VOL. X, No. 4
OCTOBER,
1933
LIGHT PENETRATION INTO FRESH WATER
I. A THERMIONIC POTENTIOMETER FOR MEASURING
LIGHT INTENSITY WITH PHOTO-ELECTRIC CELLS
BY W. H. PEARSALL AND P. U L L Y O T T .
{Received 30th January, 1933.)
(With Six Text-figures.)
INTRODUCTION.
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 A. to 20,000 A., 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.
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 millivoltmeter, 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.
JEB-X1V
2O
294
W. H. PEARSALL and P. ULLYOTT
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 (io~9 to
io~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
Light Penetration into Fresh Water
295
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
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, 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, Vlf 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. Thiscurrent builds
296
W. H. PEARSALL and P. ULLYOTT
up a potential on the grid of Vx, 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 Vt 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
-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 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.
Light Penetration into Fresh Water
297
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
toflowfor 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.
150
175
200
225
Total working potential in volts
Fig- 3-
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 niters
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 wavelengths between 3000 and 6000 A. a mercury-vapour lamp was used, and between
4000 and 7000 A. a "Pointolite" lamp and narrow-range Wratten niters. 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 galvano-
298
W. H. PEARSALL and P. ULLYOTT
iranometei
i ergs /cm.
:Hect:ion
:c.
meter) 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
Sensitivity
(sensiti
press;ed as
scale]1 per i
S>8
0-5 —
A
1' \
0-4
\ Cell W
0-3
-
\
\
0-2
/TXCeUA
/
0-1
0
3000
\ \
1
4000
5000
6000
Wave-length in Angstrom units
7000
Fig. 4.
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 thefieldtwo 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 A. 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 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 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
Light Penetration into Fresh Water
299
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, Eonerdale, 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:
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 io 6 ergs/cm.2/sec), and one for overcast conditions
(E = 1 64 x io 6 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
Sun's altitude
i8
17
13
14
15
16
12
II
10
9
8
7
6
3
4
5
i
2
S
35°
0-42
O-22
0-85
o-6o
3-12
2-28
1-69
1-20
4"3°
5-72
7-56
32-0
24-2
179
134
IO-I
463
93 0
68-3
34°
4-00
2-II
1-16
0-625
O-335
—
—
—
—
—
—
—
—
—
7-70
32-2
156
75-0
33°
—
—
—
—
—
—
—
•—
—
—
—
—
—
o-53
19-0
592
i-75
7i'4
—
36°
•
3i-4
10-5
5i5
2-62
1-45
0-77
0-40
0-21
55-6
27 xio 0
ergs/cm.2/sec.
Total
37°
—
—
—
—
—
—
—
—
—
—
—
—
—
—
0-34
50-1
930
1 70
0-92 x i o 6
ergs/cm.2/sec.
Blue
11.20-11.30 a.m.
22. ix. 32
Bassenthwaite
I I . O - I I . 2 0 a.m.
• The depths given in this column refer to Bassenthwaite only.
33°
—
—
—
—
—
—
—
—
—
0-76
3-64
2-17
1-28
518
29-5
17-3
10-4
6-i
93-5
0-93 x i o 6
ergs/cm.2/sec.
Blue
i-35-i-45P-m-
Percentage of surface intensity
4-5 xio 6
ergs/cm.2/sec.
o-8 xio 6
ergs/cm.2/sec.
4-5 x io 8
ergs/cm.2/sec.
Air intensity
Depth (m.)
Total
Blue
Total
i.2S-i-35P-m.
Light
1.40-1.50 p.m.
18. ix. 32
1.10-1.20 p.m.
23. ix. 32
Date
Windermere
Time (G.M.T.)
Ennerdale
Lake
Table I. Figures showing the penetration of light into the waters of three lakes in the Lake District.
—
4-0
—
—
—
—
—
—
—
—
—
3-5
2-5
3'O
2-O
i-5
I-O
o-5
S
Depth*
m.
H
H
r
r
o
d
a
3
o
o
Light Penetration into Fresh Water
301
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 x io 6 ergs/cm.2/sec). Light
northerly breeze. Ripples 10 cm. high. Phytoplankton moderately abundant.
3. BASSENTHWAITE. September 22nd, 1932. Sun bright—few clouds but much
haze (E = 2-55 x io 6 ergs/cm.2/sec). Wind east, moderately fresh. Waves 15-20 cm.
Water very turbid, with heavy diatom Total light intensity expressed as percentage
maximum. (These conditions are certainly of the total surface intensity (logarithmic
scale)
' not typical for this lake, since they were
100-0
10-0
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
Bassenthwaite
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
Winderinere
water. A light intensity of 1 per cent, is
found in Ennerdale between 14 and 15 m.,
at 6-7 m. in Windermere, and between .£ 110
1
2-5 to 3 m. in Bassenthwaite. Different as t
these results are among themselves they Q 12
13
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
Ennerdale
in fresh water are three in number. The
20
first is the colour of the water. This is
0
1-0
2-0
-1-0
very slight in Ennerdale, faintly yellow
Logarithms of the total light intensity
in Windermere, and distinctly yellow in
Fig. sBassenthwaite. 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
302
W. H. PEARSALL and P.
ULLYOTT
greater depths, where radiant energy of the longer wave-lengths is shown to be
predominant.
Intensity of blue light expressed as percentage of surface intensity
of blue light (logarithmic scale)
0-1
oi—
10-0
100-0
1-0
2«0
_ Bassenthwaite
AVindermere
.S 5
10
-1-0
Eiraerdale
0
Logarithms of intensity of blue light
Fig. 6.
Table II. Figures illustrating the penetration of blue light (3000-5000 A.) in different
fresh-water lakes. The intensity of the blue light is expressed as a percentage of the
total light at each particular depth.
m.
Penetration in
Ennerdale
Penetration in
Windermere
Penetration in
Bassenthwaite
Air
S
o-S
28-2
2S'3
34-2
33-8
30-6
I'O
21-2
i-5
—
279
—
173
—
20
180
143
"•4
Depth
3-0
4-0
5°
60
7-0
80
• — •
9°5
7-1
565
4-42
II-O
67
3 9
•——
—
—
10*05
S-48
2-24
088
—
—
—
—
—
—
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,
Light Penetration into Fresh Water
303
and [i the absorption coefficient. Since Io and Id are known from the data, the
corresponding value of fx 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 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, through
which 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, 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.
ENNERDALE
Total light
Range
Average
0-3 m.
3-10 m.
0-279—0-306
0292
0-221-0-236
0-226
0-420-0-519
0461
0-401-0-420
0-409
0-3 m.
3-8 m.
0-559-0-665
0-564
0-498—0-515
Blue light
Range
Average
WlNDERMERE
Total light
Range
Average
Blue light
Range
Average
°-5°3
0—4 m.
1-040—0-918
0931
BASSENTHWAITE
Total light
0-1-5 m.
1-5-4 m.
Range
Average
1-74-1-13
1-41
1-07-0-937
Blue light
Range
Average
2-56-2-70
2-64
I-OI
10-17
m
-
0-246-0-281
0-263
3<M
W. H. PEARSALL and P. ULLYOTT
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. InWindermere
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):
Ennerdale
Windermere
Total light
Blue light
4-27-3-16
3-25
0-457-0-276
0-40-0-35
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
Light Penetration into Fresh Water
305
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.
SUMMARY.
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 subaqueous 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 expenses of this enquiry have been defrayed by a grant from.the British
Association to the Fresh Water Biological Association.
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