LIGHT PENETRATION INTO FRESH WATERLight Penetration into Fresh Water 297 The potential applied to...

VOL. X, No. 4 OCTOBER, 1933

LIGHT PENETRATION INTO FRESH WATER

I. A THERMIONIC POTENTIOMETER FOR MEASURINGLIGHT INTENSITY WITH PHOTO-ELECTRIC CELLS

BY W. H. PEARSALL AND P. ULLYOTT.

{Received 30th January, 1933.)

(With Six Text-figures.)

INTRODUCTION.THE consideration of light as an environmental factor of fresh-water organismsraises a number of questions, and, even from a purely physical point of view, theproblem of measuring light and interpreting the results is by no means simple. It iswell known that the radiant energy emitted from the sun varies in wave-length from3500 A. to 20,000 A., and the absolute value of the energy which reaches the earth'ssurface is different for each different wave-length within these limits. Clearly anypapers dealing with the "measurement of light intensity" should define at theoutset the significance of their data. In this connection the range of wave-lengthsover which the photo-sensitive mechanism is operative, and its relative sensitivityto the different parts of that range must be known. Only under these conditionshave the results any real meaning.

In an aquatic environment there are additional complications arising from thefact that radiant energy of different wave-length has different powers of penetratingthrough water. The effect of this factor is to make the light environment at differentdepths in any body of water different as regards quality as well as quantity. Further,the absorption coefficient for radiant energy of a definite wave-length is not the samein different bodies of water, or even at different levels in the same body of water.It appears from the outset that the investigation of the light environment is com-plicated. This has been fully shown by the work of Birge and Juday (1929, 1930,1931, 1932), who have studied the light penetration in a number of lakes on theNorth American continent. The qualitative and quantitative differences in theradiant energy will obviously be extremely significant for the phytoplankton and theaquatic vegetation, and they, both directly and indirectly, will be important for otherorganisms.

The purpose of the present paper is to describe a transportable apparatus formeasuring light intensity, and to indicate briefly the effect of the light on submergedvegetation.

APPARATUS AND METHODS.Birge and Juday used a pyrolimnometer (thermopile sensitive to ail radiations)

and measured the potential produced when light fell on this instrument by a milli-voltmeter, or, for more accurate work, by a d'Arsonval galvanometer. This methodhas the great disadvantage that the recording instrument has to be kept on the shore.

JEB-X1V 2O

294 W. H. PEARSALL and P. ULLYOTT

This means that the distance from the shore at which observations can be made islimited by the length of cable connecting the two parts of the apparatus.

In the present research photo-electric cells of the potassium-on-copper vacuumtype were used, and preliminary trials with a number of methods of measurementwere made. Even when a potential difference of 200 volts was applied across theterminals of the photo-electric cells, they produced too small a current (io~9 toio~6 amperes) to be measured by a galvanometer which was transportable or inany sense stable under conditions such as those met with in a boat. So at first aneon discharge tube measuring device similar to that recommended by Poole andPoole (1930) was used. This had the advantage of portability, but after a time it wasdiscarded because it was neither sensitive enough nor accurate enough for the pur-pose of the work. After that, it seemed necessary to use some device which wouldamplify the minute current from the cell to such an extent that it could be recordedon a robust galvanometer, so that the whole apparatus should be very sensitive andat the same time sufficiently stable to be capable of being read accurately in a boateven during rough weather. Finally, a thermionic potentiometer used in conjunc-tion with a Unipivot galvanometer (Camb. Inst. Co.) proved very satisfactory. Thisinstrument can be read accurately under surprisingly difficult conditions.

Two photo-electric cells are necessary. One cell is lowered into the water ina container, while the other remains in an equivalent container on the deck of theboat from which the observations are made. The cell for under-water measurementsis enclosed in a water-tight gun metal casing, which has a circular glass window in itsupper surface. The two wires to the cell are enclosed in a single rubber cable whichenters the casing through a water-tight gland fitting. The second cell of the sametype in its container is exposed to the normal daylight on the deck of the boat. Thewindow of the under-water container and the similar window in the deck containerare covered with sheets of opal glass. In this way oblique rays, which otherwisemight miss the sensitive plate of the cell, are scattered and produce their effect.

Any method of quantitative amplification by means of thermionic vacuum tubesis open to criticism. Firstly, such tubes are easily affected by minute external dis-turbances, and, secondly, there are bound to be small variations in the currents whichare supplied to them. However, efficient screening overcomes the first difficulty,and compensating circuits have been designed which overcome the second, withthe result that a well-designed and well-maintained thermionic potentiometer isabsolutely accurate over long periods of time. The circuit used is shown in Fig. 1,and is a modification of one recommended by Winch (1930) and by Vickers, Sugdenand Bell (1932). The success of such a circuit depends on the careful choice of valves.The valves used in the potentiometer are of the Marconi D.E. 5 type working at afilament potential of 6 volts. The filaments are of pure tungsten and their emissionis constant under constant conditions of filament potential. Valves having thoriatedtungsten or coated filaments cannot be used for this work because their emission isinconstant. Two valves were chosen having characteristics as nearly the same aspossible, so that any small changes in filament voltage or applied anode potentialproduce similar changes in the anode currents of both valves. In this way changes

Light Penetration into Fresh Water 295

which may occur in the sources of supply of current do not affect the galvanometerzero reading. ' •

While the apparatus was being constructed the normal potential of the grid ofeach valve was set to — 3 volts. Then, with the low- and high-tension currentsswitched on, a small resistance was put into the filament circuit of one of the valves,and its value adjusted so that a zero reading was obtained on the galvanometer. Itis essential to make solid metal connections everywhere in the wiring up of anapparatus of this kind. The connecting wires were soldered directly to the valvepins and to the metal plug-in holes of the batteries. The valves are run at theirmaximum filament potential, for then the anode current is approaching its " satura-tion" value with respect to filament potential. Consequently small variations in the

Fig. 1.

filament potential will be ineffective in producing changes in the anode potential.Everything except the photo-electrical cells, the 6-volt low-tension accumulator, andthe galvanometer, was enclosed in a single box, which itself was enclosed in ascreening box of copper.

The potentiometer is essentially a series of resistances arranged in the form ofa Wheatstone's Bridge. Two of the resistances are the wire-wound fixed resistancesof 10,000 ohms each (Fig. 1 R1 and R2), and the other two are the internal (anodefilament) resistances of the two valves (V1 and V2). The internal resistance of V2 isconstant because the grid of V2 is kept at a constant negative potential (— 3 volts) bythe grid-bias battery G, so that the anode potential does not vary. The grid of theother valve, Vlf is only at a negative potential of — 3 volts when no current ispassing through the photo-electrical cell. When light falls on the cell a current whichis proportional to the incident light intensity passes through it. Thiscurrent builds


up a potential on the grid of Vx, with the consequence that the internal resistancefalls and the anode potential decreases. Within the limits used, the fall in potentialon the anode of Vt is proportional to the increase of grid potential of the valve. Infull sunlight this potential is never greater than + 1-5 volts, so that the relationbetween the grid potential and the anode potential remains linear (Fig. 2). In itsturn, the increase of grid potential is proportional to the current passing throughthe photo-electric cell, with the final result that the deflection of the galvanometeris proportional to the amount of light falling on the cell.

The galvanometer was too sensitive to be able to register the whole range of lightintensities when coupled directly between the anodes of the two valves. But sinceit was desirable to retain this sensitivity for the measurement of small light intensities,a switch was put into the circuit so that the galvanometer could be coupled eitherdirectly to the two anodes or with a resistance R3 of 100,000 ohms in series. This made

-9-0 -6-0 -3-0 0

Grid potential of V1 in volts

Fig. 2.

available two sensitivity scales, one for low intensities and one for high. The intro-duction of this resistance disturbed the linear relation between galvanometerreadings and light intensity so that a calibrated scale had to be drawn up.

The total anode current never exceeded 25 milliamperes, but this heavy currentmeant that high-tension batteries of the " super-power" type had to be used. Evenso, an inevitable running down was bound to occur, although the apparatus was neverused continuously for more than an hour at a time. Fortunately it is possible tominimise the effects of a small anode voltage drop on the galvanometer readings.Fig, 3 shows the galvanometer deflections at a constant grid potential of V1 fordifferent values of the working potential of the potentiometer. The actual voltagechosen was 195 volts, and it is plain that at this voltage a small decrease in potentialwill cause no serious decrease in the galvanometer readings. After the apparatushad been in use for four months it was found that there had been a drop to 187-5 volts,but this made a difference of less than 1-5 per cent, to the galvanometer readings.Furthermore, it is easy to correct for this voltage drop by referring to the graph.


The potential applied to the photo-electric cell is 30 volts and in the circuit isan additional resistance of 2 megohms, so that the current used cannot exceed15 microamperes. The voltmeter showed that this battery runs down so slowly thatit is, for all practical purposes, constant. The same applies to the grid-bias batterysupplying the potential to the grid of V2.

In using the potentiometer it is necessary to allow the low-tension currentto flow for a few minutes before taking readings, so that full expansion of the workingparts of the valves can occur before any measurements are made. At the beginningof each set of readings the zero of the galvanometer was noted down, and it was alsotaken again at the end. This is done as a precautionary measure to ensure that nochanges in the zero of the galvanometer were affecting the readings.

The photo-electric cells used are of the vacuum potassium type K.M.V. 6 madeby the General Electric Co. The reason for choosing such cells was that they weresupposed to be sensitive over a reasonably wide range of the spectrum.

150 175 200 225

Total working potential in voltsFig- 3-

Preliminary tests of the two cells soon showed that they differed very markedlyfrom each other, and it was consequently impossible to accept the standard spectralsensitivity curve for this type of cell as being applicable in either case. The light usedin the preliminary tests was a mercury vapour lamp, and by choosing suitable nitersthe cells were exposed to monochromatic light of different wave-lengths. Theabsolute energy value of the light was measured in each case by a sensitive thermo-pile and galvanometer. We are indebted to Mr G. H. J. Neville, of the PhysicalChemistry Laboratory, Cambridge, for supplying us with monochromatic light ofknown energy value.

The preliminary investigations only served to show that much more extensiveobservations were necessary, and, thanks to the kindness of Prof. R. Whiddington,F.R.S., the resources of the Department of Physics at the University of Leeds wereplaced at our disposal. Under these conditions it was possible to make completeseries of records of the spectral sensitivity characteristics of the two cells. For wave-lengths between 3000 and 6000 A. a mercury-vapour lamp was used, and between4000 and 7000 A. a "Pointolite" lamp and narrow-range Wratten niters. In eachcase the deflection of the Unipivot galvanometer was observed when a beam of lightof known absolute energy value (as measured by an accurate thermopile and galvano-


meter) was allowed to fall on the sensitive plate of the photo-electric cell. Thespectral sensitivity curves for the two cells are shown in Fig. 4. The cell W wasalways used as the under-water photometer.

An estimation of the sensitivity of the cell W to "total light" was made byobserving the galvanometer deflection when the "Pointolite" lamp was held atdifferent distances from the sensitive plate. In this way an arbitrary scale of readingswas obtained, in relation to light the composition of which was not very differentfrom that of daylight. Knowing the energy emission from such a lamp for differentwave-lengths and the sensitivity of the cells to those wave-lengths, it was possibleto construct a scale of values for total light. This scale represents the incidentradiant energy as ergs per square centimetre per second. The standards so obtained

ion

:Hec

t::c

.ir

anom

etei

i er

gs /c

m.

S>8

;ed

as1 p

er

ipr

ess

scal

e]S

ensi

tivi

ty(s

ensi

ti

0-5

0-4

0-3

0-2

0-1

0

—

1

-

1

A' \

\ Cell W

\

\

/TXCeUA

/ \ \

3000 4000 5000 6000 7000

Wave-length in Angstrom units

Fig. 4.

agree reasonably well with the various estimates of full daylight under differentconditions and are evidently of the correct order of magnitude.

In the observations in the field two series of readings were always made with thewater photometer, one with the opal glass only covering the window, and the otherwith the opal glass and a Wratten blue filter. This filter (No. 49 in the Wrattenseries) transmits light of wave-length 3600-5000 A. All measurements given foroutdoor conditions refer to the light incident on a horizontal surface. The measure-ments therefore represent vertical illumination, which we have called "E" in thetables.

OBSERVATIONS AND DISCUSSION.Observations on light penetration have been made in some of the lakes in the Lake

District. In each case the boat with the apparatus was taken well away from the shoreso that disturbing effects due to shading by submerged littoral vegetation, or by treesoverhanging the water, were excluded. The considerable distance between the boat


and the land also minimised the cutting down of skylight by tall objects such as treesor cliffs. The under-water container was suspended either from a boom about 2 m.long projecting from the side of the Laboratory's launch on Windermere or, on otherlakes, from the arms of a winch over the stern of a small boat. The boat was usuallyanchored from the bows only, but if necessary a second anchor was put out to moorthe boat in such a way that the support of the under-water container was keptpointing directly towards the point of maximum illumination in the sky. The effectsof the boat in cutting down skylight were thus reduced to a minimum.

The surface intensity of the light was first measured by both photometers andthe water photometer was then lowered down into the water. The galvanometerdeflection produced by the under-water photometer was recorded at each successivemetre or half-metre according to the turbidity of the water, and the lowering wascontinued until there was no appreciable deflection of the galvanometer needle. Atthis lowest limit the air photometer reading was taken as a matter of routine, nomatter how constant the light conditions appeared to be to the eye. The under-waterphotometer was then hauled up, and a reading taken at each metre on the upwardjourney. The surface intensity was again measured with both photometers. This wasthe procedure adopted under stable light conditions. If there was any tendencytowards variations an air photometer reading was taken before each photometerreading. At each observation station two series of readings were taken, one measur-ing "total light," and the other blue light only. The significance of "total light" asmeasured by this apparatus has already been made clear.

The apparatus has been extensively employed on Windermere during 1932. Thetypes of results obtained, however, may be best illustrated by the figures for lightpenetration in three lakes, Eonerdale, Windermere, and Bassenthwaite. Of theseEnnerdale is a rocky lake with clear water, and is, in fact, the clearest of the largerlakes. Windermere, on the other hand, has a fairly heavy phytoplankton, and may betaken to represent a much more silted type of lake, that is to say one in which a muchlater stage of evolution has been reached (Pearsall, 1921). The water in Windermereis faintly yellowish in colour. Bassenthwaite has the most turbid water in the LakeDistrict. This condition is partly artificial in origin, since the lake was polluted formany years by silt washings from lead mines. This silt still influences profoundly thecharacter of the lake. Bassenthwaite also bears very heavy diatom maxima at times,and its water is coloured yellow with peaty matter. The interest of these exampleslies in the fact that they cover almost the whole range of turbidity found in naturalfresh water in this country, so that the suitability of our apparatus for this type ofwork can be estimated.

The data given in Table I were obtained on the following dates and under thefollowing conditions:

1. ENNERDALE. September 23rd, 1932. Sky nearly covered with light greycloud. Wind north-east, light. Ripple 5-8 cm. high. Two series of results obtained,one for sunlight (E — 4-5 x io6 ergs/cm.2/sec), and one for overcast conditions(E = 1 64 x io6 ergs/cm.2/sec). Expressed as percentages of full light in air, theseare not significantly different at the various depths, hence only the full sunlight ones

Tab

le I

. F

igur

es s

how

ing

the

pene

trat

ion

of

ligh

t in

to t

he

wat

ers

of

thre

e la

kes

in t

he

La

ke

Dis

tric

t.

o o

Lak

e

Dat

e

Tim

e (G

.M.T

.)

Lig

ht

Air

int

ensi

ty

Dep

th (

m.)

S i 2 3 4 5 6 7 8 91

0II 1

2 13 14 15 16 17 i8

Sun

's a

ltit

ude

Enn

erda

le

23.

ix.

32

1.1

0-1

.20

p.m

.

Tot

al

4-5

x io

8

ergs

/cm

. 2/sec

.

1.4

0-1

.50

p.m

.

Blu

e

o-8

xio

6

ergs

/cm

.2/sec

.

Win

derm

ere

18.

ix. 32

i.2

S-i

-35

P-m

.

Tot

al

4-5

xio

6

ergs

/cm

.2/sec

.

i-35

-i-4

5P-m

-

Blu

e

0-93

xio

6

ergs

/cm

.2/sec

.

Bas

sent

hwai

te

22. i

x. 3

2

II.O

-II.

20

a.m

.

Tot

al

27 x

io0

ergs

/cm

. 2/sec

.

11

.20

-11

.30

a.m

.

Blu

e

0-92

x i

o6

ergs

/cm

.2/sec

.

Perc

enta

ge o

f su

rfac

e in

tens

ity

93 0

68-3

46

332

-024

-21

79

13

4IO

-I7-

565-

724"

3°3-

122-

281-

691-

200-

85o-

6o0-

42O

-22

35°

93-5

51

829

-517

-310

-46-

i3-

642-

171-

280-

76— — — — — — — — — 33

°

75-0

32-2

15

67-

704

-00

2-I

I1-

160-

625

O-3

35— — — — — — — — — 34

°

7i'4

19-0

59

2i-

75

o-53

— — — — — •— — — — — — — — 33°

55-6

3i-

410

-5 5i5

2-62

1-45

0-77

0-4

00-

21

•—

36°

50-1 93

01

70

0-34

— — — — — — — — — — — — — — 37°

Dep

th*

m.

S o-5

I-O

i-5

2-O

2-5

3'O

3-5

4-0

— — — — — — — — — —

3 a d r r o H H

• T

he

dept

hs g

iven

in

this

col

umn

refe

r to

Bas

sent

hwai

te o

nly.

Total light intensity expressed as percentageof the total surface intensity (logarithmic

scale)

10-0

.£ 10

t 11

Q 12

13

Bassenthwaite

Winderinere

100-0


are reproduced here. September is the time of the phytoplankton maximum in thislake.

2. WINDERMERE (North basin). September 18th, 1932. Sky clear, with occa-sional small white clouds. Sun bright (E = 4-4 x io6 ergs/cm.2/sec). Lightnortherly breeze. Ripples 10 cm. high. Phytoplankton moderately abundant.

3. BASSENTHWAITE. September 22nd, 1932. Sun bright—few clouds but muchhaze (E = 2-55 x io6 ergs/cm.2/sec). Wind east, moderately fresh. Waves 15-20 cm.Water very turbid, with heavy diatommaximum. (These conditions are certainly

' not typical for this lake, since they weretaken at a time of maximal turbidity.)

In all the results the intensity of light ateach depth was calculated as a percentageof the light intensity in the air. The averageof the "down " and the " up " readings wastaken (Table I). Figs. 5 and 6 show theresults for the three lakes plotted on alogarithmic scale against depth. The figuresserve to illustrate the considerable dif-ferences between the different types ofwater. A light intensity of 1 per cent, isfound in Ennerdale between 14 and 15 m.,at 6-7 m. in Windermere, and between2-5 to 3 m. in Bassenthwaite. Different asthese results are among themselves theyshould be contrasted with data which havebeen obtained for sea water by Poole andAtkins (1929), who report a light intensityof 1 per cent, at a depth of 45 m. at thesame time of the year and under similarconditions.

The factors controlling the differencesin fresh water are three in number. Thefirst is the colour of the water. This isvery slight in Ennerdale, faintly yellowin Windermere, and distinctly yellow inBassenthwaite. The second, the amount ofsediment, which is greatest in Bassenthwaite, and least in Ennerdale, and thelast, the quantity of plankton (chiefly algae in these observations) which wasalso largest in Bassenthwaite and least in Ennerdale. In all three lakes the bluelight decreases with depth more rapidly than total light, and the proportionof blue light, as a percentage of total light at each depth, can easily be estimated.This is expressed in the table (Table II). These figures clearly show that thereis a great change in the quality of the light in passing from the surface to

Ennerdale

20-1-0 0 1-0 2-0

Logarithms of the total light intensity

Fig. s-


greater depths, where radiant energy of the longer wave-lengths is shown to bepredominant.

Intensity of blue light expressed as percentage of surface intensityof blue light (logarithmic scale)

0-1oi—

10-0

_ Bassenthwaite

.S 5

100-0

AVindermere

Eiraerdale10-1-0 0 1-0

Logarithms of intensity of blue light

Fig. 6.

2«0

Table II. Figures illustrating the penetration of blue light (3000-5000 A.) in differentfresh-water lakes. The intensity of the blue light is expressed as a percentage of thetotal light at each particular depth.

Depthm.

AirS

o-SI'O

i-52 03-04-05 °6 07-08 0

Penetration inEnnerdale

28-22S'3

• — •

21-2—

1 8 01 4 3" • 49°57-15654-42

Penetration inWindermere

34-22 7 9—

1 7 3—

I I - O6 73 9•—-———

Penetration inBassenthwaite

33-830-610*05

S-482-240 8 8——————

The light transmitted by a liquid is determined by the relation -j- — e~fld> where

Io is the intensity at some particular depth 0, Id the intensity at some greater depth d,


and [i the absorption coefficient. Since Io and Id are known from the data, thecorresponding value of fx for the various depths can be calculated. Since the sun'srays strike the water at an angle, the actual depths at which the observations are madedo not represent the distances under water through which the sunlight has passed.This will always be greater than the recorded depth. The altitude of the sun canreadily be found to the nearest degree. The refractive index of water is 1-333, so thatthe angle 6 of the direct rays from the normal is given by

sin 0 =sin A

where A is the angular distance of the sun below the zenith. The distance C, throughwhich the light has actually passed, will be given by the formula

j

C =cos 6 '

For the nearly similar conditions obtaining for the three sets of observations, thisdistance is from 1-25 to 1-27 times the observed depth. The values of the absorptioncoefficient calculated on this basis vary in the manner shown in Table III.

Table III. The absorption coefficients in the different lakes.

ENNERDALETotal light

RangeAverage

Blue light

RangeAverage

WlNDERMERETotal light

RangeAverage

Blue light

RangeAverage

BASSENTHWAITETotal light

RangeAverage

Blue light

RangeAverage

0-3 m.

0-279—0-3060292

0-420-0-5190461

0-3 m.

0-559-0-6650-564

0—4 m.

1-040—0-9180931

0-1-5 m.

1-74-1-131-41

2-56-2-702-64

3-10 m.

0-221-0-2360-226

0-401-0-4200-409

3-8 m.

0-498—0-515°-5°3

1-5-4 m.

1-07-0-937I-OI

10-17 m -

0-246-0-2810-263

3<M W. H. PEARSALL and P. ULLYOTT

It will be seen from these results that there is a considerably greater absorptionof light near the surface than in deeper water. This is due to the far greater quantityof phytoplankton in the upper zone, and also to the greater absorption of blue lightin the surface layers. But in Ennerdale there is a marked increase in the absorptioncoefficient below 10 m. This has not yet been completely investigated. It maypossibly be due to the greater number of plankton animals at these depths or, on theother hand, it may be that the water of the hypolimnion is more turbid than that ofthe epilimnion, as Birge and Juday have suggested. It should be emphasised thatthe differences in the absorption coefficients indicate very considerable differencesin the amount of light passing through the water. In Ennerdale where the differencesare least, about 74 per cent, of the light passes through 1 m. of average surface water,whereas about 80 per cent, passes through water taken from between 3 and 10 m. deep.

It should be noticed that the absorption coefficients show that the surface layerrich in phytoplankton is much shallower in Bassenthwaite than in the two clearerlakes. Also the differences in the transmission of light in the different lakes can becorrelated with differences in the depths to which the littoral vegetation extends. InEnnerdale the lower limit of this vegetation in 1919-20 was at least 10 m. (Pearsall,1921); subsequently (1932) attached plants have been found at 11 m. InWindermerethe rooted plants do not grow below 4-3 m., and in Bassenthwaite not below 2-75 m.While there is evidently a general correlation between these depths and the trans-mission of light, it is impossible to attempt to find a closer agreement at present. Forwe have no justification (except in the case of Windermere) for supposing that theisolated examples- of light penetration represent average conditions during thegrowing season. The data for Bassenthwaite were obtained at a time when the lakewas at or at least very near the condition of maximum turbidity, and average lighttransmission in summer must be higher. Another difficulty is the determination ofthe depth distribution for the plants. This involves a very large number of soundings,especially where vegetation is sparse as in Ennerdale. Further, it is doubtful whetherthe results of surveys in 1919-20 which are available (Pearsall, 1921) still representthe conditions in these lakes. In Windermere, for which accurate data are available,the rooted vegetation now (1932) only extends down as far as 4-3 m., as against6-5 m. in 1919-20. It is possible that similar changes have taken place in other lakes.

Nevertheless, one striking fact is clear from the data. As one goes from shallowerto deeper water, the rooted vegetation ceases to grow at depths where the lightintensity is still high. The following figures represent percentages of surface intensityat the lower limit of rooted vegetation (September, 1932):

EnnerdaleWindermere

Total light

4-27-3-163-25

Blue light

0-457-0-2760-40-0-35

They show that the vegetation in Ennerdale and Windermere does not occur belowa zone where the light intensity is about 3-5 per cent, of that at the surface. The lightintensity at the vegetational limit seems therefore to be unusually high, especially


when it is compared with the values of 1 per cent, of full daylight, or in many casesless than 1 per cent., which have been recorded (Adamson, 1911 and 1922, and Atkinsand Stanbury, 1930) as marking the limiting light intensity for vegetation in woods.

Three possibilities have to be considered in this connection. Firstly, it is possiblethat soil conditions may determine the lower limit of vegetation in some lakes; alsoit is known that soil conditions may affect plant distribution in relation to light.Secondly, the quality of the light may have a considerable effect, and the measure-ments show that the quality is very different at different depths. Thus when the totallight in these waters has fallen to 4 per cent, of the value for full daylight, the ratioof total light to blue light is about 1 : 30, while in normal daylight it is about 1:3.In Ennerdale 2-6 per cent, of the total light at 10-5 m. is blue light, and in Winder-mere 3-4 per cent, is blue light at 6-o m. There are grounds for the belief that thischange in the quality of the light is extremely important. Lastly, most of themeasurements of light intensities in ecological work have been made with methodswhich depend wholly or chiefly on the effects produced by the blue-violet end of thespectrum. Such methods necessarily give different results from those which includea wider range of the incident energy, a point which is illustrated very clearly in theresults tabulated in this paper.

The whole question of light conditions in nature is, however, complicated byother factors to which we propose to return in later communications. The presentresults indicate that the penetration of light of wave-length less than 5000 A. maybe very important as a factor influencing organisms living in fresh-water habitats.

SUMMARY.Potassium-on-copper vacuum photo-electric cells, in conjunction with a ther-

mionic potentiometer and Unipivot galvanometer, have been found to be satisfactoryfor measuring sub-aqueous light intensities.

Some characteristic results indicate that the light intensity at the limit of sub-aqueous vegetation may be much higher than those recorded in correspondingterrestrial habitats. It is suggested that the quality of the light is of great biologicalimportance.

The expenses of this enquiry have been defrayed by a grant from.the BritishAssociation to the Fresh Water Biological Association.

REFERENCES.ADAMSON, R. S. (191 I ) . Journ. Linn. Soc. Bot. p. 264.

(1922). Journ. Ecol. 9, 114.ATKINS, W. R. G. and STANBURY, F. A. (1930). Set. Proc. Roy. Soc. Dublin, 19, 517.BIRGE, E. A. and JUDAY, C. (1929). Trans. Wis. Acad. 24, 509.

(1930). Trans. Wis. Acad. 25, 285.(1931)- Trans. Wis. Acad. 26, 383.(1932). Trans. Wis. Acad. 27, 523.

PEARSALL, W. H. (1921). Proc. Roy. Soc. B, 92, 159.POOLE, H. H. and ATKINS, W. R. G. (1929). Journ. Mar. Biol. Assoc. 16, 297.POOLE, J. H. J. and POOLE, H. H. (1930). Reprinted from the Physical and Optical Societies Joint

Discussion. Cambridge, June 4-5, 1930, p. 142.VICKERS, A. E. J., SUGDEN, J. A. and BELL, R. A. (1932). Journ. Soc. Chem. Industry, 51, 545 and

57O.WINCH, G. T. (1930). Journ. Inst. Elect. Eng. 68, 533.

LIGHT PENETRATION INTO FRESH WATERLight Penetration into Fresh Water 297 The potential applied to...

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