studies (JGOFS) core measurements (reprint). June, 1996.No. 20 Remote Sensing in the JGOFS program. September 1996No. 21 First report of the JGOFS/LOICZ Continental Margins Task Team. October 1996No. 22 Report on the International Workshop on Continental Shelf Fluxes of Carbon, Nitrogen and Phosphorus.

1996No. 23 One-Dimensional models of water column biogeochemistry. Report of a workshop held in Toulouse, France,

November-December 1995. February 1997No 24 Joint Global Ocean Flux Study: Publications, 1988-1996. October 1997.No 25 JGOFS/LOICZ Workshop on Non-Conservative Fluxes in the Continental Margins, October 1997.No 26 Report of the JGOFS/LOICZ Continental Margins Task Team Meeting, No 2, October 1997.

The following reports were published by SCOR in 1987 - 1989 prior to the establishment of the JGOFS Report Series:

• The Joint Global Ocean Flux Study: Background, Goals, Organizations, and Next Steps. Report of the InternationalScientific Planning and Coordination Meeting for Global Ocean Flux Studies. Sponsored by SCOR. Held at ICSUHeadquarters, Paris, February 17-19, 1987

• North Atlantic Planning Workshop. Paris, September 7-11, 1987• SCOR Committee for the Joint Global Ocean Flux Study. Report of the First Session. Miami, January, 1988• Report of the First Meeting of the JGOFS Pilot Study Cruise Coordinating Committee. Plymouth (UK, April, 1988• Report of the JGOFS Working Group on Data Management. Bedford Institute of Oceanography, September, 1988

Additional copies of the JGOFS reports are available from:

Ms. Judith R. Stokke, Adm. Secretary Tel: (+47) 55 58 42 46JGOFS International Project Office Fax: (+47) 55 58 96 87Centre for Studies of Environment and Resources E-mail: jgofs @uib.noUniversity of Bergen http://ads.smr.uib.no/jgofs/jgofs.htmHigh-Technology Centre. Ltd.N-5020 BergenNORWAY

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THE JOINT GLOBAL OCEAN FLUX STUDY

-JGOFS-

REPORT No. 27

PARAMETERS OF PHOTOSYNTHESIS: DEFINITIONS, THEORYAND INTERPRETATION OF RESULTS

by

Egil Sakshaug, Annick Bricaud, Yves Dandonneau, Paul G. Falkowski, Dale A. Kiefer,Louis Legendre, André Morel, John Parslow and Masayuki Takahashi

A revised version of the findings of the JGOFS Photosynthesis Measurements Task Teampublished in Journal of Plankton Research, Vol. 19 no. 11, pp. 1637-1670, 1997.Reprinted with permission of the Oxford University Press.

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Published in Norway, 1998 by:

JGOFS International Project OfficeCentre for Studies of Environment and ResourcesUniversity of BergenBergen High-Technology CentreN-5020 BergenNORWAY

The Joint Global Ocean Flux Study of the Scientific Committee on Oceanic Research(SCOR) is a Core Project of the International Geosphere-Biosphere Programme (IGBP). Itis planned by a SCOR/IGBP Scientific Steering Committee. In addition to funds from theJGOFS sponsors, SCOR and IGBP, support is provided for international JGOFS planningactivities by several agencies and organizations. These are gratefully acknowledged andinclude the US National Science Foundation, the International Council of Scientific Unions(by funds from the United Nations Education, Scientific and Cultural Organization), theIntergovernmental Oceanographic Commission, the Norwegian Research Council and theUniversity of Bergen, Norway.

Citation: Sakshaug, E., Bricaud, A., Dandonneau, Y., Falkowski, P.G., Kiefer,K.A., Legendre, L., Morel, A., Parslow, J. and Takahashi, M. 1997.Parameters of photosynthesis: definitions, theory and interpretation ofresults. Journal of Plankton Research, 19 (11), 1637-1670

Acknowledgement: Reprinted by permission of the Oxford University Press.

ISSN: 1016-7331

Cover:JGOFS and SCOR Logos

The JGOFS Reports are distributed free of charge to scientists involved in global research.

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PARAMETERS OF PHOTOSYNTHESIS: DEFINITIONS, THEORY ANDINTERPRETATION OF RESULTS

E. Sakshaug1, A. Bricaud2, Y. Dandonneau3, P.G. Falkowski4, D.A. Kiefer5, L. Legendre6 A.Morel2, J. Parslow7 and M. Takahashi8

1Trondhjem Biological Station, Norwegian University of Science and Technology, Bynesvegen 46, N-7018Trondheim, Norway. E-mail: egil.sakshaug@vm.unit.no2Laboratoire de Physique et Chimie Marines, B.P. 8, F-06230 Villefranche-sur-Mer, France3LODYC, Université Pierre et Marie Curie, 4, Place Jussieu, F-75252 Paris cedex 05, France4Oceanographic and Atmospheric Sciences Division, Brookhaven National Laboratory, Upton, New York11973, USA5Department of Biological Sciences, University of Southern California, University Park, Los Angeles,California 90080-0371, USA6Département de biologie, Université Laval, Québec, QC, Canada G1K 7P47Division of Fisheries, CSIRO, P.O. Box 1538, Hobart, Tasmania 7001, Australia8Department of Biology, University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153, Japan

All authors are members of the JGOFS Task Team of Photosynthetic Measurements

ABSTRACT

A global assessment of carbon flux in the world ocean is one of the major undertakings of the JointGlobal Ocean Flux Study (JGOFS). This has to be undertaken using historical in situ data of primaryproductivity. As required by the temporal and spatial scales involved in a global study, it can beconveniently done by combining, through appropriate models, remotely sensed information (chlorophylla, temperature) with basic information about the parameters related to the carbon uptake byphytoplanktonic algae. This requires a better understanding as well as a more extended knowledge ofthese parameters which govern the radiative energy absorption and utilization by algae inphotosynthesis. The measurement of the photosynthetic response of algae [the photosynthesis (P)versus irradiance (E) curves], besides being less shiptime-consuming than in situ primary productionexperiments, allow the needed parameters to be derived and systematically studied as a function of thephysical, chemical and ecological conditions. The aim of the present paper is to review the significanceof these parameters, especially in view of their introduction into models, to analyze the causes of theirvariations in the light of physiological considerations, and finally to provide methodologicalrecommendations for meaningful determinations, and interpretation, of the data resulting from P vs Edeterminations. Of main concern are the available and usable irradiance, the chlorophyll a-specificabsorption capabilities of the algae, the maximum light utilization coefficient (α), the maximumquantum yield (φm), the maximum photosynthetic rate (Pm), and the light saturation index (Ek). Thepotential of other, non-intrusive, approaches, such as the stimulated variable fluorescence, or the sun-induced natural fluorescence techniques is also examined.

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1. INTRODUCTION ...................................................................................... 7

2. DEFINITIONS AND THEORETICAL BACKGROUND ........................................ 82A. General......................................................................................... 82B. Irradiance .....................................................................................102C. Chlorophyll a-specific absorption coefficient of phytoplankton ......................112D. Total and partial absorption coefficients .................................................142E. Photosynthesis versus irradiance curves (P vs E curves)...............................142F. P vs E Parameters............................................................................15

2F.1 The 'maximum light utilization coefficient', α*, and the'maximum quantum yield', φm ................................................15

2F.2. The 'maximum photosynthetic rate', P*m ......................................17

2F.3. The 'light saturation index', Ek.................................................182F.4. Causes of variations in the maximum quantum yield, φm, and

other photosynthetic parameters...............................................182G. Water column productivity.................................................................19

3. METHODOLOGICAL CONSIDERATIONS......................................................213A. Penetration of photosynthetically available radiation...................................213B. Light absorption measurements............................................................223C. Carbon vs oxygen............................................................................233D. Gains and losses of POC and DOC during incubation .................................243E. Physiological acclimation...................................................................253F. Curve fitting ..................................................................................26

4. SPECIAL INSTRUMENTATION...................................................................274A. Variable fluorescence .......................................................................274B. Natural fluorescence.........................................................................294C. In situ absorption meters ...................................................................30

5. OPEN QUESTIONS...................................................................................30

6. ACKNOWLEDGEMENTS...........................................................................31

7. REFERENCES .........................................................................................31

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1. INTRODUCTION

Although phytoplankton account for only 1-2% of the total global biomass, theseorganisms may fix between 35 and 45 Pg(petagrammes = 1015 grammes = gigatonnes)of carbon per year, i.e. no less than 30-60%of the global annual fixation of carbon onEarth (Berger et al. 1989, Falkowski 1994,Antoine et al. 1996).

When studying global carbon fluxes in thesea, primary productivity may be calculatedon the basis of ocean color data provided bysatellites (Platt and Sathyendranath 1988,Sathyendranath et al. 1989, Morel and André1991, Lee et al. 1996, Behrenfeld andFalkowski 1997). Thus, whereas mostecological processes cannot be described at aglobal scale because the variables that shouldbe observed are strongly undersampled, thisis not so much the case for phytoplanktonbiomass. However, estimating marineprimary productivity from remotely sensedinformation requires regional data onphytoplankton photosynthetic characteristics,which are still much undersampled(Longhurst et al. 1995).

In order to achieve a global synthesis ofglobal carbon fluxes in the sea, mathematicalmodels must be used, with light,temperature, nutrients and chlorophyll (Chl)a concentration as input variables. What isneeded is not data for the net carbon fixationat a few given places, but a set ofmathematical relationships between theabove variables and the photosyntheticcarbon flux, i.e. primarily the parameters offunctions that relate the carbon fixation rateof phytoplankton to irradiance andchlorophyll Chl a concentration or lightabsorption.

The photophysiological responses of

phytoplankton vary as a function of lightregime, temperature and nutrient status. Amajor goal in understanding howphytoplankton photosynthesis affects carboncycles, and is affected by ocean dynamics, isto determine how the photosyntheticprocesses respond to geochemical andphysical processes. Understanding this iscritical to developing prognostic models ofthe forcing and feedbacks betweenphytoplankton dynamics and oceancirculation. Even if there presently is ageneral understanding of photosyntheticresponses to environmental variations, majordifficulties remain regarding the applicationof this knowledge to specific oceanographicregimes. One strategy for developing reliablemathematical models to calculatephotosynthetic rates under the present-dayocean forcing, as well as under climaticallyaltered forcing regimes, is to exploittheoretical constructs of photosyntheticresponses and apply these constructs toempirical measurements. Such an approachrests on the assumption that the behavior ofcomposite variables can be related togeochemical and physical processes morereadily than the complex variables derivedfrom purely empirical approaches.

This paper, which is written by the JGOFSTask Team for PhotosyntheticMeasurements, presents definitions andtheoretical considerations relevant to studiesof the relationship between carbon uptake byphytoplankton and irradiance (P vs E curves)in phytoplankton, by means of the 14Cmethod (Steemann Nielsen 1952), and forestimating light absorption by phytoplankton.The paper also discusses methodologicalproblems that may be encountered, and dealsat length with the physiological interpretationof P vs E parameters. Although obtaining asatisfactory grid of observations is a majorproblem for estimating global marineproductivity, this is not the focus of the

8

present paper. Therefore, the use of satelliteobservations, automated in situinstrumentation, etc., is cursorily treated.

2. DEFINITIONS ANDTHEORETICAL BACKGROUND

2A. General

In oxygenic photosynthesis, the term 'grossphotosynthesis' is the rate of electronequivalents that have been photochemicallyextracted from the oxidation of water.Assuming the absence of any respiratorylosses, it corresponds to the (gross) oxygenevolution rate.

If photosynthesis is measured as carbonuptake, the term 'gross carbon uptake rate'rate covers all photosynthetic carbonfixation, whether or not the organic carbonformed becomes part of the organisms or isexcreted or secreted into the environment asdissolved organic or inorganic respiratorycarbon (Williams 1993). This rate is generallylower than the gross oxygen evolution rate.The ratio of O2 evolved per CO2 fixed on amolar basis is called the photosyntheticquotient (PQ) and is larger than unity. Thisresults from not all the energy captured bythe photosystems being spent in the fixationof carbon. A fraction is used by the cells toreduce nitrate and, to a much smaller extent,reduce sulphate (Falkowski and Raven1997). Thus high photosynthetic quotientsare related to high nitrate utilization (Myers1980, Langdon 1988, Laws 1991, Williamsand Robertson 1991).

'Net photosynthesis' corresponds to the netevolution of oxygen following all autotrophicrespiratory costs. In analogy, the 'net carbonuptake rate' is the carbon uptake ratefollowing all losses of CO2 due to oxidation

of organic carbon in the cells in daylight. Thenet rates in terms of oxygen evolution andcarbon uptake (assuming that production ofextracellular organic matter is included)should be equivalent.

Primary productivity is a rate withdimensions mass (volume or surface area)-1

time-1. When dealing with phytoplankton,productivity is related to the cubic meter (m3)as the unit of water volume and the squaremeter (m2) as the unit of area.

The term 'gross primary productivity' isfrequently used for the gross carbon uptakerate over a 24 h period. The term 'netprimary productivity' is the organic carbonsynthesized by phytoplankton that issubsequently available to the next trophiclevel (Lindeman 1942). Thus, 'net primaryproductivity' represents the carbon uptakerate following all daytime and nighttimerespiratory losses. This term is thereforemost successfully expressed over a 24 hperiod. Dissolved organic carbon (DOC) thatis produced by the cells and subsequentlyreleased to the surrounding water is part ofboth net photosynthetic rate and net primaryproductivity, albeit not included in 14C-basedestimates of productivity if samples arefiltered before analysis.

Net primary productivity is related to the'growth rate', which can be defined as the netturnover rate for particulate carbon (notincluding production of DOC), provided thatthe cells are in steady-state (balanced)growth (Eppley 1981). In this definition,losses of matter/energy from the cells areincluded but not losses of cells due toexternal factors (e.g. grazing, sinking andhorizontal transport). Among the externalprocesses, grazing may represent a problemin incubation bottles (Eppley 1980).Although to some extent this may beeliminated, quite often estimates of the lossrate due to respiration reflect the communitymetabolism. It is therefore virtually

9

impossible to directly determine thecontribution of algal respiration to the totalrespiratory losses in natural planktoncommunities (Williams 1993). The depth atwhich gross photosynthesis and respirationlosses are equal is called the 'compensationdepth' (zero net photosynthesis).

The 'euphotic zone' is the portion of thewater column that supports net primaryproduction. In this regard it is important topoint out that the respiratory costs for thecalculation of the compensation depth are forthe autotrophs only and should be integratedover 24 hours. Above the compensationdepth, net primary production is positive;below it, it is negative. Due to the impact ofvariations in environmental and other factorson gross photosynthesis and respiratorylosses, the euphotic zone is easier to definethan to measure. It is commonly assumed tobe the water column down to the depth thatcorresponds to 1% of the photosyntheticallyavailable radiation at the surface. Seriousproblems are, however, associated with the1% rule: It is now acknowledged that netphotosynthesis may occur at depths down to0.1% of PAR, and at high latitudes, becauseof the extreme daylength variation, net dailyproduction may vary considerably with nochange in the 1% level.

Assuming a mixed water column: at somedepth, the gross carbon uptake rate,integrated over the above water column over24 hours, will equal the diel, water column-integrated respiratory carbon losses abovethe same depth. This depth is called the'critical depth' (Sverdrup 1953) and is alwaysgreater than the compensation depth.Although Sverdrup based his model onrespiration as the only loss factor, therealized critical depth also depends on otherloss factors, such as grazing, sinking andproduction of DOC (of which Sverdrup wasaware). These losses are incorporated inmany modern models that are extensions ofSverdrup's model.

P vs E parameters and bio-optical parametersare conveniently normalized to Chl a. Thishas been and still is the only pigmentroutinely measured at sea, using simpletechniques. Because Chl a is the terminalphotosynthetic pigment in light absorption(even if the energy has been captured byaccessory photosynthetic pigments, it mustbe transferred to chlorophyll a before it canbe utilized for the photochemical reactions),the amount of Chl a is generally used as anindex of the living, photosynthetically activephytoplankton biomass. Because of the up totenfold variation in the carbon to chlorophyllratio in natural phytoplankton communities,chlorophyll a data should not be usedwithout qualification for estimating algalcarbon. Direct measurement of algal carbonin nature is impossible in most cases becauseit is inseparable from non-algal carbon by anyconvenient and reliable approach.

In the present paper, the term chlorophyll a isabbreviated Chl a and includes the divinyl-chlorophyll a of prochlorophytes. The Chl aconcentration is denoted [Chl a], with unitsmg m-3 (or moles m-3).

We generally suggest an asterisk (*) insteadof the superscript B (with the generalmeaning of biomass) to denote the usualnormalization of productivity-relatedparameters and variables to Chl aconcentration (e.g. P* instead of PB for theChl a-normalized photosynthetic rate). Othernormalizations may be preferable andpossible in some circumstances, i.e. per cell,per unit carbon, etc.

One should note that using mass units forsome parameters and mol units for othersmay necessitate the use of molar weights inthe derivation of parameters from otherparameters. We recommend the use of molunits for carbon uptake and oxygenevolution, together with mol photons forirradiance, as the most consistent approach.

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2B. Irradiance

Photosynthesis is a photochemical process.Because any absorbed photon with awavelength in the range 350-700 nm may beequally effective in producing aphotochemical charge separation,irrespective of wavelength, it is convenient(albeit not necessary) to express the amountof radiant energy which fuels photosynthesisin terms of photons (the quanta or "particles"of electromagnetic radiation) with a specifiedwavelength or frequency.

'Photosynthetic Available Radiation' ('PAR')has been defined in reference to the abovespectral interval according to theSCOR/UNESCO Working Group 15 (Tyler1966). For reasons related to the technicaldifficulty of measuring light in the near-ultraviolet region, this interval was reducedto 400-700 nm. Neglecting the near-UV(350-400 nm) domain usually does not entaila significant error because the contribution ofthis radiation range to the total (350-700 nm)is small, of the order of 5-7% for the incidentradiation at the ocean surface. In the bluest,oligotrophic waters, however, in which thenear-UV radiation may be more penetratingthan light of wavelengths >500 nm (green,yellow, red), the UV proportion increaseswith depth and may represent up to 15% ofPAR near the bottom of the euphotic zone.

The radiometric quantity to be consideredand measured in studies of photosynthesis isthe amount of radiant energy incident per ofunit time and unit of area. This quantity istermed 'Irradiance'. It is represented by thesymbol E and is expressed in energetic units(W m-2) or quantum units (mol photons m-2 s-

1). The symbol 'I', which is often used forirradiance, should be avoided as it can beconfused with the same symbol used for'Radiant Intensity' (units W sr-1). 'Radiance',with the symbol 'L', is the radiant flux in a

given direction per unit angle per unit area,and expressed by W m-2 sr-1 (Morel andSmith 1982). Integrals of radiances over afinite solid angle and under specifiedconditions lead to the various irradiances(Table 1).

It is generally assumed that phytoplanktoncells may collect radiant energy equally fromall directions so that 'Scalar Irradiance' is therequired quantity (WG-15, SCOR/UNESCOrecommendations; see Table 1). It has the

symbol o

E (or Eo) according to IAPSO, theInternational Association for the PhysicalSciences of the Ocean (Morel and Smith

1982). o

E for a given wavelength is denoted(λ) and has also been termed PAR(λ) in the

bio-optical literature. o

E (λ) has the units Wm-2 nm-1 or mol photons m-2 nm-1 s-1. Thetotal irradiance over the whole PAR rangecan be computed in either energetic (Eq 1) orquantum units (Eq 2):

( ) λλ= ∫ dnm700

nm400

PAR

oo

EE [1]

[o

E (λ) in W m-2 nm-1]

( ) ( ) λλλ= ∫ d1nm700

nm400

PAR

oo

EhcE [2]

[o

E PAR in photons m-2 s-1, o

E (λ) in W m-2 nm-1]

To obtain mol photons m-2 s-1, the number ofphotons resulting from Eq 2 must be dividedby Avogadro's number (N = 6.022 × 1023).PAR represents roughly 40-45% of the totalsolar radiation at the sea level (Kirk 1994).The energy of a photon (ε) is related to itswavelength (λ) by Planck's law: ε = hc/λwhere h is Planck's constant (6.626 × 10-34

Joule seconds) and c is the speed ofelectromagnetic radiation in vacuo (2.9979× 108 m s-1). Thus PAR measurements interms of power cannot be accurately

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( ) ( ) λλλ⋅

= ∫

−

dnm700

nm400

*1

PAR*

oo

EaEa φφ

( ) ( ) 1*m

* −λ=λ φφφ aaA

( ) ( ) λλλ= ∫ dnm700

nm400

PUR

oo

EAE φ

PUR*mPAR*

oo

EaEa ⋅=⋅ φφ

transformed in terms of photons, and viceversa, unless the spectral distribution of theirradiance is known. Nevertheless,approximate conversions for incident solarradiation, as well as for in-water irradiance,are possible (e.g. Morel and Smith 1974).

As the air-water interface is essentially aplane, the rate of radiant energy able to enterthe ocean is represented by the symbol Ed

(Table 1), the downwelling irradiance at nulldepth (just beneath the interface). Thisirradiance is generally measured just abovethe surface and must be corrected for by theloss by reflection at the interface in order toprovide the energy actually introduced intothe water column (Section 3A).

In the following sections the term 'irradiance'is used. One should, however, bear in mindthat scalar irradiance is assumed for under-water irradiance data that will be related to

algal photosynthesis and growth (o

E inequations).

2C. Chlorophyll a-specific absorptioncoefficient of phytoplankton

The 'Chl a-specific absorption coefficient'(cross section) is crucial for calculation of theimpact of phytoplankton on the absorptioncoefficient of seawater and how much light isabsorbed by the phytoplankton in bio-opticalmodels of marine primary production. It hasthe symbol a*

φ(λ) and units m2 (mg Chl a)-1.The magnitude and the spectral shape ofa*

φ(λ) are not constant. Inter and intra-specific differences exist within rather wideintervals. They originate from chemicaleffects, i.e. pigment composition (Prézelinand Bozcar 1986) as well as physical effects,i.e. packaging. Both these effects usuallyresult from physiological acclimation(Sections 2F.4 and 3E).In the calculation of light that is actuallyabsorbed by the phytoplankton, one needs

the mean Chl a-specific absorptioncoefficient, a*

φ defined in relevance to theactual spectral composition of light sourceused in a given experiment (in situ or invitro):

[3]

The dimensionless algal absorptioncoefficient of phytoplankton, Aφ(λ), isneeded to calculate 'PhotosyntheticallyUsable Radiation' (PUR) that represents thefraction of PAR at such wavelengths that canbe absorbed by phytoplankton. Aφ(λ) isdefined in the 0-1 interval, according to:

[4]

a*φm is the maximum value of a* (λ), reached

at the wavelength λm which is generallyfound at around 440 nm. PUR is computedas:

[5]

From Eqs 3, 4 and 5, it follows that

[6]

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TABLE 1. Recommended symbols and units relevant to aquatic photosynthesis. All theradiometric quantities (part A), except PAR, can be considered as spectral quantities, with theargument ë (wavelength) added. PAR is already integrated over a wide spectral range, 350 or400 nm to 700 nm; see Section 2.B. Among the other quantities, a*, Aφ, σPSU, σPSII, α*, Ek,and β* have spectral properties; φm is usually treated as spectrally independent. An asterisk (*)denotes normalization to the amount (mg) of chlorophyll a.

Symbol Units

A. RELEVANT RADIOMETRIC QUANTITIES

Radiant energy Q J (= 1 Ws)Radiant power or flux Φ, F WRadiance

[its directional character is often depicted by a zenith angle (Θ)and an azimuth angle (φ), e.g. L(Θ,φ)]

L W m-2 sr-1

Scalar irradiance1

[o

E =∫≡ L(Θ,φ) dΩ; Ω is the solid angle and ≡ (=4π sr) the wholespace] mol photons m-2 t-1

o

E W m-2

Plane irradianceDownwardUpward[Ed =∫≡d L(Θ,φ)cosΘ dΩ; ≡d (=2π sr) represents the upperhemisphere, i.e. all downward directions. Similar integrationover the lower hemisphere, ≡u (all upward directions), providesEu]

Ed

Eu

W m-2

W m-2

Photosynthetically available radiation1,2

(as o

E , see Eqs 1 and 2)PAR mol photons m-2 t-1

(or W m-2)Irradiation (radiant exposure) H J m-2

For a duration ∆∆t, H =∆∆t E(t)dt where E iso

E , Ed orPAR

mol photons m-2

Absorption coefficient a m-1

Scattering coefficient b m-1

Attenuation coefficient (= a + b) c m-1

Vertical attenuation coefficient[for a radiometric quantity x=L, o

E ,Ed..., K = -dlnx/z, where z isdepth, measured positive downward]

K m-1

B. BIO-OPTICAL AND DARK-REACTIONPARAMETERS AND VARIABLES

Chl a-specific absorption coefficient a* m2 (mg Chl a)-1(Eqs 4, 5, 11)

Dimensionless algal absorption coefficient Aφ dimensionless (Eq 6)

Photosynthetically usable radiation PUR (as o

E , PAR) (Eqs 7, 8)

Number of photosynthetic units3 n mol X (mg Chl a)-1

Functional cross section of PSU4 σPSU m2 (mol X)-1

13

Table 1. continued

Symbol Units

Cross section of PSII σPSII m2 (mol photons)-1

Quantum yield φ mol X (mol photons)- 1

Maximum quantum yield(= α*/a*

φ = σPSII/σPSU)φm as φ (Eqs 9, 10)

Minimum turnover time for photons in PSII1 τ tInstantaneous rate of fluorescence Jf mol photons s-1

Quantum yield of fluorescence φf photons emitted (photonsabsorbed)-1

C. P-E PARAMETERS AND VARIABLES

Photosynthetic rate1,3 P* mol X (mg Chl a)-1 t-1

Maximum photosynthetic rate5

(= n τ-1)P*

m as P*

Maximum light utilization coefficient3,6

(=a*φ φm = n σPSU φm = n σPSII)

α* mol X m2 (mg Chl a)-1(molphotons)-1 (Eqs 9, 10, 12, 14)

Light saturation parameter=[P*

m /α*, = 1/(σPSII τ)]Ek as

o

E

Photoinhibition parameter β* as α*

D. WATER COLUMN PARAMETERS

Water column light utilization index7 ψ*α as α* (Eq 17)

Water column photosynthetic cross-section7 ψ*E as a* (Eq 18)

Maximum Chl a-normalized photosynthetic ratewithin a water column maximum

P*opt as P*

m

----------1the unit of time, t, should be the same for these variables and parameters; either second or

hour.2the terms photon flux or photon flux density should be avoided.3X denotes C fixed or O2 evolved; mol units are recommended, to avoid the use of conversion

factors.4or 'absorption cross section per unit of mass (mg) Chl a'; a*

φ refers to absorption byphytoplankton only; for decomposition of a* and a of water, see section 2C and Eq. 3.5also known as the 'assimilation number', not to be recommended because a 'number' implies a

dimensionless quantity and, like P*, as the 'photosynthetic capacity'.6also known as the 'photosynthetic efficiency', not to be recommended because a 'number'

implies a dimensionless quantity.7per unit Chl a mass.

14

( )[ ] ( )λ=λ φφ aaa Chl*

( ) ( ) ( ) ( ) ( )λ+λ+λ+λ=λ DSNAPw aaaaa φ

2D. Total and partial absorptioncoefficients

The 'total absorption coefficient' of seawater,a (m-1) is an 'inherent' optical property ofseawater (sensu Preisendorfer 1961). It cantherefore be expressed as a sum of partialcoefficients:

[7]

The coefficient aφ(λ) represents thecontribution by algal pigments; aw(λ) that bythe water itself, aNAP(λ) that by non-algalparticulate matter, and aDS(λ) that bydissolved colored material. The coefficientaφ(λ) is the sum of the absorption coefficientsfor photosynthetic pigments [aPS(λ)] andalgal non-photosynthetic pigments [aNPS(λ)].Absorption due to all kinds of suspendedparticles (phytoplankton, bacteria,heterotrophs, debris and various detritus,including minerogenic types) may berepresented by the sum aφ + aNAP = aP.By definition, aφ(λ) can be predicted if a*

φ(λ)and [Chl a] are known:

[8]

Phytoplankton pigments modulate, throughaφ(λ), the absorption coefficient of seawaterconsiderably, thus modifying the submarinelight field strongly (e.g. algal self shading inthe water column, ocean color), and thiseffect provides the basis for remote sensingof the pigment concentration. Dissolvedsubstances that are of organic origin (knownas 'yellow substances', 'gilvin', or 'Gelbstoff')may affect the total absorption coefficientconsiderably in some coastal waters whereriver input is prominent. The coefficient forpure seawater, aw, has been determined inlaboratory experiments; some uncertaintiesremain because of the extremely lowabsorption by water in the blue part of the

spectrum.

In aquatic systems, the bulk coefficient a(λ)can, at least in principle, be measured in situ,and the absorption spectra of filteredparticles, aP(λ), can be measured and tosome extent partitioned into components(Sections 3A,B). Finally, aDS(λ) can bemeasured on filtered water samples, using anappropriate blank.

2E. Photosynthesis versus irradiancecurves (P vs E curves)

Photosynthetic rates are related to irradiancein a non-linear fashion. To parametrize thisrelationship, P vs E data are needed. In a Pvs E determination, a series of subsamplesdrawn from a single seawater sample withknown [Chl a] is incubated in a gradient ofartificial light, at a temperature as close aspossible to natural conditions. The P vs Eresponse should ideally refer to instantaneouslight and provide information on thephotoacclimational state of thephytoplankton at the moment of sampling.However, unless incubation time is only afew minutes, some acclimation will takeplace during incubation, especially in termsof the photoprotective apparatus ofphytoplankton. Therefore "ideal"measurements fully relevant to the state ofphytoplankton at the moment of sampling arenot possible to carry out in the field.

If 14C is used to estimate photosyntheticcarbon fixation and if the duration ofincubation is so short that newlyincorporated carbon is not respired orrecycled within the cell, it can be assumedthat P vs E measurements would yield resultsthat are close to the gross carbon uptake rate(Dring and Jewson 1982, Williams 1993).Therefore, commonly employed P vs Efunctions for carbon uptake rate passthrough the origin.

15

There are at present no satisfactory methodsfor estimating the gross or the net carbonuptake rates accurately. Even shortincubations may fail in yielding the grossuptake rate (Williams et al. 1996a,b). Interms of primary productivity, however,night-time respiratory losses may be moreimportant than the daytime differencebetween gross and net carbon uptake rates.Incubations of 24 h duration or more usingthe 14C method is unsatisfactory because ofthe artefacts that may be introduced. Theoxygen method is not yet sensitive enough toresolve the variations caused byphotosynthesis in the oligotrophic parts ofthe ocean.

The P vs E response typically can bedescribed with three major regions:

i. At the lowest irradiances,photosynthetic rates are virtually linearlyproportional to irradiance, i.e. theabsorption of photons is slower than thecapacity rate of steady-state electrontransport from water to CO2.

ii. As irradiance increases, photosyntheticrates become increasingly non-linear andrise to a saturation level, at which therate of photon absorption greatlyexceeds the rate of steady-state electrontransport from water to CO2.

iii. With further increase in irradiance, areduction in the photosynthetic raterelative to the saturation level may takeplace (photoinhibition), dependent uponboth the irradiance and the duration ofexposure.

Several P vs E equations have been proposedthrough the years. Most of them fit P vs Edata adequately. Because none of them are"theoretically" perfect, one particularformulation is not recommended aboveothers. One should, however, be aware thatdifferent formulations may yield differentparameter values when fit to the same set ofdata (Section 3F).

2F. P vs E Parameters

The P vs E parameters conventionally in useare α* (the initial slope of the P vs E curve),P*

m (the 'maximum photosynthetic rate'), Ek

(the 'light saturation index', i.e. the ratio P*m

/α*), and β* (the 'photoinhibition parameter').The 'Maximum Quantum Yield' forphotosynthesis, φm, is implicit in α* (Section2F.1). The photosynthetic rate in the lowerpart of the water column (low light) isdetermined largely by α* and in the surfacelayers (strong light) by P*

m; Ek representingthe transition zone between the two regimes.

We propose some changes in the P vs Enomenclature relative to the aquatic sciencestradition, thus (i) P vs E instead of P vs Ibecause E, as already explained, denotesirradiance. We also propose (ii) that the sameunits of time should be employed for bothirradiance and the photosynthetic rate (eithersecond or hour). Finally (iii), the term'Maximum Light Utilization Coefficient' issuggested for the initial slope of the P vs Ecurve, α*, because it represents a maximumvalue, in analogy with φm.

2F.1 The 'maximum light utilizationcoefficient', α*, and the 'maximum

quantum yield', φm

The parameters α* and φm are related butdiffer in that α* is defined in terms of ambientlight (irradiance) whereas φm is defined interms of light absorbed by the phytoplankton.Because the absorption of light byphytoplankton is variable and makes up but asmall fraction of the total absorption of lightin water, α* differs greatly from φm. It would,however, approximate φm if all or most of thelight shone on the sample were absorbed bythe plant, such as in a thick leaf. To find themaximum quantum yield, α* has to bedivided by the Chl a-specific absorptioncoefficient of the phytoplankton, a*

φ, or the Pvs E data may be plotted against absorbed

16

( ) ( ) ( )

λλλλ

= ∫

−

d*m

1

PAR*

oo

EaE φφα

( ) ( ) ( )λλ=λα *m

*φφ a

PSU* σ= naφ

irradiance instead of available irradiance.

Employing the same units of time for thephotosynthetic rate and the irradiance, α*

should have the units mol O2 evolved or CO2

fixed m2 (mg Chl a)-1 (mol photons)-1,whereas φm is in the units mole O2 evolved orCO2 fixed per mole absorbed photons(Myers 1980, Falkowski and Raven 1996;Table 1). The inverse value 1/φm is called the'minimum quantum requirement'.

The maximum quantum yield, φm is, togetherwith a*

φ, frequently used as a parameter in"light-chlorophyll" models of primaryproductivity and growth rate (Bannister1974, 1979; Kiefer and Mitchell 1983,Bidigare et al. 1987, Sakshaug et al. 1989,Sathyendranath et al. 1989, Smith et al.1989, Cullen 1990, Morel 1991, Platt et al.1992, Bidigare et al. 1992, Lee et al. 1996).

α* is quantitatively related to φm and to a*φ,

the Chl a-specific absorption coefficient forthe algae; in a spectral notation:

[9]

Although the quantum yield at times hasbeen defined in terms of available radiation(PAR; Odum 1971, Dubinsky 1980), thequantum yield relevant to photobiologicalmodels should be referenced to absorbed

light. Because o

E and a*φ are spectrally

dependent, the realized value for α*, α*, is:

[10]

The maximum quantum yield, φm is inprinciple is spectrally dependent, as in Eq 10.In practice, however, it is usually treated as anon-spectral parameter. Because studies ofα*(λ) are few, there are few accuratespectral estimates of φm(λ) of photosynthesisin natural phytoplankton communities (Lewis

et al. 1985, Schofield et al. 1993, 1996,Carder et al. 1995).

Photosynthetic processes have been studiedthoroughly in the last 20 years using flashtechniques, measuring oxygen andfluorescence yields and employing targettheory for modelling (Ley and Mauzerall1982, Dubinsky et al. 1986 and Falkowski etal. 1986). Essentially, target theory describesthe photosynthetic rate as a function ofirradiance on basis of the probability that anopen reaction center of a photosystem is hitby one or more absorbed photons (excitons).[The P vs E function published by Webb etal. in 1974, identical to that of Platt et al.(1980) without a term for photoinhibition, isequivalent to target theory formulation.]These investigations have shed light on thephysiological nature of the P vs Eparameters. In the following, α* and φm isdiscussed in view of these investigations.

The light-absorbing pigments ofphytoplankton ("antennae") may be regardedas an arrangement of photosynthetic units,each containing a number of Photosystem I(PSI) and Photosystem II (PSII) reactioncenters that mediate the transformation of theabsorbed energy into a chemically usableform. According to the Emerson and Arnold(1932) definition, a photosynthetic unit(PSU) is the functional, oxygen-producingentity.

The concentration of PSUs per unit Chl a isdenoted n and has the units mol O2 (mg Chla)-1. In the terminology of target theory, thePSU has a functional cross section, σPSU, thatrelates oxygen evolution to the lightabsorbed by the entire PSU (PSII and PSI).The parameter σPSU, which is spectrallydependent, has units m2 (mol O2)

-1.The light absorption coefficient a*

φ is theproduct of σPSU and n, thus (in a non-spectralnotation):

[11]

17

1*mP −= τn

mn φα PSU* σ=

Substitution of Eq 11 into Eq 9 gives:

[12]

The maximum quantum yield, φm, can also be related to the absorption cross section of PSII, σPSII. This parameter is related to PSII (not the whole PSU) and has the units m2 (mol photons)-1, i.e. the inverse of the measured moles of photons delivered per square meter during a flash of light. Like σPSU, it is a spectrum. The maximum quantum yield is (in non-spectral notation) the ratio of σPSII to σPSU: φm = σPSII (σPSU)-1 [13] Substituting Eq 13 for φm in Eq 12 yields α* = n σPSII [14] Eq 13 implies that φm is spectrally neutral only if σPSII and σPSU have the same spectral shape. This, however, is unlikely in the presence of non-photosynthetic pigments because these pigments, which absorb mainly blue light, affect σPSU more than σPSII. Being spectra, σPSII and σPSU, like α*, should be spectrally weighted in analogy with Eq 10 or given for a defined wavelength and compared at the same wavelength. Moreover, the parameters σPSU and n may be calculated in terms of gross carbon fixation instead of oxygen evolution, with the units m2 (mol C)-1 and mol C (mg Chl a)-1, respectively. The resulting values will, however, not be the same (Section 2F.4).

2F.2. The 'maximum photosynthetic rate', P*m

The light-saturated photosynthetic rate P*m

(also known as the 'assimilation number') is independent of the absorption cross section of

the photosynthetic apparatus. It is therefore, in contrast to α*, not spectrally dependent. This implies that the maximum photosynthetic rate cannot be derived from knowledge of light absorption, except by employing empirical statistical relationships, such as in the studies of P*

m and α* by Harrison and Platt (1980, 1986). The maximum photosynthetic rate at steady-state is related to the number of photosynthetic units, n, and the minimum turnover time for electrons (τ; Dubinsky et al. 1986):

[15] The inverse of the minimum turnover time, i.e. the maximum turnover rate, τ-1, of a photosynthetic unit, represents the highest electron transfer rate for the entire photosynthetic electron transport chain from water to CO2. Both τ and n can be measured: τ may vary from 1 to >50 ms or, correspondingly, τ-1 from 1000 to <20 s-1. The rate-limiting step in the overall photosynthetic pathway has been the subject of discussion and debate. Early on it was found that the slowest step in electron transport was the reoxidation of plastoquinol, taking up to 10 ms. This suggestion ignored, however, the processes on the acceptor side of PSI related to carbon fixation. In higher plants, Pm could be related to the concentration of leaf Rubisco, indicating that carboxylation or a step closely associated with carboxylation (e.g. the regeneration of ribulose biphosphate) was the overall rate-limiting reaction under light saturation. Sukenik et al. (1987) followed changes in the pool sizes of a number of electron transport components and Rubisco in nutrient-saturated cultures of the marine chlorophyte Dunaliella tertiolecta at constant temperature and found that τ increased with

18

decreasing growth irradiance, from 3.5 to 14ms, in parallel with increases in the contentsof Chl a, PSII, PQ, cytochrome b6f, PSI andthylakoid surface density. The ratios betweenthese components were independent ofgrowth irradiance whereas τ-1 increasedlinearly with the ratio of the electrontransport components to Rubisco, suggestingthat carbon fixation rather than the electrontransport chain is rate-limiting forphotosynthesis at realistic irradiances.

2F.3. The 'light saturation index', Ek

The light saturation index is the interceptbetween the initial slope of the P vs E curve,α* and P*

m and is denoted Ek (=P*m/α*). This

ratio was introduced in the analysis ofphotosynthetic responses of freshwaterphytoplankton by Talling (1957). Ek indicatesthe irradiance at which control ofphotosynthesis passes from light absorptionand photochemical energy conversion toreductant utilisation. Ek may be a convenientindicator of photoacclimational status(Section 3E).

Substitution of Eq 14 for α* and Eq 15 forP*

m, shows that:

Ek = (σPSII τ)-1 [16]

Thus, Ek can vary by changes in either σPSII

or τ. Like α*, it is spectrally dependent.

2F.4. Causes of variations in the

maximum quantum yield, φm, and otherphotosynthetic parameters

According to well-known the 'Z' scheme,that describes photosynthetic electrontransport through PSII and PSI, and a yieldof unity for each photochemical reaction, 8photons are required to derive one moleculeof O2 (Kok 1948), hence φ has an upperthreshold of 0.125. Common values of φm forlaboratory cultures of phytoplankton may be

0.10-0.12 for oxygen evolution (Myers 1980,Ley and Mauzerall 1982). Values for φm ofgross carbon uptake are, however, typicallyas low as 0.06-0.08 (Laws 1991, Sakshaug1993). In natural communities, values of φm

based on measurements of α* and a*φ, or

derived from measurements of variablefluorescence, are highly variable and can be<0.005 in prominently oligotrophic areas -oligotrophy in combination with strong lightmay cause particularly low values (Lewis etal. 1988, Cleveland et al. 1989, Bidigare etal. 1990b, Schofield et al. 1993, Babin et al.1996b).Generally lower φm (and φ) values for carbonuptake relative to those for oxygen evolutionreflect that the photosynthetic quotient isgenerally >1 (Section 2A). Because this ismainly due to energy costs involved inuptake of nitrate, cells near the base of thenutricline in oligotrophic waters may havelower φm values for carbon uptake than cellsutilizing ammonium or urea higher up in thewater column. The lower φm for carbonuptake than for oxygen evolution impliesdifferent sets of values for every parameterthat has O2 or C (X in Table 1) in its units.Thus, α*, β*, P*

m (and P*), and n are alsolower for carbon uptake than for oxygenevolution, while σPSU is higher.

Other causes of variation in φm and the P vsE parameters include:

i. Increased absorption of light bypigments (i.e. the xanthophyll cycle) thatdissipate the absorbed energy as heatinstead of transferring it to thephotosynthetic reaction centers (antennaquenching), lowering σPSII (Demmig-Adams 1990, Olaizola et al. 1994), thusdecreasing φm and α* and increasing Ek.These non-photosynthetic pigments areoften produced at high irradiances duringnutrient deprivation.

ii. Loss of functional reaction centers,lowering n, thus also α* and P*

m (Eqs 11

19

and 15). However, if the energy transferbetween PSII reaction centers is smallbut finite, σPSII may be enhanced,increasing α* and Ek. According tostudies of the quantum yield offluorescence of cultures ofphytoplankton, n is at its maximum valuewhen cells are nutrient-replete. Thesestudies also indicate that n may beremarkably independent of species andlow in nutrient-deprived cells,presumably corresponding to the growthrate in steady state (Kolber et al. 1988,Falkowski 1992, Vassiliev et al. 1995).In the upper portion of the nutrient-impoverished central gyres of the oceans,values may be reduced by 40-70%; in thenutricline (100-125 m depth) by about25% (Falkowski and Kolber 1995).

iii. Cyclic electron flow. This increases σPSU

both for O2 evolution and C uptake andthus lowers φm and α* while increasingEk. In cyanobacteria, cyclic electron flowaround PSI, that generates ATP, isessential to support metabolism(especially nitrogen fixation). Such acycle utilizes photons but does not leadto the reduction of CO2 and, hence,appears as a reduced overallphotosynthetic quantum yield. At highirradiances, electrons can cycle aroundPSII, bypassing the oxidation of thewater-splitting complex (Prasil et al.1996). This cycle is protective because itdissipates excess excitation, accountingfor about 15% of the loss of quantumyield.

iv. Photorespiration. The major carbon-fixing enzyme, Rubisco, can accept O2 asa substrate, leading to the formation oftwo carbon molecules, especiallyglycolate. This increases thephotosynthetic quotient, thus loweringthe values of α* and P*

m for carbonuptake. Photorespiration has not beenquantified for marine phytoplankton butis known to lower the quantum yield forcarbon fixation on the order of 25% in

higher plants. Photorespiration ispresumably high at elevated oxygenlevels.

v. Packaging. The packaging of pigmentsinside the cell may reduce the absorptionefficiency of pigments. Thus the Chl a-normalized parameters a*

φ and σPSII maybe somewhat smaller for shade-acclimated than for light-acclimated cells.Because α* is Chl a-normalized itbehaves similarly, while Ek may increase.This effect is physical: pigments packedinto chloroplasts are less efficient inabsorbing light per unit pigment massthan when in an optically thin solution.The packaging depends both on the cellsize and pigment concentration/ratios inthe cell (Kirk 1975, Morel and Bricaud1981) and is wavelength-dependent,being most pronounced whereabsorption is highest. It is generally mostpronounced in large and pigment-rich(shade-acclimated) cells; this may causelower α* in shade-acclimated than light-acclimated cells. This loss of efficiencyper unit pigment mass is, however,smaller than the absorption gain throughincreased cellular pigment content. Thusshade-acclimated cells in the end absorbmore light per cell or unit carbon thanlight-acclimated cells.

2G. Water column productivity

The ability to derive basin-scale maps of thedistribution of phytoplankton Chl a in theupper ocean from satellite color sensors(Lewis 1992) has progressively led to thedevelopment of models relating biomass toprimary productivity (Falkowski 1981, Plattand Sathyendranath 1988, Morel 1991,Bidigare et al. 1992, Cullen et al. 1993,Behrenfeld and Falkowski 1997). Theamount or concentration of Chl a, however,represents a state variable. Therefore, tocalculate primary productivity, which is aflux, a variable that includes the dimension

20

( ) *s ChlPAR αψ⋅= tottot aP

time-1 is needed. Such models relating carbonfixation to Chl a incorporate irradiance and a

normalized to the amount of Chl a (ψ*) andis a bulk yield function for the whole watercolumn, valid for a lapse of time, e.g. oneday. These so-called "light-chlorophyll"models (Ryther and Yentsch 1957, Cullen1990) are difficult to verify in the ocean,hence, their usefulness lies in understandingthe underlying biological processes and howthose processes are regulated.

The transfer function ψ includes thephysiological response of phytoplankton tolight, nutrients, temperature, etc (Falkowski1981). It merges both the absorption and thephotosynthetic response of the entirepopulation exposed to decreasing irradiancefrom the surface down to the bottom of theeuphotic zone. Thus, understanding ψ*

requires the knowledge of each level of thespectral absorption capacity ofphytoplankton, of their photosynthetic (P vsE) responses, etc., as well as the spectralcomposition and amount of available energy.

If PAR is expressed as photons, thecoefficient ψ*, then denoted ψ*α (the 'watercolumn light utilization index'), has the sameunits as α*, i.e. mol C m2 (mg Chl a)-1 (molphotons)-1. It is smaller for carbon uptakethan for oxygen evolution (cf. thephotosynthetic quotient, Section 2A). Watercolumn production (Ptot) may be related tothe amount of Chl a in the water column andincident irradiance via the factor ψ*α ,thus:

[17]

Traditionally, Ptot is in the units g C m-2 andfor a given period (e.g. one day), and PARs isPAR at the ocean surface during the sameperiod. The fraction of PAR which isabsorbed by phytoplankton depends on theamount of Chl a within the water column,Chl atot (g m-2), and on the absorptioncharacteristics of the algae in question.

Phytoplankton biomass may be expressed inenergy units (Platt and Irwin 1973). Thus, ifPARs is expressed in energy units, therealized Ptot during the same period can betransformed into its energetic equivalentPSRtot ('Photosynthetically Stored Radiation')by assuming an energetic equivalent for thefixed carbon and using a transfer function,ψ*

E, with the same units as the Chl a-specificabsorption coefficient a*, i.e. m2 (mg Chl a)-

1:

PSRtot = PARs Chl atot ψ*E [18]

Here, both PARs and PSRtot are in energyunits, e.g. J m-2, for a given duration (Morel1978, 1991). The quantity ψ*

E may be termedthe 'Water Column Photosynthetic CrossSection' per unit of Chl a mass for the watercolumn.

Assuming an annual global phytoplanktoncarbon fixation of 40 Pg (a midpoint of therange given in the Introduction), aconversion factor of 4.3 mmol photons (kJ)-1

(Morel and Smith 1974), an energy densityof 39 kJ (g C)-1 in phytoplankton (actually avalue for carbohydrate; Morel 1978) andPAR over the oceans averaging 9.76 × 1020

kJ yr-1, the global ratio of PSR:PAR is0.16%, or about 1370 photons per fixedcarbon atom on an annual basis (Morel 1991,Behrenfeld and Falkowski 1997). Thisestimate is about 1/3 of the apparentconversion efficiency of land plants.

3. METHODOLOGICALCONSIDERATIONS

The vertical distribution of Chl a in theeuphotic zone and the penetration of light areassumed to be known. At each depth, theradiant energy absorbed by thephotosynthetic pigments in phytoplankton isusually represented by the product of [Chl a],

o

E and a*, the latter two being spectral

21

variables. PUR may be computed byintegrating Eq 5 from 400-700 nm. Thecalculation of PUR takes into account thespectral distribution of the light and requiresthat a*(λ) be measured (see below). Themaximum quantum yield, φm, over the PARregion is then calculated as φm = α*/a*

φ thefactor α* being weighted for the spectraldistribution of the incubator light.

3A. Penetration of photosyntheticallyavailable radiation

Above the surface, measurements ofirradiance have to be made in terms ofdownwelling irradiance, Ed, i.e. with aninstrument equipped with a flat (cosine)collector. Use of a spherical collector alwaysimplies an overestimate of the penetratingradiant flux. The magnitude of thisoverestimate is considerable and mainlydepending on solar altitude. It may, forinstance, reach a factor of about 2 for azenith-sun angle of 60° and a dark-blue sky.

Because scalar irradiance, o

E , is the soughtquantity in the water column (Section 2B), aspherical (4π collector must be used for the

in-water measurements. Both o

E (λ) ando

E PAR, i.e. the integrated irradiance over the400-700 nm range (Eq 1), are consideredand measured as function of depth. Becauseof the fluctuations (originating from wave-induced "lens effects" and from variations inimmersion depths), the measurements in theupper layers are unreliable and theirextrapolation toward the "null depth" veryuncertain. As a consequence, the origin(100%) of the vertical irradiance profileremains poorly determined, so that all therelative irradiances (such as the 1% depth,which is much used to fix the depth of theeuphotic zone) are inaccurately known. Thecommonly adopted solution consists inmeasuring the incident irradiance in air,

above the surface (Ed), and in correcting themeasured value for the loss due to reflection.This loss amounts to only 3-5%, and morethan 10% of incident irradiance for low solarelevation, slightly depending on the sea stateand on the sky radiation (Morel and Antoine1994).

Historically, and because of instrumentallimitations (now overcome), in-water data

for o

E have been replaced by measurementsof Ed. This is not crucial in terms of relativeirradiance profiles, as the attenuationcoefficients for both kinds of irradiance areclose. It is, however, important in terms of

absolute values of available energy: the o

E :Ed

ratio, always >1, can be as high as about 2 insome instances, e.g. in highly scatteringwaters with low absorption. Phytoplanktonblooms are relevant examples (Morel 1991).By relying on exact calculations of radiativetransfer (e.g. Mobley 1994), or onapproximations, Ed can be transformed into

o

E with reasonable accuracy.Approximations have been developed byKirk (1984); a method is presented in Morel(1991).

If penetration of light into the sea cannot bemeasured, it may instead be predicted fromdata for incident PAR radiation recordedabove the surface, and from the verticaldistribution of Chl a, at least in Case I waters(Baker and Smith 1982, Morel 1988). Theprediction in Case II waters is morecomplicated and requires additionalinformation, that is generally not available,on the other optically active constituents.

3B. Light absorption measurements

Methods that separate light absorption intocomponents have been much discussed inrecent years. The present section deals withmeasurements of total particulate absorption

22

(aP), absorption by phytoplankton only (aφ),and photosynthetically relevant absorptiononly (aPS).

Spectral absorption by total particulatematter, aP(λ), represents a majormethodological problem because of the lowconcentration of particulate matter inseawater (Yentsch 1962). To overcome this,the most widely used technique over the pastyears has been the glass filter technique firstproposed by Trüper and Yentsch (1967). Itinvolves measuring the absorption spectrumof particles retained on a glass-fiber filter(with a blank filter as a reference), using aspectrophotometer equipped with anintegrating sphere or another opticalarrangement for collection of light scatteredby particles. This simple, rapid, andconvenient measurement for routine use inthe field is, however, strongly affected bypathlength amplification, induced by multiplescattering within the filter and between thefilter and particles. The pathlengthamplification factor (β sensu Butler 1962),varies with optical density, and thereforewith wavelength, and with filter type(Mitchell 1990). Although previous studies(Mitchell 1990, Cleveland and Weidemann1993) proposed species-independentalgorithms, a recent study (Moore et al.1995) suggests that these algorithms maylead to significant errors for somephytoplankton groups such asprochlorophytes and cyanobacteria.

An alternative technique, based on amodification of the filter-transfer-freeze(FTF) technique used for microscopicobservations (Hewes and Holm-Hansen1983), has been recently proposed (Allali etal. 1995). It involves concentrating particlesonto a Nuclepore filter, transferring thefiltered material to a glass microscope slide.After removing the filter, the absorptionspectrum of particles is measured directly onthe slide. Thus, the pathlength amplificationeffect is avoided.

Because the quantum yield is classicallydefined by reference to light absorption byliving phytoplankton, aφ(λ), the factor aP

must be corrected for absorption by non-algal particles, aNAP (biogenous and non-biogenous detrital particles, heterotrophicbacteria, etc). Various techniques have beensuggested, e.g. washing the sample with amixture of organic solvents, applying UVradiation in the presence of hydrogenperoxide (Konovalov and Bekasova 1969),bleaching the cells with peracetic acid(CH3CO3H; Doucha and Kubin 1976) orwith sodium hypochloride (Tassan andFerrari 1995). The most frequently usedprocedure at present is that proposed byKishino et al. (1985). It involves immersionof the filter in methanol for extractingpigments, measuring residual absorption onthe bleached filter, yielding aNAP(λ), andsubtracting this residual absorption spectrumfrom aP(λ) to obtain aφ(λ). A modifiedprocedure has been described in the case ofmeasurements with the "glass-slide"technique. The Kishino method, althoughconvenient, has some obvious limitations: theabsorption spectrum of living phytoplanktonis only approximated because (i) the aNAP(λ)spectrum may include absorption by depig-mented phytoplankton cells in addition tothat by non-algal particles; (ii) aNAP(λ) alsoincludes water-soluble pigments such asphycobilins, only weakly extracted or not atall by methanol, and (iii) aφ (λ) erroneouslyincludes detrital pheopigments andcarotenoids that were extracted by themethanol. Therefore, numerical or statisticaldecomposition methods, based on variousassumptions (Morrow et al. 1989, Roesler etal. 1989, Bricaud and Stramski 1990,Cleveland and Perry 1994) have beenproposed as alternatives to Kishino'schemical method.

To gain insight into the variability of φm thatis due to algal non-photosynthetic pigments,it is desirable to partition aφ(λ) into

23

absorption by photosynthetic pigments,aPS(λ), and that by non-photosyntheticpigments, aNPS(λ). The factor aPS(λ) can beapproximated, using the detailed pigmentcomposition (Bidigare et al. 1990a, Johnsenet al. 1994, Sosik and Mitchell 1995). The invivo absorption spectra of phytoplankton are,however, affected also by the packaging ofpigments, so that accurate reconstruction ispossible only for very small and weaklypigmented phytoplankton cells in which thiseffect can be neglected. Besides, variations inenergy transfer efficiency among the variouspigments may cause difficulties.

In many cases, a good proxy for theabsorption coefficient of photosyntheticpigments, aPS(λ), is the fluorescenceexcitation spectrum (emission measured ataround 730-740 nm); it is very similar inshape to the action spectrum for oxygenrelease (Haxo 1985, Neori et al. 1986). Suchspectra, however, are usually measured on arelative scale, so they have to be scaled to theunits of a*

φ(λ). Maske and Haardt (1987) andSakshaug et al. (1991) scaled thefluorescence excitation spectrum to the redabsorption peak at 676 nm where theabsorption is almost exclusively caused byChl a, to distinguish aPS(λ) from aφ(λ) incultures. An underlying assumption for doingthis is that the scaled fluorescence should besmaller than a*

φ(λ) at any wavelength becausethe fraction of light energy which istransported to PSII cannot be larger than thetotal energy absorbed. Johnsen and Sakshaug(1993) noted that three main problems in thisscaling technique: (i) the cells should betreated with DCMU under actinic light toavoid variable fluorescence; (ii) the lightenergy received by PSII relative to PSI,which is dependent on the pigmentcomposition of the two systems and theirrespective light-harvesting complexes(LHCs), affects the scaling; and (iii) the lightenergy transfer efficiency at 676 nm thereforemay be considerably less than 100%.

On the basis of studies on dinoflagellates,Johnsen and Sakshaug (1993) suggested a80-85% scaling against the red peak of a*

φ at676 nm for chromophytes. As an alternative,however, scaling may be carried out to 100%at a wavelength chosen so that no"overshoot" relative to a*

φ(λ) occurs at theother wavelengths. For dinoflagellates, ascaling to 100% at around 545 nm (theshoulder of the peridinin spectrum) fulfils thisrequirement and implies a 80% scaling at 676nm (Johnsen et al. 1994). Forphycobiliprotein-containing organisms, 100%scaling at around 570 nm may beappropriate. This scaling corresponds,however, only to a 15% scaling at 676 nm(e.g. Synechococcus; Johnsen and Sakshaug1996), reflecting the small amount of Chl abound in the highly fluorescent PSII relativeto PSI which is virtually non-fluorescent atroom temperature. The "no overshoot"approach may be the more general andrecommendable procedure for scaling offluorescence excitation spectra to a*

φ.

3C. Carbon vs oxygen

In algal cultures, carbon uptake must belower than rates for oxygen evolution(Section 2F.4). For natural communities,however, the oxygen budget of a P vs Esample, or a body of water, is related to thenet community production, i.e. the grossphotosynthesis minus respiratory losses in allorganisms, heterotrophs included. Thismakes it difficult to detect the small changesthat arise, due to photosynthesis, in oxygenconcentration after short (<24 h) incubations,except under bloom conditions (Williams etal. 1983). We therefore concentrate here onguidelines which refer to measurements ofcarbon uptake.

3D. Gains and losses of POC and DOCduring incubation

24

In some of the methods used to estimatephytoplankton productivity or to determinephotosynthetic parameters, cells are retainedon a filter and DOC is in the filtrate (thetraditional 14C technique for estimatingmarine carbon uptake rates - e.g. linearincubators) whereas in other methods,measurements are conducted on wholeseawater samples (e.g. photosynthetrons).Radioactivity retained on filters is related toparticulate production, whereas analysis ofwhole seawater samples ideally yieldestimates for the production of bothdissolved (DOC) and particulate organiccarbon (POC). The production of dissolvedorganic carbon may, however, beunderestimated for short incubation times(few hours) because DO14C increases withtime until isotope equilibrium is reached. Thedistinction between POC (particulate organiccarbon) and DOC is arbitrary and dependson the filter used to separate the two sizefractions. However, all organic mattersynthesized by phytoplankton, whetherparticulate or dissolved, is part of the primaryproduction. Estimates of phytoplanktonproduction and/or photosynthetic parametersmay sometimes differ significantly if they arederived from carbon fixed in POC only or inboth DOC and POC.

The various pathways through which carbonfixed by phytoplankton is transformed toDOC and DOC is oxidized to CO2 (respired)include:

i. Photorespiration, which leads toproduction of glycolate and oxidation ofpart of it into CO2. Some glycolate isexuded into the surrounding water; 14Ctaken up by phytoplankton may appear inthe exuded glycolate within 5-10minutes.

ii. Exudation of various polysaccharides,from low to high-molecular weight. Thismay be particularly important at highlatitudes, e.g. blooms of theprymnesiophyte Phaeocystis (Wassmann

et al. 1991) and some diatoms (e.g.Chaetoceros socialis and C. affinis var.willei) that release abundantlycarbohydrates, especially when nutrient-deficient (Myklestad 1974, Zlotnik andDubinsky 1989).

iii. Spontaneous lysis of cells, which wouldrelease cellular material in the water. Thismay occur when cells are nutrient-deficient at the conclusion of a bloom(von Boekel et al. 1992), but thelikelihood of spontaneous autolysisduring incubations is not documented.

iv. Lysis of cells following viral attacks,which releases cellular material into thewater (e.g. Cottrell and Suttle 1995).

v. Grazing by mesozooplankton ("sloppyfeeding"), which is accompanied byrelease of cell material (e.g. Roy et al.1989). In most cases, however, thisproblem is minimized by screening outthe mesozooplankton before incubation(possibly causing stronger nutrientlimitation than in the naturalenvironment).

vi. Grazing by microzooplankton, whichdoes not generally transfer phytoplanktoncarbon to the DOC pool becauseautotrophic organic matter becomesincluded in heterotrophic organisms.Hence, there is little loss of tracer fromthe particulate phase. Respiration byheterotrophs following grazing causes,however, loss of carbon.

Uptake by heterotrophic bacteria of DOCreleased by phytoplankton during the courseof incubation could result in transferringdissolved tracer back to the particulatephase. The actual rate of re-incorporation oftracer into POC through this pathway willdepend on the relative concentrations ofPOC and heterotrophic bacteria. Dependingon the duration of the incubation, some ofthe tracer taken up by bacteria could berespired before the end of the incubation.

The above considerations stress that

25

comparison of photosynthetic parametersdetermined using different methods, orcomparison of carbon uptake ratesdetermined at sea using the conventional 14Cmethod with estimates derived from P vs Emeasurements, should take into account thefollowing differences in approaches:

i. Duration of incubation: short incubations(order of 1 hour) provide estimates thatare (with qualification) closer to thegross carbon uptake rate than longincubations, because the likelihood oflabelled carbon to be respired to CO2

increases and/or recycled within the cellwith the duration of incubation (Dringand Jewson 1982).

ii. Filtered vs whole samples: this problemmight be resolved if uptake of tracer inthe DOC and POC fractions were bothdetermined in cases involving filtration.This, however, is generally not done inthe field.

Both points suggest that estimates ofphotosynthetic parameters from aphotosynthetron, an incubator that that usessmall whole- water samples (Lewis andSmith 1983) should lead to higher estimatesof productivity than those from linearincubators which involve filtering of samplesand no determination of DOC (althoughsome DOC may be adsorbed on the filter Maske and Garcia-Mendoza 1994), and thatthey should be higher than those resultingfrom long incubations at sea. It should benoted that mitochondrial respiration mayoccur simultaneously with photosynthesis,thus causing too low values for gross oxygenevolution rates (Weger and Turpin 1989,Weger et al. 1989).

3E. Physiological acclimation

Physiological acclimation of thephotosynthetic apparatus during incubation

may cause P vs E curve variability, i.e. as aresult of variations in light, temperature andnutrients. This is another reason, in additionto respiratory loss of labelled carbonmentioned above, to keep incubation timesas short as possible, or using non-interferingmethods (Section 4A).

The acclimation strategies appear to havecommon molecular biological causes that aresignalled by the redox status of specificelements in the photosynthetic electrontransport chain (Escoubas et al. 1995). Inessence, physiological acclimation serves tominimize variations in the growth rate whenenvironmental growth-controlling factorsvary (Sakshaug and Holm-Hansen 1986);this is reflected in, at times major, changes inthe pigment contents and composition of thecells, and in the P vs E parameters.

Phytoplankton strive to maintain an optimumbalance between light and dark reactions ofphotosynthesis, i.e. a balance between therate of photon absorption by PSII and therate of electron transport from water to CO2.This balance happens at the irradianceindicated by the light saturation parameter,Ek (Escoubas et al. 1995). At lowerirradiances, the quantum yield ofphotosynthesis is higher, but thephotosynthetic rate is lower; at higherirradiances, there is no major increase in thephotosynthetic rate and, hence, nothing to begained, and potentially much to be lost.Consequently, if the irradiance increases, thealgae adjust their Ek upwards, and vice versa.Thus, Ek is a convenient indicator of thephotoacclimation state of phytoplankton.Because irradiance in the field is fluctuatingand acclimation takes some time, Ek (likeother acclimation-sensitive parameters) iscontinually changing and in principle neverentirely matching the instantaneousirradiance. This is particularly true forphytoplankton in well-mixed waters.

The changes in acclimation state are due to

26

different processes that have evolved on anumber of time scales. To one extent oranother, they affect either σPSII or τ, i.e. thetwo factors that determine Ek. For example,the xanthophyll cycle affects σPSII within atime scale of <60 minutes in a highlyreversible fashion (Olaizola et al. 1994). Asdescribed before, changes in τ are related tothe ratio of Rubisco to the electron transportcomponents. In situ observations (Falkowskiand Kolber 1995) suggest that both non-photochemical quenching of fluorescencedue to photoprotective mechanisms anddamage to reaction centers may occursimultaneously in the marine environment.

On time scales of tens of minutes, non-photochemical quenching by photoprotectivepigments may lower σPSII (hence α*

decreases and Ek increases) while damage tophotosynthetic units may lower n (henceboth α* and P*

m), as mentioned earlier(Maxwell et al. 1994, Olaizola et al. 1994,Vassiliev et al. 1994).

On time scales of hours to days, the redox-signalling pathways can lead to thegeneration of specific signal molecules thatcan repress or enhance the expression ofchloroplast and/or nuclear encoded genes.These alterations are responsible for (e.g.)the light-dependent changes in cellular Chl acontent and the C:Chl a ratio and similarlyforced changes in response to thermalchanges (Escoubas et al. 1995). In thediatom Skeletonema costatum, there appearsto be a virtually linear relationship betweenthe C:Chl a ratio and the number of absorbedphotons per unit Chl a, irrespective of thespectral composition of light (Nielsen andSakshaug 1993).

In the oligotrophic upper ocean, lowphotochemical energy conversion efficienciestypically resulting from photoinhibition canoften be restored within two days to near-maximum values by incubating subsamplesunder moderate irradiance (adding

supplemental inorganic nitrogen if the cellsare also nutrient-deficient; Falkowski andKolber 1995). Such restoration occursnaturally in the open ocean in conjunctionwith an increase in the nutrient supply wheneddies and storms are passing. Eddy andstorm-enhanced productivities may beindicated in transects of variable fluorescenceand seem to be correlated with temperaturechanges as low as ca 0.1°C.

3F. Curve fitting

Problems related to the fitting ofmathematical functions to P vs E data toestimate α*, β* and P*

m have been adressedby Frenette et al. (1993). The user hasseveral choices at this step but, according tothe approach that is adopted, the resulting Pvs E parameters can be markedly different.

A first problem is the dark fixation of carbon,which is related to β-carboxylation (Geiderand Osborne 1991). The dark bottle valuesmay constitute a significant fraction of lightbottle values, especially in oligotrophicwaters (Banse 1993). It is generally admittedthat fixation in clear bottles minus darkfixation represents the effect ofphotosynthesis. Therefore, the dark fixationrate, which is typically not null, is subtractedfrom the light bottle readings and the P vs Ecurve is forced through the origin, as forcommonly used P vs E formulations, such asthose of Webb et al. (1974) and Platt et al.(1980). This may be the most reasonableapproach; however, the difference betweenvalues from a dark and a clear bottle doesnot necessarily exactly representphotosynthesis. There are some indicationsthat the non-photosynthetic carbon fixationrate is not the same in the dark as in the light(Legendre et al. 1983, Li et al. 1993).

Knowledge of the dark fixation rate isimportant in P vs E determinations becauseφm for carbon uptake occurs at vanishing

27

irradiances (derived from α*). The carbonfixation rates in the dark bottle and at thelowest irradiances have the largest weight inthe determination of this slope. Usingfunctions that allow the curve to have a non-null intercept at the origin, may lead to widedispersion of α* values.

A second problem concerns the choice of a Pvs E model. Several models or mathematicalrepresentations exist (Webb et al. 1974,Jassby and Platt 1976, Jamart et al. 1977,Platt et al. 1980), which include or not aterm for photoinhibition. At present, it ispremature to recommend one model aboveanother as long as they fit the data reasonablywell - a theoretically "correct" functionwould yield a curvature anywhere betweenthose of the Michaelis-Menten and Blackmanfunctions. One should, however, be aware ofthis source of variation and therefore takecare to follow a protocol that includearchiving of the original data (i.e. the carbonfixation at each irradiance) so that thesecould be fitted to other models for purposesof comparison. As stressed by Frenette et al.(1993), systematic differences in α* and P*

m

are found between the models of e.g. Webbet al. (1974), Jassby and Platt (1976) andPlatt et al. (1980), α* being particularlysensitive. This results from the regressionsforcing mathematical functions with differentcurvatures to the data. One also has to beaware of this when comparing data in theliterature.

Published protocols usually containrecommendations concerning precision andaccuracy of the results. This cannot beachieved for the photosynthetic parameters,which are estimated from several differentmeasurements and thus lack an absolutereference.

4. SPECIAL INSTRUMENTATION

4A. Variable fluorescence

Considering all the artefacts indeterminations involving incubations,emphasis should be put on developing non-manipulative methods, preferably profilingmethods that do not require water samplingat all. The variable fluorescence yield ofphotosystem II (Falkowski and Kiefer 1985),together with the development of new bio-optical instruments, are possible approacheswhich permit new insights into thephysiology of the phytoplankton and do notrequire filtration, thus avoiding a time-consuming and error-generating step inoperations at sea.

The usefulness of variable fluorescencemethods to studies of photosynthetic rates inthe sea lies not so much in the quantitativevalue of the measurements but inunderstanding the parameters that influencethe photosynthetic behavior ofphytoplankton. The changes in variablefluorescence can be extremely helpful ininterpreting or apportioning causes ofvariations in φm. While such measurementscan be made using specialized instruments,such as the fast-repetition rate fluorometer(FRRF), the pump and probe fluorometer(PPF), and the pulse-amplitude-modulated-fluorescence meter (PAM), similar measure-ments can be made using simple fluorometersby determining the fluorescence yields priorto and following the addition of the electrontransport inhibitor, DCMU. Yields ofstimulated fluorescence can be used, inaddition to the above, to determine thefunctional absorption cross section of PSII(σPSII) in situ and to derive Ek in the watercolumn.

The FRRF, PPF and PAM instruments arebased on the progressive closure ofphotosystem II reaction centers andsubsequent increase of fluorescence, by abrief series of strong (pump) and weak(probe) excitation flashes. The characteristics

28

and evolution of the fluorescence yieldduring this brief series of flashes is then usedto estimate [Chl a], the fraction of openreaction centers, the maximum change in thequantum yield of fluorescence, and theabsorption cross section of PSII (Falkowskiand Kolber 1993). These parameters can beentered in models of photosynthesis and usedto estimate the primary productivity. Thegreat advantages of the FRRF and PPFfluorometers is their great sensitivity and thatthey are profiling, i.e. they can be attached toa CTD and provide vertical profiles ofphotosynthetic parameters at the same rate asvertical profiles of temperature and salinity,making it possible to study the response ofprimary production to physical forcing atsmall space and time scales. A hand-heldPAM instrument for divers is available andoffers new possibilities for in situ studies ofphotosynthesis of both phytoplankton andseaweeds.

The measurement of variable Chl afluorescence can also be done in a surveymode on a ship by diverting a stream of near-surface sea water from the hull pump into aflow-through cuvette of the FRRF,configured with a blue excitation source anda red emission detector (Kolber et al. 1994).In this instrument, the excitation pulse isprovided as a burst of subsaturating flashes inthe microsecond time domain. Thecumulative excitation provided by the flashesleads to saturation of PSII within ca 75 µs;the saturation profile can be used to derivethe initial fluorescence yield, Fo, themaximum fluorescence yield, Fm, and,importantly, σPSII (Greene et al. 1994). Thesemeasurements can also be made in situ witha submersible version of the FRRF, equippedwith 2 excitation channels. The FRRF ismuch more efficient than the formerly usedPPF, as σPSII can be derived virtually instantly(within 150 µs), instead of over a period ofminutes. Moreover, in a vertically profilingconfiguration, the submersible FRRF can beused to derive the fraction of open reaction

centers at any instant. From knowledge ofthe cross sections, the quantum yield ofphotochemistry, and the simultaneouslymeasured instantaneous spectral irradiance,which provide an estimate of the absoluterate of linear photosynthetic electrontransport, can be derived and translated intoa P vs E curve after calibration againstoxygen evolution or carbon uptake rate(Kolber and Falkowski 1993).

Finally, the FRRF can be mounted on anundulating platform that permits both verticaland horizontal profiling. All three types ofsampling strategies can be used to derivevertical and horizontal sections offluorescence parameters along shiptracks(Falkowski and Kolber 1995). In conjunctionwith satellite images, these in situmeasurements can be used to infer howchanges in the physical environment affectphotosynthetic energy conversion efficiency.

4B. Natural fluorescence

The contribution of phytoplanktonfluorescence to the upward irradiance wasfirst documented by Morel and Prieur (1977)and Neville and Gower (1977). Since then,natural fluorescence (also known as passive,solar or sun-induced phytoplanktonfluorescence) has been used to estimate seasurface [Chl a] (Gordon 1979, Topliss 1985,Gower and Borstad 1990) and photosyn-thetic activity in the water column of marineenvironments (Kiefer et al. 1989, Chamberlinand Marra 1992, Abbott et al. 1995). A largevariety of instruments containing passivefluorescence sensors have been developed;some for water column profiling and somefor drifters (Chamberlin et al. 1990, Marra etal. 1992). By the end of this century, threesatellites in orbit (MODIS, MERIS, GLI)will measure sea-surface sun-stimulatedfluorescence rates.

The underlying theory for predicting the

29

( ) ( )λλ= ∫ dnm700

nm400φφ aEP

o

( ) ( )λλ= ∫ dnm700

nm400

φφ aEJo

ff

photosynthetic rate on the basis of naturalfluorescence, has been elaborated by Kieferet al. (1989) and Kiefer and Reynolds(1992). The instantaneous rate offluorescence (Jf, mol photons s-1), as well asthe gross rate of photosynthetic carbonfixation (P, mol C m-3 s-1), can beapproximated as the product of the rate ofenergy absorbed by the photosystem and thefraction of this energy re-emitted asfluorescence or stored as photosyntheticcarbon, respectively. These fractions aredetermined by the probability that energyharvested by the photosystem be channeledinto carbon fixation or fluorescence:

[19]

[20]

φ is the quantum yield of photosynthesis andφf the fluorescence quantum yield. The use ofnatural fluorescence to estimate grossphotosynthetic rates is appealing because it isa non-intrusive method. Because rates ofsolar-induced fluorescence andphotosynthetic carbon fixation in the watercolumn appear to be equally dependent onthe energy harvested by photosyntheticpigments, the estimates may be assumed tobe independent of spectral variation inirradiance and light absorption coefficient ofphytoplankton (reabsorption of emitted lightmay constitute a problem, see Collins et al.1985). It is, however, necessary to know thevariability in the φ: φf ratio in order tocalculate accurate photosynthetic rates frommeasurements of natural fluorescence:

P = (φ:φf) Jf [21]

Chamberlin et al. (1990) used an empiricalapproach based on field observations todescribe the variability in the φ:φf ratio due tochanges in PAR. Their observations suggest

that, when combining the results from avariety of ecosystems and light regimes (from2-150 m depth), the variability inmeasurements of natural fluorescenceaccounts for 84% of the observed variabilityin photosynthetic rates; i.e. Jf and P asexpected, are largely dependent on theirradiance. However, they found that the φ:φf

ratio increases with increasing temperatureand may decrease almost two orders ofmagnitude with increasing irradiance; asimilar result was also obtained noted byStegmann et al. (1992). By taking thisvariation into account, Chamberlin et al.(1990) were able to account for 90% of thevariability in photosynthetic rate related tonatural fluorescence.

One must be careful when extrapolatingresults to various seasons and oceanicregimes (Stegmann et al. 1992). Forexample, species composition may play animportant role in the variability in the φ:φf

ratio; e.g. this ratio may be higher incommunities dominated by Synechococcus,as a result of its low PSII:PSI ratio, than inother communities. Moreover, there is strongevidence of deviations betweenphotosynthetic rates estimated at sea andthose predicted on the basis of naturalfluorescence under the high-light conditionsoften observed in the upper ocean (Stegmannet al. 1992), emphasizing the need toaccount for photoprotective pigments (i.e.the xanthophyll cycle).

In summary, variations in the grossphotosynthetic carbon fixation rate, P, maybe strongly correlated with variations in Jf

(Babin et al. 1996a) over a wide range ofirradiances, suggesting that the rate of lightabsorption by the phytoplankton, i.e. theproduct of aφ and irradiance, is the mainvariable controlling Jf and P. This correlationmay, however, be considerably weakened inthe upper part of the water column wherephotosynthesis is light-saturated.

30

4C. In situ absorption meters

In situ spectral absorption meters are nowavailable (Moore et al. 1992, Zaneveld et al.1992). These are 25-cm path lengthtransmissometers that have been modified toinclude a reflecting tube and a large areadetector, so that most of the scattered light iscollected by the detector (residual scatteredlight is estimated using an infra-red channel).One version of this instrument, called the"chlam", measures light absorption in thechlorophyll red peak region, at 676, 650 and712 nm; the measurement at 676 nmprovides the necessary absorption value, andthe other two wavelengths allow correctionfor the absorption by degraded chlorophylls.Such an absorbance meter operated onvertical profiles, coupled to fluorescenceexcitation spectra at discrete depths, wouldallow scaling of fluorescence excitationspectra measured at discrete depths. Asimilar scaling can also be achieved withanother version, the AC-9®, that measuresabsorption and attenuation at 9 user-definedwavelengths. Although this instrument yieldsmore complete spectral information than the"chlam", at all wavelengths except in the redpart of the spectrum, measurements ofabsorption of light by phytoplankton is notstraight-forward: absorption by dissolvedmatter has to be corrected for, e.g. using asecond AC-9 with a filter as well asparticulate detrital absorption, e.g. bynumerical methods.

5. OPEN QUESTIONS

In principle, light-photosynthesis modelsprovide estimates for the gross rate ofphotosynthesis if irradiance and the quantumyield for photosynthesis are known. Bysubtracting the daily rates for respiration andproduction of DOC, net daily production ofphytoplankton can be calculated. Results

from such models can be expressed in termsof oxygen, nitrogen or carbon, using Redfieldratios (assuming that nutrient deficiency doesnot skew the ratio in the cells relative to thisratio). In prognostic models, i.e. if we wantto use the predicted increase in biomass asthe basis for the next-step prediction of themodel, the increase in biomass (e.g. C) mustbe converted into an increase in [Chl a]which is the input of the P vs E function; forthat, we need the C:Chl a ratio (as incalculations of carbon biomass). The C:Chl aratio, however, is highly variable and apotential source of error. Similarly, the ratiobetween carbon fixed and oxygen evoluted(i.e. the photosynthetic quotient) and the roleplayed by respiration, need to be betterunderstood.

Experiments, and especially incubations inartificial conditions, modify the environmentof the phytoplankton, which immediately,and more or less rapidly, start to acclimatethemselves to the new conditions, whereasthe results of the experiment often arereferred to initial physiological conditions.The parameter σPSII may, for instance, changein the course of a few minutes (Falkowski etal. 1994), with consequences for φm and α*

(Mitchell and Kiefer 1988, Morel et al.1987). The effect on community productionestimates may, however, be slight (Falkowskiet al. 1994).

Not all the light absorbed by thephytoplankton is transferred tophotosynthesis, the remaining fraction beingabsorbed, especially by the photoprotectivepigments. This fraction is included in thedetermination of a*

φ but does not contributeto photosynthesis, thus not to α* (Sakshauget al. 1991, Sosik and Mitchell 1995).Hence, if the cells have large concentrationsof photoprotective pigments (i.e. are high-light acclimated), the estimate of φm (= α*/a*

φ)will be low and necessarily spectrallydependent because the photoprotectivepigments absorb mainly in the blue region

31

(Sakshaug et al. 1991, Morel et al. 1996).Once again, as much information as possibleconcerning the lamp emission spectrum andthe phytoplankton absorption spectra shouldbe given in published papers and recorded indata bases.

Over the past decade, phytoplanktonecologists have tended to develop ever moreelaborate models to relate the photosyntheticrate to [Chl a] via irradiance. In so doing, theimportance of P*

m or its analogue, themaximum Chl a-normalized photosyntheticrate within a water column (also known asP*

opt, in the same units as P*m ), tends to be

neglected. P*m varies by more than one order

of magnitude in the ocean. It cannot inprinciple be predicted from a*

φ or φm, hencenot from α*. While, to a first order, P*

m isrelated to temperature, the simple Arrhenius-type of relationship described by Eppley(1972) does not appear to describe thevariations in P*

m in the ocean (Balch andByrne 1994, Behrenfeld and Falkowski1997). Certainly, much more attention needsto be paid to the sources of variation in P*

m

and P*opt, if global scale productivity models

are to be developed with an acceptabledegree of physiological representation ofphotosynthetic processes in situ.

Finally, the P vs E parameters and thepigment content of phytoplankton, and thusthe C:Chl a ratio - which are essential inmodels of algal growth rate - arecontinuously changing as a result of thefluctuations in environmental conditions,because they are subjected to acclimation,typically with delayed responses at differenttime scales. This implies a need for models inwhich the P vs E parameters and pigmentcontent are dynamical variables. Thedevelopment of such models has alreadybegun (Geider et al. 1996).

6. ACKNOWLEDGEMENTS

Thanks are due to Drs Marcel Babin, MikeBender, Geoffrey Evans, Jacques Gostan,Geir Johnsen and Ricardo Letelier for theircontributions to this paper, and for valuablediscussions. We thank JGOFS for sponsoringthe meetings of the Task Team forPhotosynthetic Measurements and Dr. GuyJacques for hosting one of the meetings inBanyuls-sur-Mer, France, in September1995.

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