10 Productivity and Food Webs in the Sea

Notes for Marine Biology: Function, Biodiversity,

EcologyBy Jeffrey S. Levinton

©Jeffrey S. Levinton 2001

Productivity vs biomass

Biomass the mass of living materialpresent at any time, expressed as gramsper unit area or volume

Productivity is the rate of production of living material per unit time per unit areaor volume

Productivity

Primary productivity - productivity due toPhotosynthesisSecondary productivity - productivity due toconsumers of primary producers

Food Chain

Food chain - linear sequence showingwhich organisms consume which otherorganisms, making a series of trophic levels

Food web - more complex diagram showingfeeding relationships among organisms, notrestricted to a linear hierarchy

Food Chain Abstraction

Adultherring

Phytoplankton

Barnaclelarvae

Mollusklarvae Small

copepodseuphausid tunicate

cladocerans

amphipodsand eel

Young herring

arrowworm

Largercopepod

Phytoplankton

Copepod

Herring

Food chain Food Web

Transfer Between Trophic Levels

Transfer from one trophic level to thenext is not complete:

1. Some material not eaten2. Not all eaten is converted with 100%

efficiency

Transfer Between Trophic Levels 2

Budget for ingested food (use energy units):

I = E + R + G

I amount ingestedE amount egestedR amount respiredG growth (partitioned between somatic

growth and reproduction)

Transfer Between Trophic Levels 3

Incomplete transfer up a food chain:

Measure by food chain efficiency:

E = amount extracted from a trophic level amount of energy supplied to that levelOften in range of as little as 10%

Transfer Between Trophic Levels 4

Use food chain efficiency to calculateenergy available to highest trophic level:

P = BEn

B = primary productionP = production at highest levelE = food chain efficiencyN = number of links between trophic levels

Transfer Between Trophic Levels 4

Use food chain efficiency to calculateenergy available to highest trophic level:

P = BEn

Let E = .1, B = 1, n = 2,3,4If n = 2, P = ?

Transfer Between Trophic Levels 4

Use food chain efficiency to calculateenergy available to highest trophic level:

P = BEn

Let E = .1, B = 1, n = 2,3,4If n = 2, P = ?P = 1 x (0.1)2 = 1 x 0.01 = 0.01

Transfer Between Trophic Levels 4

Use food chain efficiency to calculateenergy available to highest trophic level:

P = BEn

Let E = .1, B = 1, n = 2,3,4If n = 3, P = ?P = 1 x (0.1)3 = 1 x 0.1 x 0.1 x 0.1 = 0.001

Transfer Between Trophic Levels 5

Use food chain efficiency to calculateenergy available to highest trophic level:

P = BEn

With 5 trophic levels, a change of E from0.1 to 0.2 magnifies P by a factor of 16

Oceanic Food Webs

Food webs in the oceans vary systematically in food chain efficiency,number of trophic levels, primary production

Oceanic Food WebsFood Chain

TypePrimary

Productivity

gCm-2y-1

Trophic Levels

Food Chain

Efficiency

Potential

Fish Production

mgCm-2y-1

Oceanic 50 5 10 0.5

Shelf 100 3 15 340

Upwelling 300 1.2 20 36,000

Oceanic Food WebsFood Chain

TypePrimary

Productivity

gCm-2y-1

Trophic Levels

Food Chain

Efficiency

Potential

Fish Production

mgCm-2y-1

Oceanic 50 5 10 0.5

Shelf 100 3 15 340

Upwelling 300 1.2 20 36,000

Oceanic Food WebsFood Chain

TypePrimary

Productivity

gCm-2y-1

Trophic Levels

Food Chain

Efficiency

Potential

Fish Production

mgCm-2y-1

Oceanic 50 5 10 0.5

Shelf 100 3 15 340

Upwelling 300 1.2 20 36,000

Oceanic Food WebsFood Chain

TypePrimary

Productivity

gCm-2y-1

Trophic Levels

Food Chain

Efficiency

Potential

Fish Production

mgCm-2y-1

Oceanic 50 5 10 0.5

Shelf 100 3 15 340

Upwelling 300 1.2 20 36,000

Oceanic Food Webs

Note: Great potential of upwelling areasdue to combination of high primary production,higher food chain efficiency, lower numberof trophic levels

Oceanic Food Webs

Stable, low nutrient Turbulent, high nutrient

Few trophic levels

Manytrophiclevels

Open ocean,gyre centers

Shelf, upwelling

Measuring Primary Productivity

Gross primary productivity - total carbon fixedduring photosynthesis

Net primary productivity - total carbon fixed during photosynthesis minus that part whichis respired.

Measuring Primary Productivity 2

Net Primary productivity most interesting:gives that part of the production available to higher trophic levels

Measuring Primary Productivity 3

Oxygen technique -

Principle - relies upon fact thatoxygen is released during photosynthesis

CO2 + 2H2O ---> (CH2O)n + H2O + O2

Measuring Primary Productivity 4

Oxygen technique 2 - there is an addition fromphotosynthesis and a subtraction fromrespiration

Measuring Primary Productivity 5

Oxygen technique 3 -

Measurement of oxygen:

Winkler technique - chemical titration of Oxygen

Polarographic oxygen electrode -

Measuring Primary Productivity 6

Oxygen technique 4 -

Light-Dark bottle technique:

Light bottle gives oxygen from photosynthesisminus oxygen consumed in respiration

Dark bottle gives oxygen consumed from Respiration

Measuring Primary Productivity 7

Oxygen technique 5 -

Light-Dark bottle technique:

Start light and dark bottles with water sample, waita short amount of time

At end of experiment: oxygen in light minus that in dark bottle gives you gross photosynthesis

Measuring Primary Productivity 8

Oxygen technique 6 -

Light-Dark bottle technique:

Get change of oxygen, but need to convertto units of carbon. If phytoplankton are synthesizing sugars, multiply change inoxygen atoms by 375 to get equivalent incarbon

Oxygen technique - effect of depth

Measuring Primary Productivity 9

Radiocarbon technique -

Principle: carbon is taken up by cellsDuring photosynthesis, so if you label that carbon you can trace it as it isincorporated into cells during photosynthesis.

Measuring Primary Productivity 10

Radiocarbon technique 2 -

Method: add bicarbonate to solutionWith phytoplankton that is labeled with14C

Measuring Primary Productivity 11Radiocarbon technique 3 -

Method: add bicarbonate to solutionWith phytoplankton that is labeled with14CIncubate phytoplankton in the radiocarbonSolution

Then filter phytoplankton and count radiocarbonTaken up by phytoplankton, using a scintillationcounter

Measuring Primary Productivity 12Radiocarbon technique 4 -

Calculation: 1. Know the amount of bicarbonate that wasin container2. Know the amount of radiolabeled bicarbonateyou added and the amount that was taken up by Phytoplankton

allows calculation of amount of bicarbonatetaken up in photosynthesis

Measuring Primary Productivity 13Radiocarbon technique 5 -

Correction:14C is taken up more slowly than much

more common stable isotope 12C. Therefore,

--> need to multiply results by 1.05 to get amount in photosynthesis

Measuring Primary Productivity 14Radiocarbon technique 6 -

What you get with this measure:

Carbon incorporation into phytoplankton(net photosynthesis)

Measuring Primary Productivity 15Compare Oxygen technique with radiocarbon:

Oxygen technique - used where primary production is high in estuaries, shelf

Radiocarbon technique - useful where primaryproduction is low such as open ocean

Measuring Primary Productivity 16Compare Oxygen technique with radiocarbon 2:

Oxygen technique tends to give higher estimatesof primary production, perhaps becausecells are leaking sugars during photosynthesis,resulting in loss of radiocarbon when cellsare filtered and counted

Measuring Primary Productivity 17Satellite Approaches:

Satellites can use photometers specific towavelength to measure chlorophyll, Seawater temperature

Need ground truthing to get relationshipBetween chlorophyll concentration and primary production; varies with region

Measuring Primary Productivity 18Satellite Approaches 2:

Satellites can use photometers specific towavelength to measure chlorophyll, Seawater temperature

Need ground truthing to get relationshipBetween chlorophyll concentration and primary production; varies with region

sun

Satellite

Color scanner

IrradianceRadiance

Phytoplankton

Geographic Variation of Productivity

1. Continental shelf and open-ocean upwellingAreas are most productive2. Convergences and fronts often are sites ofrise of nutrient rich deep waters (e.g., shallowwater seaward of slope3. Central ocean, gyre centers are nutrient poor,low primary production

SouthPacific

NorthPacific

NorthAtlantic

SouthAtlantic

Antarctic

IndianOcean

Satellite image of world productivity, from SeaWiFS satellite

The End