Download the liberation and utilisation of oxygen by the population of rock

162
THE LIBERATION AND UTILISATION OF OXYGEN
BY THE POPULATION OF ROCK-POOLS
BY T. A. STEPHENSON, D . S C , A. ZOOND, PH.D.
AND
JOYCE EYRE, M.Sc.
(Department of Zoology, University of Cape Town.)
{Received ibthjwte, 1933.)
A. P. ORR (1933). in the course of a study of the physical and chemical conditions
prevailing in the sea near coral reefs, and in shallow pools on their surface, found
that the oxygen content of the water in pools containing coral may rise to a very high
figure (sometimes as high as a supersaturation of 278 per cent.) by the end of the period
of low water during the daytime, falling to a low figure (e.g. 18 per cent, saturation)
by the end of low water at night. Respiration by plants and animals uses up oxygen
in both light and darkness; photosynthesis produces an excess of oxygen during
daylight. The photosynthesis is due not only to free-living algae, but also (and in
some places predominantly) to the symbiotic algae known as zooxanthellae which
abound in the tissues of nearly all reef-corals. The effect can be detected in open
water over coral, but is naturally much less marked there than in pools. C. M.
Yonge, M. J. Yonge and A. G. Nicholls (1932) and S. M. Marshall (1932) studied
the oxygen production of corals and their larvae experimentally. Oxygen determinations on coral reefs, and experiments on corals, have also been made by other authors—
for references see Yonge, Yonge and Nicholls (1932) and Verwey (1931).
In temperate seas where there are no corals, but where there is usually a much
more bulky population of non-calcareous free-living algae than occurs on coral reefs
(on which, in places where coral is most abundant, large algal growth is apt to be
singularly scanty), it is to be expected that the oxygen content of the water in rockpools containing algae should behave in the same way as that of pools containing
coral; and it should also be possible to record a less marked effect over Laminaria
beds in open water, and in other regions rich in plant life.
The effect of the presence of abundant plant life in raising the oxygen content
of the sea in its vicinity is now well known as regards the larger water masses, whether
inshore or farther out, and whether the vegetation in question be diatoms and other
planktonic forms, attached algae (either microphytic or macroscopic), or marine
phanerogams1. So far as we have been able to determine2, however, the oxygen
1
See Legendre, 1909, 1909a and 1922; McClendon, 1918; Powers, 1920; Atkins, 1922; Allee,
1923; Marshall and Orr, 1928 and 1930; Thompson, Miller, Hitchings and Todd, 1929; Gran and
Thompson, 1930; from these papers further references may be obtained.
• Unfortunately we cannot obtain in South Africa a paper entitled " A study of the tide-pools on
the West Coast of Vancouver Island" by I. Henkel, published in Postelsia, 1906.
Liberation and Utilisation of Oxygen by the Population of Rock-pools 163
conditions in rock-pools, where all natural fluctuations are exaggerated, have been
much less fully investigated; and although data on oxygen in such pools are not
entirely lacking (Powers, 1920; Humphrey and Macy, 1930; etc.), this aspect of the
subject has been comparatively neglected. With regard to pH there is rather more
information, and readings as high as />H 8-9, for instance, are recorded for pools
in which photosynthesis had been in progress (Johnson and Skutch, 1928), with
contrasting values as low as 7-6 in small pools containing animals but no plants.
Gail (1919) found the pH in pools containing vegetation often as high as 8-8 in the
afternoon, the average value before sunrise being 7-43.
The present paper gives an account of the variations in oxygen content and
hydrogen-ion concentration in rock-pools situated on the False Bay side of the Cape
Peninsula; and a quantitative account of the population of some of these pools, for
correlation with the oxygen determinations. The paper forms one of a projected
series of publications intended to throw light upon the general problem of the
distribution of organisms in the sea surrounding this Peninsula, an area in which
a warm and a cold ocean current meet one another.
In this paper the populations of the pools are partly catalogued under broad
headings (e.g. sponges are counted collectively and not sorted into species), since
this treatment is adequate for the present purpose. In another paper we hope to give
detailed lists of the species, together with an account of the ecological succession of
the organisms which establish themselves in the denuded pools.
METHOD.
In the present paper only a limited number of determinations, carried out on
water samples from three pools, are given in full. These determinations are by no
means all that were made, but are the final ones, made on the most suitable pools,
after a series of preliminary experiments by Letitia Starke and S. S. Alexander as well
as by the present authors. Since, however, all the determinations tell the same story,
and those quoted in full are perfectly typical, there is no reason for presenting a
larger number in detail.
Determination of the population.
The volume of water in the pools was measured either by pumping out the water
and measuring it in a graduated bucket; by dipping it out direct with the bucket;
or by emptying the pool first and measuring the amount of water required tofillit up
to normal level. In determining the population, the whole content of plants, animals,
and gravel was removed, after the water. The algae were weighed while still fresh,
the weight given representing their wet weight after free water had been drained
away; gravel and animals were removed from among their roots as far as possible.
The gravel, in the case of two of the pools, was present in such large quantities
(hundreds of pounds) that it was possible neither to preserve it nor yet to extract the
amphipods and other small animals from it. It was therefore sifted through a J-inch
sieve, and the animals remaining in the sieve were preserved. All animals were
164
T. A. STEPHENSON, A. ZOOND and JOYCE EYRE
subsequently sorted and counted or, in the case of sponges, compound ascidians and
ova, weighed. When the population had been removed, all that remained in the pools
were thin crusts of Melobesiae or of Hildenbrandtia, with which nothing practicable
could be done, and fragments of algal roots, etc.; in fact nothing of any significance
to the present issue.
It will be obvious that the details of populations given refer only to the contents
of the pools on the day on which they were cleaned out. Since each pool had to be
used several times over for oxygen determinations, clearly the population could not
be removed until the series of readings was complete. There must therefore have
been some fluctuation in personnel during the period of experiment, affecting
migratory forms to a certain extent, but sedentary forms hardly at all. The pools
were under observation all the time; and we should judge that the fluctuations from
day to day were not considerable enough to produce any marked difference in oxygen
content, and that the counts given provide a fair indication of the average population.
It has been shown experimentally by Gersbacher and Denison (1930) that the
number of motile animals remaining in rock-pools is more or less constant.
It will further be noted that the counting of animals, especially of those such as
limpets and barnacles, which have heavy shells, gives only an indirect and imperfect
index to the weight of living organic matter contained in them. It was not our aim,
however, to make a precise comparison of the net weight of organic matter contained in the three pools, since even if this very elaborate determination (involving
decalcification, removal of gut contents, etc.) were made, it would provide nothing
which could be closely correlated with the observations on fluctuations in oxygen
content. For the present purpose, which is to give an intelligible means of comparing the biota of the pools, counting was judged to be a sufficiently refined
method.
Choice of pools.
The intertidal zone at St James, where the determinations were made, is very
narrow, and the tidal range at extreme spring tides is only about 7 \ ft. Consequently
the number of rock-pools available is limited, and among those whose populations
were suitable for this work only one (pool A) could be found which was entirely
stable at low water—i.e. which had no inflow into or outflow from it, once the tide
had receded. Of the other two pools chosen, one (pool B) originally communicated
by means of very shallow horizontal grooves with other pools, and had also a trickle
from its seaward end which reduced its volume slightly, after the tide had ebbed.
The trickle was allowed to continue, as it was too slight to cause any appreciable
effect; the grooves were filled in with cement and clay before the pool was emptied.
The other pool (C) also lost a little in volume after the tide had left it, from a small
invisible outlet; and this pool developed two small inflow trickles when it was
cleaned out, which were probably not there before. It is evident, however, that the
water in these pools was virtually stagnant in spite of any slight variations introduced
by the factors mentioned, as was demonstrated by introducing colouring matter into
the water and watching the effect; we do not think that the extremely slight water
Liberation and Utilisation of Oxygen by the Population of Rock-pools 165
movements present had any significant effect on the oxygen determinations. Even
in the most isolated pools a slight stirring of the water occurs if there is the least
breeze.
Oxygen and pH determinations.
The determinations were always made as near as weather permitted to the lowest
tide of any set of springs. The tidal range is so slight that only the more extreme
tides are useful for such purposes.
Dissolved oxygen was determined by the Rideal Stewart modification of the
Winkler method as given in the Standard Methods of Water Analysis of the American
Public Health Association (sixth edition). Approximately 0-02 Alodium thiosulphate
was used in the titrations, the solution being standardised with potassium dichromate
before each series of determinations. All titrations were done in duplicate. The
samples were collected in narrow-necked glass-stoppered bottles of 280 c.c. capacity. During the actual collecting of the sample the bottle was fitted with a stopper
holding two glass tubes, one extending to the bottom of the bottle and the other
having its lower end flush with the bottom of the stopper and projecting about 4 in.
above the rim of the bottle. Thus, when the bottle was held about 3 in. under the
surface of the pool, the water could flow in, and the displaced air could pass out
without any bubbling. When the bottle was full the ground glass stopper was immediately inserted. After each collection the samples were carried a distance of
about 50 yards to a house on the sea front, and analysed at once.
Hydrogen-ion concentration was measured by the colorimetric method of
McClendon (1917). Cresol red was used for the range pH 7-45-8-20, and thymol
blue for pH 8-00-9-00. The colour standards were sealed in tubes of 15 mm.
diameter, and, when not in use, were stored in light-proof boxes. The determinations were carried out independently by two workers, with very good agreement.
After about three months it was found that the cresol red range had faded, and could
not be matched with the sea-water samples. Thus, in the readings taken on September 17th, 1932, the thymol blue range only was used. Several of our samples (i.e.
those with very high oxygen content) were found to lie on the alkaline side of the
thymol blue range, and consequently only approximate values could be obtained.
For work on rock-pools, where the variations in pH are relatively very considerable,
an additional range of buffer standards more alkaline than McClendon's thymol-blue
range is required.
DESCRIPTION OF THE POOLS AND OF THEIR POPULATIONS.
Of the three pools used for the final determinations, one lies some 150 yards from
the other two, but at approximately the same level on the shore, on a sloping outcrop
of rock. The remaining pools are situated within a few feet of each other, on a broad
platform of rock which becomes uncovered for several hours at low water of springs,
and is uncovered for a shorter time, or just awash, at low water of neaps. The pools
will be designated respectively A, B, and C.
166
T. A. STEPHENSON, A. ZOOND and JOYCE EYRE
Pool A. A basin-like pool of neat form. Length 95 in., breadth 85 in., greatest
depth 17^ in. Volume of water 418 litres (approximately). The pool is entirely
hollowed out of rock, and usually contains no loose sand or shingle, except for a
small amount of shell gravel in the crevices. It sometimes contains one or two large
loose stones. It is fully exposed to the sun from shortly after sunrise until such time
as the sun disappears behind the coastal mountains.
Pool B. A large pool of irregular shape, entirely floored by rock. Length 25 ft.,
breadth 9 ft. 4 in., greatest depth i2f in. Volume of water ca. 800 litres. The pool
contained a considerable amount of shell gravel in its pockets and crevices, and
caught in the roots of algae. It is fully exposed to the sun as in the case of pool A.
Pool C. A small elongate pool, floored by rock, and overhung by a rocky mass at
one end, so that this end lies in a small cave; apart from which the pool is partly kept
in shadow by surrounding rocky masses and gets much less sunshine than the other
two. Length 4 ft. 2 in., breadth 1 ft. n in., depth 4-7 in. to the top of the shell
gravel with which the pool is half filled, above its rocky floor. Volume of water
ca. 112 litres.
The population.
Pool A. This pool presented a good example of a mixed population, containing
not only a reasonable growth of plants but also a considerable number of animals.
The principal algae were the brown leafy Gigartina radula (1962-7 gm.); the green
Ulva lactuca (220-0 gm.); and species of coralline, Jania, Amphiroa and Cheilosporum
or Corallina1 (387-2 gm.). There were also thin encrustations of Melobesiae. The
animal population is listed in Tables I and II.
Pool B. This pool provided a case in which plant growth reached the maximum
amount to be found among the pools available; the growth of algae being so bulky as
to fill many parts of the pool almost completely. The principal alga was Pycnophycus
brassicaeformis, a whip-like brown species, together with which was mingled a certain
amount of Gigartina radula (total weight of brown algae 41,753-4 gm.)3. There was
also a good deal of Ulva lactuca (3755-1 gm.), and encrusting the rock the brown
HUdenbrandtia pachythallos. Details of the animal population are given in Tables I
and II.
Pool C. This pool formed the most complete contrast available to pool B—i.e.
instead of containing a predominance of plant life it contained hardly any algae at
all, the total weight of these present (apart from thin crusts of Melobesiae) being
3-29 gm. of the red alga Pleonosporium Harveyanum. The population of animals,
however, was large for the size of the pool and consisted, apart from vagrant forms
such as sea-urchins, crabs and fishes, of (a) animals concealed in the gravel, and
(b) sedentary forms encrusting the rock. Among these latter the worm Pomatoleios
crosslandi was conspicuous, forming large masses of calcareous tubes partly sub1
The material of one species was sterile; it cannot therefore be stated with certainty whether
it belonged to Cheilosporum or Corallina.
1
This figure represents perhaps go per cent. Pycnophycus, the remainder Gigartina »nd
HUdenbrandtia.
Liberation and Utilisation of Oxygen by the Population of Rock-pools 167
merged in the pool and partly uncovered at low water. There were also considerable
numbers of barnacles {Tetraclita serrata, Chthamalus dentatus and Balanus trigonus)
and, on the overhanging walls of the cave, sponges and a variety of other forms.
Further details are given in Tables I and I I .
Table I. Showing the number {or weight) of each of the principal kinds of animals in
the three pools. Limited to animals large enough to be held back by a sieve of
\-inch mesh.
Animals
Sponges
Anemones
Pomatoleio8
Other worms
Parechinus
Asteroids
Holothurians
Patella
Siphonaria
Other gastropods
Mytilus
Acanthochites
Chiton
Barnacles
Crabs
Fishes
PoolB
Pool .4
256 gm.
289 pn.
23 individuals
966
352
9
143
»
2
29
7
,
,
58 individuals
44
.
22
34
10
669 gm.
20 individuals
3 I: H
215
28
114
I41
276
15
>
1
,
101
,
3
.
0
63
.
.
67
129
413
144
PoolC
,
9
,
24
15
3
54
490
19
3
49
9
69
>
t9
6
»
12
I
,
,
Table I I . Showing the number {or weight) of organisms per 100 litres of water, in the
three pools. Calculated to the nearest whole number. Animals limited, as before,
to those large enough to be held back by a sieve of ^-inch mesh.
Organisms
Pool A
Pool B
Sponges
Anemones
Worms
Echinoderms
Mollusca
Crustacea
Fishes
69 gni.
5 individuals
315
37
59
32 gm.
7 individuals
Total number of
animals other
than sponges
Algae
5
2
•
,,
.,
423 individuals
619 gm.
20
,,
It13 ::
9
189 individuals
5688 gm.
PoolC
597 gm.
18 indiv duals
2981
86
84
44 8
1
,
3618 individuals
3 gm.
Reviewing the populations of the three pools, therefore, we have one pool containing animals almost exclusively; one containing an enormous abundance of
plants and, compared with the other pools, relatively few animals; and a third
containing a moderate abundance of both. In the subsequent text these pools
will be referred to respectively, for convenience, as the animal, plant and mixed
pools.
JBB-Xlii
12
168
T. A. STEPHENSON, A. ZOOND and JOYCE EYRE
The temperature in rock-pools.
In order to obtain some idea of the variation of temperature in rock-pools round
the coasts of the Cape Peninsula, a series of readings was taken on a sunny day
(April 10th, 1932), in twenty-three pools situated at six localities at fairly regular
intervals along the opposing Atlantic and Indian Ocean shores of the Peninsula1.
For each pool the temperature was recorded once an hour from 9 a.m. to 5 p.m.
The readings show that, as has been found in other parts of the world, the number
of degrees which the temperature in any pool rises above the temperature of the sea
at the same place depends entirely upon the local conditions of the particular pool—
upon its depth and the volume of water which it contains, the period of its isolation
from the sea, and whether it is entirely exposed to the sun or partly or wholly
shaded. Generally speaking, pools high up the shore become hotter than those lower
down, but this does not apply in cases where, for instance, a pool high up is well
shaded and one lower down is exposed to continuous sunshine. The greatest change
observed on the day in question was in a sunny pool at Hout Bay (Atlantic coast),
where the rise was 13-4° C. above the maximum temperature of the sea at the same
place (from n-8 to 25-2° C ) . The least change observed was in a large deep pool
very near low-water level at Cape Point (the tip of the Peninsula), where the rise
was only o-i° C. above the maximum temperature of the sea at the same place (from
11-8 to 11-9° C). We agree with Brooker Klugh (1924) in thinking that the number
of degrees which the temperature in a given pool rises above the local temperature
of the sea is one of the most important, if not the foremost, of the factors which
determine which plants and animals will be able to establish themselves there.
Effect of oxygen variation on organisms.
Although a good deal is known of the effect on the respiration of animals in sea
water of different oxygen tensions, and of other factors2, it would nevertheless be
a very complicated calculation, for which there are not yet enough data available,
to determine the probable limits of oxygen tension between which the inhabitants
of a rock-pool containing a complex population of given constitution should be
expected to feel no ill effects. There is no sign, however, that the range of variation
here recorded, although it is very wide, acts as a limiting factor in the distribution of
animals among the pools of the peninsula. In the case of the pools here described, also,
salinity appears to be a factor of very secondary importance; in pool A, in which
evaporation would have most effect, the chlorinity of the pool on April 30th, 1933 (a
day of continuous sunshine), rose only 0-2 per thousand above that of the open sea.
1
We wish to express our sincere thanks to a number of workers who made this experiment
possible by taking temperatures simultaneously, throughout the day, at places far removed from one
another.
' It is known, for instance, that in certain fish and invertebrates oxygen consumption is lowered
at low oxygen tensions; that oxygen consumption may vary with temperature and pH; that water
very much supersaturated with oxygen has harmful effects on certain animals; and that some marine
animals can survive for long periods under anaerobic conditions. See Collip, 1920 and 1921;
Haempel, 1928; Hyman, 1929; Galadziev and Malm, 1929; F. G. Hall, 1929 and 1931; Keys, 1930;
Koller, 1930; V. E. Hall, 1931; Raffy, 1931; etc.
Liberation and Utilisation of Oxygen by the Population of Rock-pools 169
THE RESULTS OF THE DETERMINATIONS,
(i) Determinations made during daylight.
First we may consider a typical set of readings taken from the mixed pool and the
plant pool on the same day; these are recorded in Table III. From the table it may
be seen that as the tidal period progressed both temperature and oxygen content
rose steadily, the oxygen value at the penultimate reading (which is chosen because
at the final reading the pools had been invaded by the sea) being 193 per cent, of its
initial value in the mixed pool, and 251 per cent, of its initial value in the plant pool.
The/>H in both pools behaved correspondingly, rising to about 9-0 in the plant pool
and nearly as high in the mixed pool. Further, although the oxygen content became
decidedly higher in the pool with a superabundance of plants than in the one with
fewer plants, yet this difference is probably not of great significance, because in the
case of both pools much oxygen was evolved as bubbles and lost from the surface
during the tidal period. Only by taking a series of pools containing progressively less
and less vegetation than the mixed pool would it be possible to obtain a graded series
of oxygen readings which could be correlated with the exact plant content of the pool.
Table III. May 24th, 1932. Cloudy day with intermittent sunshine and little wind.
At the last sampling the tide was beginning to enter pool B and possibly also pool A.
Low water occurred at 12.17 p.m.*, sunrise at 7.38 a.m.
Plant Pool (B).
Algae 5688 gm. per ioo litres
Mixed pool (A).
Algae 619 gm. per ioo litres
Time
Temp.
0
C.
Oxygen
in mg.
per litre
9.45 a.m.
" • 1 5 ..
12.45 p.m.
2.0
„
143
14-8
165
1305
16-2
1995
8-4
PH
8-025
8-2S
8-55
8-775
Time
9-45 a.m.
U-I5
,.
12.45 p.m.
2.0
„
Temp. Oxygen
in mg.
°C.
per litre
130
145
152
15 2
8-95
i8-45
225
22-65
8-075
8-50
8-95 (approx.)
900 (approx.)
• The times given for low water in this paper apply to Table Bay. We have no means of applying
an accurate correction for St James, but the difference is a matter of minutes only.
Next may be considered a series of readings taken from the plant and animal
pools on the same day, and recorded in Table IV. This table demonstrates the fact
that whereas in the plant pool the oxygen rose markedly as before (the highest
reading being 197 per cent, of the initial one), in the animal pool, where respiration
but practically no photosynthesis was in progress, the oxygen content was not only
much lower to begin with (photosynthesis had been in progress in the plant pool
before the first reading), but also fell decidedly. In both pools the pH behaved
accordingly. With this result may be compared an isolated pair of determinations
made at the end of the period of low water on October 3rd, 1932, when the following
readings were obtained from samples taken in sunshine, at 1245 p.m., i£ hours after
the time of low water and 6£ hours after sunrise:
Plant pool (B): temperature 200 C , oxygen 23-8 mg.
Animal pool (C): temperature 150 C , oxygen 3-45 mg.
170
T. A. STEPHENSON, A. ZOOND and JOYCE EYRE
A further point in Table IV requires explanation. It will be noted that the last
reading for pool B and the second reading for pool C are not strictly in series. This
discrepancy can easily be accounted for by the fact that the water in these pools is
not completely homogeneous either in temperature or in oxygen content, but is
patchy; so that readings cannot always be strictly progressive.
Table IV. Sept. 1 jth, 1932. Sunshine. Pools isolated all the time. Open sea: oxygen
8-7 mg.; pH 8-13. Low water occurred at 10.21 a.m., sunrise at 6.44 a.m.
Plant pool (B). Algae 5688 gm. per 100 litres
Time
8.0 a.m.
9-3° ,.
11.0
,,
12.0 noon
12.30 pjn.
Oxygen in
mg. per litre
13-3
22-3
25-1
26-2
26-0
Animal pool (C). Algae 3 pm. per 100 litres
Time
8-30
8-85
>QOO
> 9OO
8.0 ajn.
9-3O „
11.0 „
12.0 noon
12.30 pjn.
Oxygen in
mg. per litre
s
<8-oo
3-6
3'9
3-8
(ii) Determinations made at night.
All these determinations concern the plant and animal pools only, and are summarised in Table V. From this table it is evident that on all four occasions the
oxygen content in both pools was low or fairly low to begin with (with a tendency to
be higher in the plant pool than in the other); that it had in all cases become lower
at the last reading and always reached a lower value in the animal pool than in the
plant pool. The fall in oxygen content is continuous from the beginning of the
experiment to the end, with two exceptions; one of these is a case of a rise in value
due to waves washing over the pool; the other may be due to the same cause, but we
cannot be certain.
It may also be noted that we have not been able to demonstrate any significant
difference between the diminution of oxygen in the pools on moonlight and moonless
nights; there is thus no indication of the occurrence of photosynthesis on moonlight
nights. If we compare the amount of oxygen utilised on the night of June 10-1 ith
(moonless) with the amount utilised on the night of June 19th (full moon), we find
that the two pools lost respectively 1-5 and 2-1 mg. per litre on the moonless night,
and 1 -5 and 4-5 mg. on the moonlight night, in approximately the same time (2 hou rs).
Comparing similarly the nights of October 1st (moonless) and October 17th
(moonlight), wefindthat in a comparable time (the 2 hours preceding midnight) the
loss was i-6 and 0-7 mg. on the moonless night and 2-1 and 1-2 mg. on the moonlight
one.
It does not follow from these results that no photosynthesis occurs in the pools
in moonlight; but, if it does occur, it may well be of too small an amount to appear
clearly in determinations of quantities of a dissolved gas in a pool, since in any case
these quantities vary considerably from day to day and even from one part of the pool
to another.
Since colorimetric pH determinations can only be carried out satisfactorily in
daylight, the samples collected at night were kept until the following morning. It was
Liberation and Utilisation of Oxygen by the Population of Rock-pools 171
then found that all the samples were on the acid side of pH 8-o, and thus fell outside
the range of our colour standards. We are therefore unable to give accurate values;
and can only say that during all night experiments thepH fell below 8-o in both pools.
Table V.
Plant pool (#). Algae 5688 p n . per 100 litres
Time
Oxygen in mg.
per litre
Animal pool (C). Algae 3 gm per 100 litres
Time
Oxygen in mg.
per litre
June ioth-nth, 1932. No moon. Water temperature 140 C. Low water occurred at
1.40 a.m., sunset a t 5-43 P m 12.0 midnight
60
12.0 midnight
7'2
1.0 a.m.
1.0 a.m.
6-8»
7-7*
7-0
2.0 „
2.0 ,,
5'7
62
3-o „
i-° „
4'7
June 19th, 1932. First night after full moon. Water temperature 13-141 C. Low water
occurred at 10.12 p.m., sunset at 5.43 p.m., moonrise at 6.38 p.m.
7.50 p.m.
7.50 p.m.
7'4
7-4
9-O
9-o
7'3
5-6
,,
10.0
100
„
59
2-9
II.O
,,
6-of
II.O
4-4t
0
Oct. ist, 1932. No moon. Starlight. Water temperature 14 C. Low water occurred at
10.3 p.m., sunset at 6.48 p.m.
60
7.45 p.m.
7.45 p.m.
5-2
9-'5 .,
915 „
26
3'9
i-S
10.15 ,»
10.15
>>
2-8
11-i.S ..
II.15
„
i-5
i-7
a m
I-I
12.15 a.m.
12.15
- 1-2
Oct. 17th, HJ32. 'I 'hird night after full moon. Water temperature 14-5 C. Low water
occurred at 10.43 p.m., sunset at 7.1 p.m., moonrise at 10.4 p.m.
40
2-9
10.0 p.m.
10.0
p.m.
11.0 ,,
28
„
II.O
2-2
12.0 midnight
12.0 midnight
1-9
i-7
• The reason why these readings are higher than the preceding ones is unknown; possibly due to
splashing from waves.
f These readings are higher than the preceding ones because the pools had been invaded by the
sea. At the first reading on this date the pools were still covered by the sea, and only fully isolated
by the third reading.
SUMMARY.
The present paper shows that in rock-pools containing a good growth of seaweed the oxygen content rises markedly during low water in the daytime, as a result
of photosynthesis, but falls equally definitely at night in the absence of the latter.
Our highest daytime record, in a pool containing a very dense growth of algae, was
26-2 mg. per litre; the lowest night record in the same pool being 1-2 mg. (The
value for the open sea at the same place may be taken as about 87 mg.) In a pool
containing animals but no appreciable growth of plants, the oxygen content falls
both in daylight and in darkness; our lowest record for such a pool being I-I mg.
per litre (at night). No appreciable difference was observed between the oxygen
values obtained on moonless and on moonlight nights. ThepH in the pools behaves
in accordance with the amount of CO2 liberated in respiration or utilised in photo-
172
T. A. STEPHENSON, A. ZOOND and JOYCE EYRE
synthesis (though these are not the only factors which determine it), and rises to o,-o
or higher during strong photosynthesis, falling well below 8-o in the absence of the
latter. The populations of three of the pools used are described quantitatively, for
correlation with the oxygen data.
We are very much indebted to Letitia Starke and S. S. Alexander for carrying
out preliminary series of determinations on a number of pools. We also offer our
sincere thanks for invaluable help to the systematic workers who have identified
plants and animals for us. The algae were determined by Dr A. D. Cotton and
Miss C. I. Dickinson, the animals by Dr K. H. Barnard (crustacea, echinoderms
and fishes), Mr C. C. A. Monro (polychaetes), and Mr J. R. le B. Tomlin (mollusca).
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