A Quantitative Study of Some Factors Affecting Tide Pools By Margaret P. Ambler and V. J. Chapman, Auckland University College [Read before the Auckland Institute, May 23, 1947; received by the Editor, August 11, 1948.] I. Aim of Study The object of the present study was to obtain some quantitative information about certain factors affecting the flora and fauna of selected tide pools, and to ascertain the extent of the daily changes that occur in the factors and throughout the year in different pools at different levels on the shore. Although previous workers have carried out similar observations on rock pools elsewhere, no data are available so far for rock pools in New Zealand. II. The Area and Pools Studied The pools studied were on Narrow Neck reef and headland, at the northern end of Narrow Neck beach, Auckland. The shallow, broad reef which extends from the base of the headland is about 200 yards long and is completely covered at every high tide. At extreme low water the reef has an elevation of six feet and varies in width from 20 to 30 feet near the shore to 60 to 70 feet at its broadest. Near the headland there is an independent area locally known as Crab Island. The rock comprising the reef is a heterogeneous fine volcanic agglomerate, known as “Parnell Grit,” conformably interbedded with the Waitemata sandstone, which forms the bulk of the Auckland Isthmus. At the base of the headland, and extending from beach level to approximately high water mark (H.W.M.), is a belt of the hard “grit” immediately above which a niche has been cut in the softer sandstone, and here pools 1 and 2 are located on the south side. Pool 3 is also on the headland, pool 4 on Crab Island, and pools 5 and 6 on the reef. The selected pools were at varying levels on the shore and contained different algal populations. They varied in size, depth and slope of their walls. Those located at high water mark (pools 1 and 2) were not more than 6 inches deep; for contrast, the pools selected on Crab Island and the main part of the reef were 2 to 3 feet deep and up to 6 feet in diameter. In comparing these pools, size and degree of shading as well as level has to be taken into consideration, e.g. the shallow high-level pools are subject to a much greater change in environment than the majority of lower ones. Only one pool (3) has a permanently sandy bottom; all the remainder have a rocky floor and walls.

All the pools lie between 10·15 and 3·8 feet. Auckland Harbour Board (A.H.B.) datum, and those below 8·75 feet are covered and uncovered twice daily. Each pool was drawn to scale and the plant positions plotted. Vertical sections were made in order to determine the slope of the walls and the levels at which the species were growing. The volume of water in the pools was also measured, since it plays a major part in determining the extent of temperature changes. Pool 1. High-level pool with a dominant Enteromorpha population (Fig. 1). Oscillatoria nigroviridis Filamentous diatoms Enteromorpha sp.* This species is being studied in some detail. At certain seasons it has the appearance of a dwarf E. acanthophora, whilst at other seasons it is more or less unbranched This is the highest pool studied in detail and is situated at 10·25 feet A.H.B. datum. It has a volume of approximately 2 litres. The pool is characterised by transient communities of Enteromorpha, Fig. 1–Plan and section of pools 1 and 2. filamentous diatoms and a more or less permanent growth of Oscillatoria nigroviridis. This pool may be quite dry for several successive days both in summer and winter. It is much more subject to the influence of rain-water than the other pools. Pools 1 a, b, c. Three high-level pools with transient communities. Symploca laeteviridis Scytosiphon lomentarius Enteromorpha sp.* Ulva liza

These pools are small hollows in the sandstone, each approximately 12 inches in diameter and 2 to 3 inches deep. Since they are located at 11·5 feet A.H.B. datum,* All subsequent levels are based on A.H.B. datum. they are only occasionally filled with sea-water by very high tides. Pool 2. High-level pool with Corallina (Fig. 1). Corallina officinalis Ceramium sp. Enteromorpha procera f. minuta Animals: Sypharochiton pellissepentis Vermilia carinifera Actinia sp. Pool 2 is a shallow, irregular pool located at 9·25 feet, but it is not shaded by the cliff to the same extent as Pool 1. The volume is approximately 18·5 litres. It has a sparse population of Corallina officinalis with occasionally some epiphytic Enteromorpha procera. Ceramium sp. appears seasonally, whilst there is a permanent animal population of chitons, Vermilia carinifera and Actinia, sp. Pool 3. Sand pool at high level (Fig. 2). Polysiphonia variabilis Ceramium sp. Lophosiphonia macra Corallina officinalis Enteromorpha procera f. minuta Animals: Actinia sp. Zeacumantus subcarinatus Acanthoclinus littoreus Fig. 2—Plan and section of pool 3. Along the cliff side of pool 3, located on the headland at 9·75 feet, there is a broad, overhanging ledge of rock which casts a shade quite early every day over half the pool. The algal population of Polysiphonia, Lophosiphonia and filamentous diatoms is usually partially buried by sand. Pool 4. Intermediate level pool (Fig. 3). Corallina officinalis Enteromorpha intestinalis Ecklonia radiata Enteromorpha procera f. minuta Dictyota dichotoma Laurencia botrychioides Hormosira banksii Symphyocladia marchantioiodes Colpomenia sinuosa Rhodymenia leptophylla Diatoms Aphanocladia delicatula This pool, situated at the 7·5 feet level on Crab Island, has a volume of approximately 315 litres. The walls and bottom are well covered by Corallina so that there is very little bare rock. Hormosira forms a fringe round the edge of the pool, but

Fig. 3–Plan and section of pool 4. The entire floor is covered with Corallina (shown only in the vertical section). individual plants are also found at a depth of 7 to 10 inches. Between one to four plants of Ecklonia radiata could be counted at different times of the year, but their growth suffered somewhat from the attack of molluses. This large brown alga is normally found at the 1·4 feet level on the open shore, where it grows permanently submerged. Thus, when it grows

under pool conditions, it may be elevated by as much as six feet. Seasonal species found in the pool include Colpomenia sinuosa, Dictyota dichotoma, Laurencia botrychioides, Enteromorpha intestinalis, and Enteromorpha procera, which grows epiphytically on the Corallina. Pools 4a, 5, and 6. Low-level pools with rich brown algal population (Figs. 4 and 5). Entcromorpha procera Carpophyllum maschalocarpum f. minuta Corallina officinalis Cladophora feredayi Hymenena berggreniana Hormosira banksii Gelidium caulacantheum Carpophyllum plumosum Gigartina chapmanii Carpophyllum flexuosum Laurencia botrychioides Ecklonia radiata Symphyocladia marchantioides Zonaria velutina Rhodymenia leptophylla Colpomenia sinuosa Pandorea traversii Dictyota dichotoma Pterocladia capillacea Dictyota ocellata Diatoms Fig. 4—Plan and section of pool 5. Pool 4a is a large pool, approximately 12 feet in diameter, situated on Crab Island at 5·25 feet. This pool and pools 5 and 6, which are at 4 feet and 3·8 feet respectively on the main part of the reef, have each a dense algal population with large brown algae predominating.

Fig. 5—Plan of pool 6. The sides of all these pools are covered with Corallina, whilst a Hormosira-Corallina association fragment* The association fragment is part of the general Hormosira-Corallina association covering the greater part of the rocks at low levels: the term has been used earlier by Cranwell and Moore (1938). fringes the edge. Pool 6 has a rather denser population of Ecklonia and Carpophyllum (including C. phyllanthus) than the other two pools, but the bottom is devoid of vegetation other than pink patches of the encrusting Corallines. The delicate red algae, such as Pandorea traversii, are found beneath the big kelps along with Cladophora feredayi and young plants of Carpophyllum. III. Tidal Phenomena The varying fluctuations of the environmental conditions in the pools can only be fully appreciated after determining the hours of exposure and submergence. By the term exposure is meant the period during which the pool is not covered by the tide. In order to determine the approximate length of time a pool would be exposed, the levels of the pools were found by means of a levelling survey. At the same time a tide pole was established near Crab Island and included in the levelling survey. The height of the tide on the pole at high water was noted on a number of days and compared with the corresponding

readings from the Auckland Harbour Board tide gauge. From the results it was found that the maximum range at Queen's Wharf, where the A.H.B. gauge is situated, is about 0·4 feet greater than that at Narrow Neck. Use of the A.H.B. marigrams might therefore involve an error of up to ± 2%, but this slight difference in range might well be due to swell rendering the tide pole readings slightly inaccurate. The tidal conditions at Narrow Neck were therefore considered to be identical with those in Auckland Harbour. When the tide pole readings and the A.H.B. levels had thus been correlated, the levels of the pools could be read off from the tide pole, and the values thus obtained were checked by levelling to a bench mark. Once the pool levels had been reduced to A.H.B. datum it was possible to use the A.H.B. tide gauge marigrams in order to calculate the tidal phenomena. Marigrams for several years would be desirable, but useful and fairly accurate data can be obtained, as in the present case, from a complete marigram for one year. (a) Hours of exposure and submergence per diem. Levels above mean high water of neaps are free from submergence for periods of from one to several days; these periods will be termed periods of continuous exposure. Similarly, levels below mean low water of neaps are not exposed for two or more days at a time; these are periods of continuous submergence. In order to determine the difference between hours of exposure and submergence during spring and neap tide periods, the hours of exposure and submergence between two successive spring tides and two successive neap tides were read off from A.H.B. marigrams for 1945. High waters of the tides selected were 11·25 and 11·75 feet for the spring tides and 9·26 and 9·2 feet for the neaps, with the respective low waters at 0·5, 0·75, 3·5 and 3·75 feet. The results showed that the lower levels (4·0 ft.) are exposed for a greater length of time during spring tides than during neap tides. They also undergo a much shorter period of submergence during spring tides. On the other hand, the high levels are submerged for a relatively long time during spring tides and are exposed continuously during neap tide periods. (b) Monthly exposures. The monthly exposures and submergences were calculated on a percentage basis, because each marigram covered a different number of hours, depending on the time it was started and concluded on the tide gauge. The calculations showed that at 10·25 feet the greatest percentage exposure occurred in March and April. At 9·75 feet, 9·25 feet and 7·5 feet there was an increase in exposure during April, whilst at the first two of these levels there was an even greater increase in November. At the lower levels (5·25 feet and 4·0 feet) there is a decrease in exposure in September, the decrease being more pronounced at 5·25 feet. The increased exposure at the two 9 feet levels in November may be sufficient, to result in significant temperature and salinity changes in the water of pools. Furthermore, November is an important month,

because considerable growth in many algae takes place then, so that changes in the environment may be more effective than at other times. (c) Continuous exposure (Fig. 6). Only levels above 9·0 feet experience any pronounced periods of continuous exposure. At 9·75 feet and 10·25 feet there is a marked increase of continuous exposure in April and May and an even more pronounced rise in November. Fig. 6—Number of days per month of continuous exposure at +9.25, +10.25, +11.5 feet levels. Rain during May and November could bring about significant changes in salinity, and increases in both air temperature and hours of sunshine during November might also have a profound effect upon a pool undergoing continuous exposure, although this would depend upon the weather. IV. Tide Pool, Factors Other Than Tidal. Hourly observations were made on a number of days at different times of the year. Temperature readings and pH determinations were carried out at the locality and sea-water samples were brought back for chloride and oxygen determinations. The readings were repeated once a month during May, June, July, September, November, and December, 1946, and on several days in January, February, and March, 1947. A series of observations extending over twenty-four hours was also made in May, 1947, principally in order to determine the effects of photo-synthesis and respiration of the living organisms

on pH. Additional readings were secured occasionally from pools 1a, b, and c on the headland and pool 4a on Crab Island. (1) Temperature Diverse factors affect the temperature of tide pools and fluctuations occur according to the conditions prevailing at the time of determination. A high-level pool, which has been exposed for a long time may show a considerable rise in temperature on a fine sunny day, though the extent of the rise will depend very largely upon the size and depth of the pool. A shallow pool at low levels, although exposed for a relatively short time, may attain quite high temperatures during the summer months. At Narrow Neck is was found that topography of the rocks and aspect of the pool influence the temperatures to a considerable extent, and may even mask the effect of level. Fluctuations in air temperature may also be reflected in the temperature of a pool. This is demonstrated by readings for pools 1–3 on May 30 (Fig. 7) and for pools 4 and 4a in January (Fig. 8), though it will be noted that, in general, air temperature is below that of the pools. Fig. 7—Hourly changes in pool temperatures for May 30 (A), June 30 (B), July 31 (C). Air temperature changes are shown on May 30. The temperature reached by a pool on any given day depends very largely on the time of low water. If low tide occurs in the middle of the morning, the temperature will not reach a high value before the pool is flooded once more. If low tide occurs at mid-day or early afternoon a much higher value will be reached before the pool is reflooded. (a) Daily change. The daily change in temperature depends mainly on local conditions. Those pools which remain in the sun during the period they are exposed show a steady increase in temperature, whereas those which become shaded soon show a decrease. This fact is illustrated in the graphs for pools 1, 2, and 3 (Fig. 7). These high-level pools are on the south side of the headland and thus receive the sun's rays only up till mid-day or earlier, depending on the season of the year.

The water temperature in most cases therefore rises up till noon or 1 p.m., after which there is a fall. Pool 1 is the first to become shaded, and it shows the fall in temperature first. With one or two exceptions the greatest changes in temperature occurred in the high-level pools, whilst the maximum temperature recorded during the days of observation (32°C.) was in pool 2 on January 20, 1947. There may be a difference of several degrees between the top and bottom layers of a pool on a hot day, e.g. in pools where there was a dense brown algal population (pools 5, 6) the temperature of the bottom layer may be as much as 4°C. less than that of the surface. On the other hand, pool 4, which is a moderately deep pool, never showed, much difference in temperature between top and bottom layers. In this case, however, there is no surface covering of large brown kelps. This example illustrates the important part large algae may play in reducing circulation of water in a pool. (b) Pool and sea temperature. Fig. 8—Temperature changes in Pools 1, 2, 4, 4a at successive low tides, January 16–30, 1947. Pool 1 was dry from 18th–21st. (Air temperature is given as a separate curve.) Johnson and Skutch (1928) found that the temperature of pools may be higher than that of the sea during the day and lower at night. A similar result was obtained at Narrow Neck during May 3 and 4, when readings were obtained over a 24-hour period. Pool 1 was in the shade when readings commenced and had a lower temperature than the sea. Pool 2 was in the sun until 1 p.m., when its temperature

was 2·7° above a sea temperature of 18·5°C., but by 2 p.m. the temperature had fallen to 17°C. (sea temperature 18·4°C.). The temperatures of pools 4 and 5 were higher than that of the sea until the pools were submerged. During the night pools 4 and 5 did not show such a great decrease below sea temperature as pools 1 and 2. These latter are shallow pools and therefore are more influenced by air temperature, whereas the deeper pools follow sea temperature more closely. High tide occurred at 6.36 a.m., and by 10 a.m. the temperature of pool 2 had risen to 4°C. above sea temperature, and by 11 a.m. the temperature of all four pools had risen above that of the sea. Fluctuations such as the above will only occur when the time of high water is at mid-day or later. (c) Temperature at successive low tides (Fig. 8). The data obtained show that the greatest sudden changes in temperature (i.e. on flooding by the colder sea-water) can be experienced in the high-level pools 1 and 2 when high tide occurs in the middle of the day, because their maximum temperature is reached at mid-day or just after. On the other hand, any sudden changes in pools 4 and 4a (low level) only occur if they are flooded between mid-day and 5 p.m. (d) Discussion. The temperature readings of the different pools were compared in the following manner: i. Pool 1 (small shaded high-level pool) compared with pool 2 (larger unshaded high-level pool). ii. Pool 2 (high-level shallow pool) compared with pool 4 (intermediate level deep pool). iii. Pool 1 (high-level pool) compared with pool 5 (low-level pool). iv. Pool 4 (intermediate level pool) compared with pool 5 (low level pool). i. In all cases pool 2 reached a higher temperature throughout the year. This is a result of the differential shading of the two pools, because pool 2 is exposed to the sun for one to two hours longer than pool 1. ii. In May, June, and July, pool 4 attained the higher temperature, but this also is a direct result of shading. In the winter months, once the sun leaves the pools, the water cools very quickly because of the lower air temperature. In the summer, however, the cooling is not so rapid and in September, November, and December both pools were affected more or less equally. iii. In May, June, July, and September, pool 5 reached higher temperatures than pool 1; the pools had approximately the same temperature in November, but pool 5 attained the greater temperature in November. This difference can be accounted for by the degree of shading that pool 1 undergoes. iv. The two pools were affected more or less equally by temperature, so that the difference in water volume (100:315 litres) is not significant.

(f) Conclusions. The only conclusions that can be drawn from the temperature changes of this limited number of pools are: (i) That temperature increases occur daily and when pools are not shaded by the sun there is a greater increase in the high-level shallow pools. There may, however, under certain conditions, be quite a considerable change in low-level pools during the short time they are exposed in summer months. (ii) The temperature of the surface layers of certain pools may be several degrees higher than that of the bottom layers. In such pools it seems that the plants shade the bottom layers and also inhibit circulation of water. (iii) The temperature of the pools may be higher than that of the sea during the day and lower at night. (iv) The pools usually have temperatures above that of the air, but fluctuations in the air temperature are reflected in the pool temperatures. (2) Hydrogen Ion Concentration The hydrogen ion concentration is, to a large extent, a measure of the O2/CO2 ratio in the pool and is also an indication of whether assimilatory or respiratory processes are more rapid in the pool in question. (a) Method. The pH of the pools was tested colorimetrically using a set of “B.D.H.” standards. (b) Daily change (Fig. 9). When the pools are just uncovered the pH is the same as that of sea-water, but in some cases it rises very rapidly, especially where vigorous photo-synthesis is taking place. Quite frequently there was an increase from pH 8·0 to pH 9·0 or more within an hour (pools 1 and 2 on June 30, pool 2 on December 11), although further exposure usually did not result in any very appreciable additional rise. The high-level pool 1 with a green algal population of Enteromorpha always showed the greatest increase in pH, whilst pool 3 with a population of Polysiphonia and a greater volume of water, showed the least change. The plant population of this pool is probably not sufficiently large to cause the pH to rise as much as it does in the other pools where green algae or large Phaeophyceae are present. (c.) The effect of plants on pH. An example of the effect of the plant population on the pH value is illustrated by the readings for pool 1 in January (1947), where the daily drift of pH at successive low tides is shown (Table I). On the 22nd there were no visible Enteromorpha plants present and the pH was recorded as 8·0, but by the 27th a sufficient Enteromorpha population had developed to cause the pH to rise to 9·4. Thus when plants were absent there was no change in pH above that of sea-water, but with an Enteromorpha population there was a substantial increase.

Fig. 9—Hourly change in hydrogen ion concentration for June 30, July 31, September 28, November 12, and December 11. On June 30 the pH of pools 1, 2, and 3 at 11 a.m. was that of sea-water soon after uncovering. Table I—ph of Four Pools During Successive Days at Low Tide January Pool No. 8.40 16th 9.30 17th 11.30 19th 12.15 20th 1.0 21th 1.20 22nd 3.10 24th 3.20 25th 5.0 27th 5.0 28th 6.0 30th 1 8.0 8.9 dry dry dry 8.0 8.1 8.0 9.4 9.4 9.1 2 8.0 8.5 8.8 9.0 9.0 9.0 8.9 8.8 9.0 8.8 8.8 4 8.0 8.5 8.8 8.9 8.9 8.8 8.8 8.9 8.8 8.6 8.4 4a 8.0 8.8 9.0 8.9 8.9 8.9 8.8 8.8 8.8 8.6 8.4 A further example of the effect of plants on pH is seen in the diurnal changes of the pools (Table II). Immediately pools were uncovered after the high tide on May 3 the water in the pools had the same pH as sea-water, and throughout the hours of darkness the readings remained the same. After 9 a.m. on May 4, when active photosynthesis was again taking place, the pH of four pools out of five began to increase.

Table II—Hourly ph Readings Over 24 Hours 3rd May, 1947 Pool 11.45 1 p.m. 2 p.m. 3 p.m. 4 p.m. 5 p.m. 6 p.m. 7.30 p.m. 8.30 p.m. 9.30 p.m. 10.30 p.m. 11.30 p.m. 1 9.4 9.5 9.5 9.5 9.5 9.5 Flooded 8.1 8.0 8.0 8.0 8.1 2 8.8 9.0 9.2 9.2 9.2 9.2 by 8.0 8.0 8.0 8.0 3 8.2 8.2 8.2 8.2 8.2 8.2 tide 4 9.1 9.2 9.4 9.3 9.4 8.0 8.0 5 9.1 9.3 9.3 8.0 8.0 4th May, 1947 Pool 12.30 a.m. 1.30 a.m. 2.30 a.m. 3.30 a.m. 4.30 a.m. 5.30 a.m. 6.30 a.m. 7 a.m. 8 a.m. 9 a.m. 10 a.m. 11 a.m. 1 8.0 8.0 8.0 8.0 8.0 8.0 Flooded 8.0 8.0 8.0 8.1 8.4 2 8.0 8.0 8.0 8.0 8.0 by 8.0 8.0 8.0 8.1 8.4 3 tide 8.0 8.0 8.0 4 8.0 8.0 8.0 8.6 5 8.0 8.0 8.6 (d) pH and temperature. The actual temperature of the pools seems to make very little difference to the pH values, e.g. on June 30, in pool 1, when the average temperature was 12·1°C., the pH reached 9·4, and on January 27, when the temperature was 22·5°C., the pH was again 9·4. (e) Discussion. All the pools studied (with the exception of pool 3) show more or less the same daily increase in pH. From the data available, pool 1, which contains a growth of green algae, reached a higher pH in most cases than pools 4 and 5, which contain Corallina and large brown algae respectively. This seems to support the theory that the green algae in the well-lighted pools photosynthesise more rapidly than the red and brown algae. Pool 3, with its dominant small red algal population, nearly always had a pH below that of other pools. Much more work is required on this problem, and at least one requisite is a correlation of the total plant material to water volume of the pool. It is only possible to conclude from these results that the nature and extent of the algal population is the principal determining factor for hydrogen ion concentration. The range of pH observed does not appear to be outside the values most algae can withstand (Biebl, 1937). (3) Oxygen Content (a) Method. Winkler's method was used for estimating oxygen and pool 4 was selected for intensive study. Unless otherwise stated, all observations were made on pool 4. (b) Daily change. The oxygen content rose during the daytime in the same way as did temperature and pH. The maximum value recorded was 13·1 c.c. of oxygen per litre on June 30, 1946, at 4 p.m., whilst the minimum value was 7·16 c.c. O2 per litre at 2 p.m. on November 14. It would seem, therefore, that the water of the pools must on occasion be

supersaturated. Pool 3, on the headland, with its rather scanty population, showed, on the one occasion investigated (February 23), a much lower value than pool 4, which contained a denser algal population; this low oxygen value corresponds with the lower pH values for the same pool. The maximum depth in pool 4 is 20 inches, but no significant difference could be detected at any time between the oxygen content of the surface and bottom layers. (4) Chloride Concentration (a) Method. The chloride concentration was determined volumetrically by titrating sea-water with standard silver nitrate solution, using potassium dichromate as an indicator. (b) Daily change. The range recorded at Narrow Neck on the limited number of determinations made was from 6·67 to 2·20% for pool 1, 1·96 to 2·04% for pool 2. 1·90 to 2·09% for pool 3, and 1·84 to 2·05% for pools 4, 4a, 5, and 6. On the whole, the observed chloride values reveal very little change and all fall within the ranges given by Biebl (1937) us tolerated by characteristic algae of European tide pools. (5) Other Environmental Factors Other factors beside level, temperature, pH, etc., play a part in determining the distribution of the tide pool algae. The chance distribution factor must play a large part, especially in pools of approximately the same level, where other factors are more or less uniform. Currents and splash may play some part in this connection: thus there may be pools so located that they do not receive zygotes or sporelings, whilst there may be other pools so subject to spray that they contain species thrown in that are lacking in adjacent pools protected from spray. Drainage at Narrow Neck follows the cracks and slope of the reef and most channels or cracks carry a growth of Corallina and Hormosira. After the tide has uncovered a pool, the level of the water may fall half an inch and uncover the fringing algae. Fronds of Carpophyllum and Ecklonia, which are left exposed at the surface of the pool, often show signs of bleaching. It is very difficult to determine the part that light plays in the distribution of the algae in the pool. No readings were taken of light intensity in the pools, but where there is a surface covering of large brown algae light reaching the plants of the lower layers must be considerably reduced. V. Summary A number of rock pools at different levels on the shore at Narrow Neck are described. The level of the pools was related to Auckland Harbour Board datum and the marigrams for one year used to determine the tidal phenomena. At some levels the tidal phenomena may at certain seasons expose the pools to the possibility of marked environmental changes in temperature and salinity.

Changes in water temperature of the pools are determined by the time of high water, the degree and time of shading during the day and the air temperature. Large brown algae appear to inhibit water circulation and thus promote temperature layering. There is also a seasonal trend in the water temperatures. The pH and oxygen content of the pools rises during the day as a result of algal photo-synthesis, the nature and extent of the algal population being the principal determining factor. The changes in chloride concentration all fall within the range tolerated by characteristic European tide pool algae. Acknowledgments Thanks are due to Mr. J. Morton and Mr. V. W. Lindauer for assistance in identifying the animals and some of the algae respectively. Thanks are also due to Miss L. B. Moore for help and criticism over the preparation of the manuscript. References Bebl., K., 1937. Okologische und Zellphysiologische Studien an Rötalgen der englishe Sud Kuste. Bei. Bot. Cent., 57, 381. Johnson, D. S., and Skutchi, A. F., 1928. Littoral Vegetation on a Headland of Mount Desert Island, Maine. Ecology, 9, 188.