Potassium-on-copper vacuum photo-electric cells, in conjunction with a thermionic potentiometer and Unipivot galvanometer, have been found to be satisfactory for 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 corresponding terrestrial habitats. It is suggested that the quality of the light is of great biological importance.

The consideration of light as an environmental factor of fresh-water organisms raises a number of questions, and, even from a purely physical point of view, the problem of measuring light and interpreting the results is by no means simple. It is well known that the radiant energy emitted from the sun varies in wave-length from 3500 Å. to 20,000 Å., and the absolute value of the energy which reaches the earth’s surface is different for each different wave-length within these limits. Clearly any papers dealing with the “measurement of light intensity” should define at the outset the significance of their data. In this connection the range of wave-lengths over which the photo-sensitive mechanism is operative, and its relative sensitivity to the different parts of that range must be known. Only under these conditions have the results any real meaning.

In an aquatic environment there are additional complications arising from the fact that radiant energy of different wave-length has different powers of penetrating through water. The effect of this factor is to make the light environment at different depths 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 same in 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 complicated. 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 the North American continent. The qualitative and quantitative differences in the radiant energy will obviously be extremely significant for the phytoplankton and the aquatic vegetation, and they, both directly and indirectly, will be important for other organisms.

The purpose of the present paper is to describe a transportable apparatus for measuring light intensity, and to indicate briefly the effect of the light on submerged vegetation.

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 method has the great disadvantage that the recording instrument has to be kept on the shore. This means that the distance from the shore at which observations can be made is limited by the length of cable connecting the two parts of the apparatus.

In the present research photo-electric cells of the potassium-on-copper vacuum type were used, and preliminary trials with a number of methods of measurement were made. Even when a potential difference of 200 volts was applied across the terminals of the photo-electric cells, they produced too small a current (10−9 to 10−6 amperes) to be measured by a galvanometer which was transportable or in any sense stable under conditions such as those met with in a boat. So at first a neon discharge tube measuring device similar to that recommended by Poole and Poole (1930) was used. This had the advantage of portability, but after a time it was discarded because it was neither sensitive enough nor accurate enough for the purpose of the work. After that, it seemed necessary to use some device which would amplify the minute current from the cell to such an extent that it could be recorded on a robust galvanometer, so that the whole apparatus should be very sensitive and at the same time sufficiently stable to be capable of being read accurately in a boat even during rough weather. Finally, a thermionic potentiometer used in conjunction with a Unipivot galvanometer (Camb. Inst. Co.) proved very satisfactory. This instrument can be read accurately under surprisingly difficult conditions.

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

Any method of quantitative amplification by means of thermionic vacuum tubes is open to criticism. Firstly, such tubes are easily affected by minute external disturbances, and, secondly, there are bound to be small variations in the currents which are supplied to them. However, efficient screening overcomes the first difficulty, and compensating circuits have been designed which overcome the second, with the result that a well-designed and well-maintained thermionic potentiometer is absolutely 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, Sugden and 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 a filament potential of 6 volts. The filaments are of pure tungsten and their emission is constant under constant conditions of filament potential. Valves having thoriated tungsten or coated filaments cannot be used for this work because their emission is inconstant. Two valves were chosen having characteristics as nearly the same as possible, so that any small changes in filament voltage or applied anode potential produce similar changes in the anode currents of both valves. In this way changes which may occur in the sources of supply of current do not affect the galvanometer zero reading.

While the apparatus was being constructed the normal potential of the grid of each valve was set to—3 volts. Then, with the low-and high-tension currents switched 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. It is essential to make solid metal connections everywhere in the wiring up of an apparatus of this kind. The connecting wires were soldered directly to the valve pins and to the metal plug-in holes of the batteries. The valves are run at their maximum filament potential, for then the anode current is approaching its “saturation” value with respect to filament potential. Consequently small variations in the filament potential will be ineffective in producing changes in the anode potential. Everything except the photo-electrical cells, the 6-volt low-tension accumulator, and the galvanometer, was enclosed in a single box, which itself was enclosed in a screening box of copper.

The potentiometer is essentially a series of resistances arranged in the form of a Wheatstone’s Bridge. Two of the resistances are the wire-wound fixed resistances of 10,000 ohms each (Fig. 1,R1 and R2), and the other two are the internal (anode filament) resistances of the two valves (V1 and V2). The internal resistance of V2 is constant because the grid of V2 is kept at a constant negative potential (—3 volts) by the grid-bias battery G, so that the anode potential does not vary. The grid of the other valve, V1, is only at a negative potential of—3 volts when no current is passing through the photo-electrical cell. When light falls on the cell a current which is proportional to the incident light intensity passes through it. This current builds up a potential on the grid of V1, with the consequence that the internal resistance falls and the anode potential decreases. Within the limits used, the fall in potential on the anode of V1 is proportional to the increase of grid potential of the valve. In full sunlight this potential is never greater than + 1·5 volts, so that the relation between the grid potential and the anode potential remains linear (Fig. 2). In its turn, the increase of grid potential is proportional to the current passing through the photo-electric cell, with the final result that the deflection of the galvanometer is proportional to the amount of light falling on the cell.

The galvanometer was too sensitive to be able to register the whole range of light intensities when coupled directly between the anodes of the two valves. But since it 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 either directly to the two anodes or with a resistance R3 of 100,000 ohms in series. This made available two sensitivity scales, one for low intensities and one for high. The introduction of this resistance disturbed the linear relation between galvanometer readings and light intensity so that a calibrated scale had to be drawn up.

The total anode current never exceeded 25 milliamperes, but this heavy current meant that high-tension batteries of the “super-power” type had to be used. Even so, an inevitable running down was bound to occur, although the apparatus was never used continuously for more than an hour at a time. Fortunately it is possible to minimise 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 for different values of the working potential of the potentiometer. The actual voltage chosen was 195 volts, and it is plain that at this voltage a small decrease in potential will cause no serious decrease in the galvanometer readings. After the apparatus had 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 is an additional resistance of 2 megohms, so that the current used cannot exceed 15 microamperes. The voltmeter showed that this battery runs down so slowly that it is, for all practical purposes, constant. The same applies to the grid-bias battery supplying the potential to the grid of V2.

In using the potentiometer it is necessary to allow the low-tension current to flow for a few minutes before taking readings, so that full expansion of the working parts of the valves can occur before any measurements are made. At the beginning of each set of readings the zero of the galvanometer was noted down, and it was also taken again at the end. This is done as a precautionary measure to ensure that no changes 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 made by the General Electric Co. The reason for choosing such cells was that they were supposed to be sensitive over a reasonably wide range of the spectrum.

Preliminary tests of the two cells soon showed that they differed very markedly from each other, and it was consequently impossible to accept the standard spectral sensitivity curve for this type of cell as being applicable in either case. The light used in the preliminary tests was a mercury vapour lamp, and by choosing suitable filters the cells were exposed to monochromatic light of different wave-lengths. The absolute energy value of the light was measured in each case by a sensitive thermopile and galvanometer. We are indebted to Mr G. H. J. Neville, of the Physical Chemistry Laboratory, Cambridge, for supplying us with monochromatic light of known energy value.

The preliminary investigations only served to show that much more extensive observations 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 were placed at our disposal. Under these conditions it was possible to make complete series of records of the spectral sensitivity characteristics of the two cells. For wave-lengths between 3000 and 6000 Å. a mercury-vapour lamp was used, and between 4000 and 7000 Å. a “Pointolite” lamp and narrow-range Wratten filters. In each case the deflection of the Unipivot galvanometer was observed when a beam of light of known absolute energy value (as measured by an accurate thermopile and galvanometer) was allowed to fall on the sensitive plate of the photo-electric cell. The spectral sensitivity curves for the two cells are shown in Fig. 4. The cell W was always used as the under-water photometer.

An estimation of the sensitivity of the cell W to “total light” was made by observing the galvanometer deflection when the “Pointolite” lamp was held at different distances from the sensitive plate. In this way an arbitrary scale of readings was obtained, in relation to light the composition of which was not very different from that of daylight. Knowing the energy emission from such a lamp for different wave-lengths and the sensitivity of the cells to those wave-lengths, it was possible to construct a scale of values for total light. This scale represents the incident radiant energy as ergs per square centimetre per second. The standards so obtained agree reasonably well with the various estimates of full daylight under different conditions and are evidently of the correct order of magnitude.

In the observations in the field two series of readings were always made with the water photometer, one with the opal glass only covering the window, and the other with the opal glass and a Wratten blue filter. This filter (No. 49 in the Wratten series) transmits light of wave-length 3600-5000 Å. All measurements given for outdoor conditions refer to the light incident on a horizontal surface. The measurements therefore represent vertical illumination, which we have called “E” in the tables.

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 shore so that disturbing effects due to shading by submerged littoral vegetation, or by trees overhanging 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 trees or 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 other lakes, from the arms of a winch over the stern of a small boat. The boat was usually anchored from the bows only, but if necessary a second anchor was put out to moor the boat in such a way that the support of the under-water container was kept pointing directly towards the point of maximum illumination in the sky. The effects of the boat in cutting down skylight were thus reduced to a minimum.

The surface intensity of the light was first measured by both photometers and the water photometer was then lowered down into the water. The galvanometer deflection produced by the under-water photometer was recorded at each successive metre or half-metre according to the turbidity of the water, and the lowering was continued until there was no appreciable deflection of the galvanometer needle. At this lowest limit the air photometer reading was taken as a matter of routine, no matter how constant the light conditions appeared to be to the eye. The under-water photometer was then hauled up, and a reading taken at each metre on the upward journey. The surface intensity was again measured with both photometers. This was the procedure adopted under stable light conditions. If there was any tendency towards variations an air photometer reading was taken before each photometer reading. At each observation station two series of readings were taken, one measuring “total light,” and the other blue light only. The significance of “total light” as measured by this apparatus has already been made clear.

The apparatus has been extensively employed on Windermere during 1932. The types of results obtained, however, may be best illustrated by the figures for light penetration in three lakes, Ennerdale, Windermere, and Bassenthwaite. Of these Ennerdale is a rocky lake with clear water, and is, in fact, the clearest of the larger lakes. Windermere, on the other hand, has a fairly heavy phytoplankton, and may be taken to represent a much more silted type of lake, that is to say one in which a much later stage of evolution has been reached (Pearsall, 1921). The water in Windermere is faintly yellowish in colour. Bassenthwaite has the most turbid water in the Lake District. This condition is partly artificial in origin, since the lake was polluted for many years by silt washings from lead mines. This silt still influences profoundly the character 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 examples lies in the fact that they cover almost the whole range of turbidity found in natural fresh water in this country, so that the suitability of our apparatus for this type of work can be estimated.

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

Table I.

Figures showing the penetration of light into the waters of three lakes in the Lake District.

Figures showing the penetration of light into the waters of three lakes in the Lake District.
Figures showing the penetration of light into the waters of three lakes in the Lake District.
  1. Ennerdale. September 23rd, 1932. Sky nearly covered with light grey cloud. 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 × 106 ergs/cm.2/sec.). Expressed as percentages of full light in air, these are not significantly different at the various depths, hence only the full sunlight ones are reproduced here. September is the time of the phytoplankton maximum in this lake.

  2. Windermere (North basin). September 18th, 1932. Sky clear, with occasional small white clouds. Sun bright (E = 4·4 × 106 ergs/cm.2/sec.). Light northerly breeze. Ripples 10 cm. high. Phytoplankton moderately abundant.

  3. Bassenthwaite. September 22nd, 1932. Sunbright—few clouds but much haze (E = 2·55 × 106ergs/cm.2/sec.). Wind east, moderately fresh. Waves 15−20 cm.

Water very turbid, with heavy diatom maximum. (These conditions are certainly ‘not typical for this lake, since they were taken at a time of maximal turbidity.)

In all the results the intensity of light at each depth was calculated as a percentage of the light intensity in the air. The average of the “down” and the “up” readings was taken (Table I). Figs. 5 and 6 show the results for the three lakes plotted on a logarithmic scale against depth. The figures serve to illustrate the considerable differences between the different types of water. A light intensity of 1 per cent, is found in Ennerdale between 14 and 15m., at 6−7 m. in Windermere, and between 2·5 to 3 m. in Bassenthwaite. Different as these results are among themselves they should be contrasted with data which have been obtained for sea water by Poole and Atkins (1929), who report a light intensity of 1 per cent, at a depth of 45 m. at the same time of the year and under similar conditions.

The factors controlling the differences in fresh water are three in number. The first is the colour of the water. This is very slight in Ennerdale, faintly yellow in Windermere, and distinctly yellow in Bassenthwaite. The second, the amount of sediment, which is greatest in Bassenthwaite, and least in Ennerdale, and the last, the quantity of plankton (chiefly algae in these observations) which was also largest in Bassenthwaite and least in Ennerdale. In all three lakes the blue light decreases with depth more rapidly than total light, and the proportion of 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 there is a great change in the quality of the light in passing from the surface to greater depths, where radiant energy of the longer wave-lengths is shown to be predominant.

Table II.

Figures illustrating the penetration of blue light (3000-5000 Å.) in different fresh-water lakes. The intensity of the blue light is expressed as a percentage of the total light at each particular depth.

Figures illustrating the penetration of blue light (3000-5000 Å.) in different fresh-water lakes. The intensity of the blue light is expressed as a percentage of the total light at each particular depth.
Figures illustrating the penetration of blue light (3000-5000 Å.) in different fresh-water lakes. The intensity of the blue light is expressed as a percentage of the total light at each particular depth.
The light transmitted by a liquid is determined by the relation where Io is the intensity at some particular depth o, Id the intensity at some greater depth d, and μ the absorption coefficient. Since Io and Id are known from the data, the corresponding value of µ for the various depths can be calculated. Since the sun’s rays strike the water at an angle, the actual depths at which the observations are made do 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 can readily be found to the nearest degree. The refractive index of water is 1·333, so that the angle θ of the direct rays from the normal is given by
where A is the angular distance of the sun below the zenith. The distance C, through which the light has actually passed, will be given by the formula

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

Table III.

The absorption coefficients in the different lakes.

The absorption coefficients in the different lakes.
The absorption coefficients in the different lakes.

It will be seen from these results that there is a considerably greater absorption of light near the surface than in deeper water. This is due to the far greater quantity of phytoplankton in the upper zone, and also to the greater absorption of blue light in the surface layers. But in Ennerdale there is a marked increase in the absorption coefficient below 10 m. This has not yet been completely investigated. It may possibly be due to the greater number of plankton animals at these depths or, on the other hand, it may be that the water of the hypolimnion is more turbid than that of the epilimnion, as Birge and Juday have suggested. It should be emphasised that the differences in the absorption coefficients indicate very considerable differences in the amount of light passing through the water. In Ennerdale where the differences are 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 layer rich in phytoplankton is much shallower in Bassenthwaite than in the two clearer lakes. Also the differences in the transmission of light in the different lakes can be correlated with differences in the depths to which the littoral vegetation extends. In Ennerdale 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. In Windermere the 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 transmission of light, it is impossible to attempt to find a closer agreement at present. For we have no justification (except in the case of Windermere) for supposing that the isolated examples of light penetration represent average conditions during the growing season. The data for Bassenthwaite were obtained at a time when the lake was at or at least very near the condition of maximum turbidity, and average light transmission in summer must be higher. Another difficulty is the determination of the 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 whether the results of surveys in 1919-20 which are available (Pearsall, 1921) still represent the 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 against 6·5 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 shallower to deeper water, the rooted vegetation ceases to grow at depths where the light intensity is still high. The following figures represent percentages of surface intensity at the lower limit of rooted vegetation (September, 1932):

They show that the vegetation in Ennerdale and Windermere does not occur below a zone where the light intensity is about 3·5 per cent, of that at the surface. The light intensity 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 cases less than 1 per cent., which have been recorded (Adamson, 1911 and 1922, and Atkins and Stanbury, 1930) as marking the limiting light intensity for vegetation in woods.

Three possibilities have to be considered in this connection. Firstly, it is possible that soil conditions may determine the lower limit of vegetation in some lakes ; also it 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 measurements show that the quality is very different at different depths. Thus when the total light in these waters has fallen to 4 per cent, of the value for full daylight, the ratio of 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 Windermere 3·4 per cent, is blue light at 6·o m. There are grounds for the belief that this change in the quality of the light is extremely important. Lastly, most of the measurements of light intensities in ecological work have been made with methods which depend wholly or chiefly on the effects produced by the blue-violet end of the spectrum. Such methods necessarily give different results from those which include a wider range of the incident energy, a point which is illustrated very clearly in the results tabulated in this paper.

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

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

Adamson
,
R. S.
(
1911
).
Journ. Linn. Soc. Bot
. p.
264
.
Adamson
,
R. S.
(
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
.
Birge
,
E. A.
and
Juday
,
C.
(
1930
).
Trans. Wis. Acad
.
25
,
285
.
Birge
,
E. A.
and
Juday
,
C.
(
1931
).
Trans. Wis. Acad
.
26
,
383
.
Birge
,
E. A.
and
Juday
,
C.
(
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 570
.
Winch
,
G. T.
(
1930
).
Journ. Inst. Elect. Eng
.
68
,
533
.