algae of two somersetshire pools standing crops of phytoplankton and epipelic algae as measured by...

J. Phycol. 5, 158-168 (1969)

ALGAE OF TWO SOMERSETSHIRE POOLS: STANDING CROPS OF PHYTOPLANKTON AND EPIPELIC ALGAE AS MEASURED BY CELL NUMBERS AND CHLOROPHYLL A 1

Brian Moss2 Department of Botany, The University, Bristol, United Kingdom

SUMMARY

Standing crops measured as cell numbers and as chlorophyll a content of phytoplankton and epipelic algal communities (those free-living on sediment surfaces) in 2 small water bodies of contrasted nutrient status were measured for 27 months. Mean yearly crops, o n an areal basis, of phytopiankton were 4-13.4 times greater than those of epipetic algae in the nutrient-rich pool, but were only 0.65-1.8 times as great in the nutrient-poor pool, which, however, was shallower than the rich pool. T h e role of cells < 5 p in diameter in the composition of standing crops was minor. Effects of phytop!nnkton in limiting growth of epipelic algae by light attenuation are shown. Compilation of available data shows a dil-ect relationship between epipelic algal crops i n various water bodies and nutrient-status of the ambient water, similar to that already established for phytoplankton.

INTRODUCTION

Interest in benthic algal communities in aquatic ecosystems is increasing. Much has been done on phytoplankton (11) but far less is known about the algal communities living attached to submerged substrates, and those (epipelic) free living on mud or sediment surfaces (20,?2,?3,?9). Measurements of benthic algal standing crops and productivities are infrequent (4,9, 20,25,29,39,40), and most studies have been concerned with attached algae, whose importance, especially in streams and shallow water bodies, is increasingly rec- ognized. Crop sizes of epipelic algae, measured on an areal basis, are discussed in refs. 4,6,18,20,25 and other studies of epipelic algae are recorded in refs. 5,14,26$2,41. This paper includes area-based seasonal data on crops of epipelic algae, and of contem- porary phytoplankton in 2 small water bodies.

The algal crops were measured as cell numbers and as chlorophyll a content. Limitations to the useful- ness of chlorophyll a include a variable relationship to total organic matter in different species, and under varying culture conditions in the same species. Con- tamination of extracts of living cells with “inactive” chlorophyll degradation products from dead cells may also invalidate the pigment estimates if they are not taken into account. Despite these drawbacks, chloro-

1 Received December 6,1968; reoised February 2,1969. 2Present address: Dept. of Botany and Plant Pathology,

Michigan State University, East Lansing, Michigan 48823.

pllyll a remains the one substance, entirely specific to plant cells, measurable with ease, precision, and accuracy (see 17 for references). I n small bodies of water, much detritus may be present in the seston (the particulate matter, living and dead, suspended in the water), from inwash of soil material and resus- pension of bottom sediments. Animal and bacterial populations may also be present. Nonplant-specific parameters such as dry weight, carbon, nitrogen, lipid, protein, or carbohydrate content then can only be used occasionally to measure phytoplankton standing crops with accuracy. Whenever it is necessary to measure standing crops of epipelic algae, only 2 parameters are suitable since it is virtually impossible to separate epipelic populations completely from sediment. These are cell numbers (together with cell volume to give more information) and pigment content, if the method used can distinguish between pigments in living cells from those of dead plant material in the substrata1 sediment.

DESCRIPTION OF SAMPLING SITES

Abbot’s pond (Z”4O’W; 51°28’N; National grid reference ST 536732) and Priddy pool (2”39?W; 51”21’N; S T 547517) are near Bristol, England, and experience similar weather. Bathymetric maps are given in Fig. 1 and 2. A detailed introductory account of the pools is given in ref. 3.

As a result of its sheltered position-closely surrounded by tall dcciduous trees in a narrow valley-semipersistent thermal and chemical stratification occurs in the water column of Ab- bot’s pond (2192) despite its shallowness. Large quantities of leaves fall into the pond in autumn and the bottom sediment is rich in organic matter, particularly in the deeper parts of the pool. Vascular macrophytes are not abundant. The pond was formed by building a stone dam across a small stream. The water is relatively rich in dissolved ions (Table 1). Previous studies on the algae and limnology of the pond include 3,5,7,8,

Sampling stations are shown in Fig. 1. Stations 2 and 3 were similar, with about 4 m2 of brown mud covered by u p to 50 cm of water. Station 3 was sometimes disturbed by surface seiches generated by winds, occasionally penetrating the woodland, but constrained by the topography along the long axis of the pond. Station 3 was also less shaded by tree foliage in summer than was station 2 and most of the E and W pond margins, which were similarly undisturbed by seiches. The sediment at the “deep water” stations 5, 6, 7, and 8 was liquid and flocculent, and a t stations 5 and 6 was covered by anaerobic water during part of the summer.

Priddy pool (Fig. 2) is more elevated and exposed than Ab- bot’s pond, is shallower, and thermally stratifies only diurnally. Its catchment area consists of acid grassland and its ionic levels (Table 1) are relatively low. About 85% of the pool is occu-

1 0,13,18,19,21-23,34.

158

ALGAL STASDING CROPS 159

ABBOT‘S POND PRIDDY POOL

o u t f l o w

34 Irn iiOO’l

FIG. 1. Bathymetric map of Abbot’s pond.

pied by emergent vascular rnacrophytes (mainly Equketunz fluviatile L.), leaving a smaller area of open water. The algal biology of the weed bed is being studied by J. G. Brown (personal communication) and is not considered here. Previous studies on the pool include 3,5J3,18,23,34.

Sampling stations utilized are shown in Fig. 2. Station 4 was an open water station with about 115 cm of owrlying water. Station 2 was about 3 m2 in area, with a predominantly inorganic sandy sediment, with some flocculent organic matter at the sediment surface. Station 3, 4 m2 and with 10-40 cm of water overlying, had a highly inorganic sandy sediment. T h e station was frequently and rigorously disturbed by water mow- ments induced by strong north and west winds.

METHODS

An approximately constant time schedule was adhered to in removal and subsequent treatment of the samples, which were taken at weekly or fortnightly intervals.

Phytoplankton sampling. Surface samples from station 4 proved representative of Priddy pool open water. At station 5 in Abbot’s pond, from May 1966 to April 1967, water samples were taken a t 0.5-m depth intervals, with a small closing sam- pler. Previous to May 1966, only surface samples were taken. Water samples were stored in polyethylene containers protected from direct sunlight, before processing in the laboratory within 3 h r of sampling.

Epipelic algal population sampling. Quantitative estimation of epipelic algal populations presents greater difficulties than those involved in the estimation of phytoplankton. Spatial differences in distribution are common to both, and can be elim- inated by sufficiently comprehensive sampling; the greater possibilities of mixing in open water, and the heterogeneity of sediment surfaces, result in this needing to be more extensite for the epipelic populations than for phytoplankton. However, whereas natural mixing processes facilitate valid sampling of the phytoplankton, they complicate that of epipelic algae by erosive removal of celIs (31). In Abbot’s pond only a small proportion of the marginal sediment, represented by station 3, was subjected to relatively severe erosion, but other factors, e.g., light limitation and deoxygenation (18), resulted in marked horizontal differences between populations of cpipelic algae in different parts of the pond.

Samples were taken from the marginal stations (2 and 3) in both pools, using the area-based sampling method of Eaton S- Moss (6). Three separate samples, each from an area of 61 cm’, were normally removed. During Aug. and Sept. 1965, when clumps of Spirogyia and Cladopkora were present over part of station 2 in Abbot’s pond, only sediment not occupied by these

682rnI200’l I

FIG. 2. Bath>metric map of Prickly pool.

mats was sampled. Three short, undisturbed, surface mud cores (4.5 cm2 in area) were removed from each of stations 5, 6, 7 , 8 on each occasion. Estimates from stations 5 and 6 and 7 and 8 were averaged, as being representative of sediment below more than 2.5 m of water, and under 1-2.5 m, respectively. Extensive sampling over the whole of the pond bottom showed these to be valid assumptions. As a check on randomness in sampling marginal populations, a standard sample of surface sediment was taken by drawing a long glass tube over the sediment surface (6), on most occasions. Tithe sainples were drawn from relatively large areas of sediment couiparetl with those sampled by the area-based method, and, by eliminating much of the error resulting from heterogeneity within the sampling station, provided a useful comparison with the area-based samples.

Phytoplankton sample processing. Up to three 100-1000 ml samples of thoroughly shaken water to which about 0.1 g MgCO, had been added were filtered in subdued light through Milli- pore filters (AA grade, pore size 0.8 p). Glass fiber filters (What- man GF/C grade) were used in some analyses from Jan. 1967 to April 1967. Loss of material through these filters was found to be negligible and substantial discrepancies in chlorophyll a estimates between replicate samples filtered through Millipore and glass fiber filters, as recorded by Spencer (36) for marine phytoplankton, were not found.

The filtered seston was stored overnight at -20 C in the dark or was treated immediately with 90% analjtical grade acetone (not redistilled), after mechanical grinding with washed sand. Extraction of pigments was complete within 24 hr, in darkness, at 3 C.

Several hundred iodine-sedimented cells from each water sample were counted with an inverted microscope.

TAIILE 1. Ranges of iitnjor ions (nig/Eiter) in Abbot’s pond and Pridriy pool. Dntn front Enton (5).

Ion Abbot’s pond Priddy pool

Ca++

Na+ I(+ C1- SO,-- IVeak acid salts (meq/liter) PH

Mg++ 3644 12-1 7 1&13 2-3

14-22 9-38

2.9-5.5 7.0-9.2

4-8 1-1.5 5-8

0.2-2 7-1 1 6-14

0.054.4 5.74.7

160 BRIAN MOSS

r = = - - - -- - - - - w = I TABLE 2. Pronlinent species in the surface water phytoplank- toll of Abbot's pond, 1965-1967.

1965 1966 1967

I I 250 1

J I i i u " z - I \. " 0

I I =- I I - 1 J F M A M J J A S O N D J F M A M J J A S O N D J F M A

1965 IS6

FIG. 3. Seasonal distribution of cell numbers and chloio- phjll a content of the phytoplankton in the surface water of Abbot's pond.

Processing of epipelon. The sediment saniples from Priddy pool and from stations 2 and 3 in Abbot's pond were treated, and the epipelic algae were subsequently estimated, using the tissue-trapping technique of Eaton & Moss (6). The coefficient of variation for counts was 20.8%. The method has the advan- tage of sampling only actively motile epipelic cells, so that sedimented phytoplankton does not interfere with the estimates. Crops from tube samples were estimated by cell counts only, on a unit mass of sediment basis.

An alternative method of sediment preparation for the sediments from stations 5-8 in Abbot's pond was devised, since much loss of sediment and cells occurred when the supernatant water was pumped off after settling the sediment (6). The liquid sediment was drained through a glass fiber filter molded into the bottom of a perforated Perspex dish. Although suitable for light organic sediments, the method was not usable for inorganic sediments in which compaction on drainage impeded epipelic algal movements to the sediment surface.

Chlorophyll a estimation for all samples was spectrophotometric. Calculation of chlorophyll a content, including a correction for any pheopigment present in the extracts, was carried out using the equations of Moss (17) from Jan. 1966 onward. Before this, the equation of Parsons & Strickland (ZS), which does not include a correction for pheopigments, was used. The 196.5 measurements may be expected, as a result, to be over- estimates.

RESULTS

Phytoplankton-A bbot's pond. Cell counts and chlorophyll a estimations for the surface water samples are plotted in Fig. 3. The counts have been divided into 3 categories corresponding to numbers of cells with all diameters < 5 p, numbers of cells with their largest diameter 5-25 p, and numbers of cells with at least 1 diameter > 25 p. These categories are to some extent arbitrary, but a major distinction lies at 5 p, which separates very small green flagellates

Jan-Mar. Cryptomonas sp. Cryptornonas Chrysococcus Chluniydornonus sp. sp. diaphanus

Skuja

Mar.-June AJterronella forniosa Asterionellu Asterionella Hass.; Synedra actis forniosa forinosa Krttz bar. angustissin~a Hass. Hass. Grun.; Ankistrodesnitis falcatus var. aciculuris (A. Braun) G . S. West.

June-Aug. Pandorina momnt Pandol-ina (Mull) Bory. morunz (Mull) I7oZu0~ uwevs Ehr. Bory.

Aug.-Sept. Asterionella formosa Cyclotella Hass. ni enegh in iana

Kutz.

0ct.-Dec. None prominent. Stephanodis- cus rotula (Kutz) Hen- dey; Chryso- coccus diaphanus Skuja.

~~ ~~~

tending to form numerically very large populations from other phytoplankters forming numerical populations 1 or 2 orders of magnitude smaller. Distinc- tions between the 5-25 and 25 p categories have less significance. Factors causing initiation and limitation of individual species populations are not discussed here. There are theoretical reasons why interpreta- tion of the causes of seasonal cycles from total population estimates may be invalid (18), but the general succession of abundant species is given in Table 2.

The bulk of the annual crop was present in July- Aug. in 1965. The relationship between chlorophyll n and total organic matter under natural conditions (18) in the algae of Abbot's pond and Priddy pool was much less variable than found for cultured algae (2). In 1966, although the summer crop was larger than in 1965, it was superseded by a large crop of Ch?y.rococcus diaphanus (lo), which was present during winter 1966/ 1967. Comparably substantial winter crops of any species had not occurred in the surface water in previous years (1962-1965).

Comparison among the cell numbers (Fig. 3) re- veals that numerically large crops of organisms, < 5 p in size in April/May 1965, NOV. 1965, Mar. 1966, and at the end of May 1966 were not associated with substantial rises in the chlorophyll a content of the phytoplankton. Conversely, the graphs for organisms > 5 could be matched, for most of the period, peak for peak with changes in chlorophyll a level. Some- times the numerical peaks were of organisms 5-25 p, especially in late 1966, and sometimes, e.g., July, Aug. 1965, 1966, of those in the > 25

The ratio cell numbers : chlorophyll a on any given occasion was not constant, but this was to be

category.

16 I ALGAL STANDING CROPS

I - -- -- ?- ---

lv lo i JJ 2 u l A$ Ser OCI “I/ I ) tc Jon Ccb Mi- Upr

,566 !?67

FIG. 4. The phytoplanktonic chlorophyll a content of Ab- bot’s pond, calculated as described in the text. The iippcr graph represents the contcnt of the total watcr mass, thc lowcr line that of the 2-4 m stratum, and thc difference between tlir 2 graphs that o f tlie upper, 0-2 m, stratimi.

expected considering tlie range of ccll sizes encoun- tered even within a single size category.

During 1966167, when the vertical distribution of cell numbers in d4bbot’s pond was ii~vestigated (ZZ), equally detailed estimates of chlorophyll a content of the phytoplankton were made regularly only during the final 3 months. Besides these estimates, isolated ohservations during 1966 and detailed observations in Jan. 196‘7 were made. Then, distribution of cell riurribers and chloropliyll a content of tlie phytoplankton at 8 depths was investigated at iritervah ol a few Iionrs during a 24-hr period. These data showed that on a given occasion the numbers of cells > 5 in size at a given depth were closely related to the chlorophyll a content at that depth-especially true if, as was frequent, a single species predomiiiated.

From the cell numbers at depths in the water column, arid from tlie surface phytoplankton chlorophyll a content, the chlorophyll (2 depth profile coulcl be calculated, therefore, for occasions when it was not measured directly. Chlorophyll I( contents as amounts per water column 0-4 m deep by 1 111” area, arid for the upper arid lower halves of this coliinin were calculated with respect to tlie volumes of these straia in the poiid (Fig. 4).

Samples of surface water, taken from stations other tlian no. 5, revealed no marked differences in hori- Lontal distribution of the phytoplankton, except on one occasion, for which allowaricc lias been made, in Aug. 1966, when Pundoyina moriim was concentrated markedly toward tlie north end of the pond.

On most sampling occasions (here w s niucli inore phytoplanktonic chlorophyll a in the iippcr 2 111

stratum of water than in the lower 2 r n stratum (Fig. ‘I), partly owing to density stratification of the water column (ZI), arid partly to phototactic aggregation

of motile phytoplankters near the surface (10,22). No evidence was found of variation in chlorophyll a content per cell with depth. I n the short water col- uinn considered, sucli variation would he unexpected compared with deep water situations from which the plienonienon lias previously been reported, where cells may spend considerable periods in low light intensities before being recirculated to the surface.

T h e surface water was often turbid with plankton arid especially in summer the 2-4 m slratum was alniost dark (18). The bathyinetry of the pond does not account for the differences in the upper and lower halves of the water mass sirice the lower stratimi occupied nearly 50% of tlie pond volume, hut contained only 19.7% of the year’s average stancling crop, as rrieasured by chlorophyll a. T h i s average was calculated by dividing the area under the upper graph (Fig. 4) by the number of occasions on which mea- siirements were made: it was found 4.0 x 105 mg cliloropliyll n for the total water mass during May 1966-April 1967. Expressed on a colunin/area basis the mean standing crop was 108 mg/iiiz.

Altliough only siirface water samples were taken (luring May 196.5-April 1966, an estimate of mean standing crop for tlie total water mass during this period of 1.3 x 10; nig (35.8 mg/mz) can be made, assuming the relationship between mean cldorophyll n content at the surface to mean total chlorophyll u was similar in both years. This crop is less than was obtained in 196,6167, despite a possible overestimate owing to the presence of pheopiginents. T h e increased crop in 1966167 resulted mainly from tlie large winter growth of Chquococc?(s diri$JhamLs, which ac- coiinted for just over half tlie year’s mean st.aiidirig crop.

I K ~ the set of observations made during a 24-hr period i n Jan., there was no indication of diurnal rhythmic fluctuations in chloropliyll (i content per cell, as reported for niarinc phytoplankton (35,43,44).

P ~ ~ ~ ~ o ~ ~ a ~ ~ ~ ~ o ~ - ~ ~ ~ z ~ ~ ~ y pool. Cell counts and chlorophyll n estimations for the phytoplankton o f Priddy ji001 have been plotted (Fig. 5) as for Abbot’s pond. A notable feature is the riuirierical predominance of cells < 5 I* in sire, tlie larger cell categories being represented b y small siiniiner populations of desmicls ant1 b y I l znob~yon spp. from Jan. to May. Relation- ships between cell counts and chlorophyll n content are mire difficult to interpret than for Abbot’s pond data, because of the general lack of major peaks in populations, However, i t is notable that the numerical peak of < 5 ,,, organisms in April 1966 was not reflected in a comparable increase in chlorophyll a cont cn t.

The Eq71U~f2i~72 growing in the pool bears large populations of epiphytic filamentous Chlorophycean algae, in which filaments are entangled numerous tlesinitls rid small green algae. The contribution of iliese to tlie phytoplankton, as a result of wind dis-

162 BRIAN MOSS

TUBE SAMPLES

A I

J F hi A M J J A s o N D J F rd A M J J A s o N D J

1965 l9FL

FIG. 5. Seasonal distribution of cell numbers and chlorophyll n content of the phytoplankton in the open water of Priddy pool.

turbance, is probably great (23). The area of deepest water in the pool (adjacent to the dam) is perma- nently clear of macrophytes, and is treated as an entity separate from the macrophyte bed, in assessing the relative abundances of phytoplankton and under- lying epipelic algal crops in the mixed open water area.

The mean phytoplankton crop for Jan.-Dec. 1965 was 3280 mg chlorophyll a per total open water mass (3.7 mg/m2), and for Jan.-Dec. 1966 was 2580 mg per total open water mass (2.5 mg/m2).

Epipelic algal populations-A bbot';, pond. Fig. 6 includes counts and chlorophyll u contents of epipelic algal populations at station 2 . Counts of diatom and flagellate cells (including Cryptophyta, Eugleno- phyta, and Chlorophyta) and blue-green algal filaments are given for tube samples. Chlorophyll a contents and counts of diatom cells and of flagellates when numbers were significant in area-based samples are also given. Area-based cell counts and chlorophyll a contents of the epipelic algal populations at stations 5 and 6 (2.5+ m) and 7 and 8 (1-2.5 m) are given in Fig. 7.

Diatoms generally predominated at the little-dis- turbetl station 2 (Fig. S), but there was a considerable population of cryptomonads in Aug. and Sept. 1965. There was very good correspondence between flagellate counts from both area-based and tube samples, and good agreement between diatom counts, the only major discrepancy being an underestimation in area-

o c L I L - - - N O J F M A M J J A S O N D J F M A M J J A S O N D J F M

965 I966

FIG. 6 . Seasonal distribution of cell numbers and chlorophyll a contcnt of epipelic algal populations at ytation 2 in Abbot's pond.

based samples of the peak during April 1966. The area-based samples can therefore be considered representative of the general area of the sampling station. Comparison between graphs of area-based cell counts and of chlorophyll a content shows very good agreement in the seasonal population distribution as measured by the 2 parameters. Diatoms made up the bulk of the populations; the numerically large population of flagellates in autumn 1965 contributed only a small amount of chlorophyll a.

3 0 C

E t /. 5 20

Moy Jm JLI Aug Sep Ucl NOV Dec Jon Feb "4or Apt

Y66/67

FIG. 7. Seasonal distribution of cell numbers and chlorophyll a content of epipelic algal populations at stations 5 and 6 (2.5 + in) and 7 and 8 (1-2.5 m) in Abbot's pond. For counts, circles = number of cells of diatoms, triangles = number of filaments of blue-green algae, and squares = number of cells of flagellates.

.lI‘ADLE 3. hlC’On S t 0 T l d i 7 l g C 7 0 ~ l . S Of C’f) i f )<’ / iC {Llgne in AbbUi’S f ) (JJft l .

Total area of Mean epipelic Meail epipelic algal srdimcnt algal stancline stariding crop ( mg

( m2) in each crop, C’phyll a, chlorophy!l u per Water depth depth mg/rn. in total area in each

calegor? category each categon- depth category

- ~~

0-1 in (Stn. 2) 926 11.7 10.800

2 . 5 4 111 (Stns. 5 , 0) 12113 4 .o 4,880 1-2.5 m (Stns. 7, 8) 1573 7.3 1 I .500

I otal foi wliole pond: 2T.180

‘The situation of station 3 w;ts similar to t h a t of station 2, rather better correspondence between cell counts froin tube and area-based sarnples being obtained. A relatively smaller crop a t station 3 during winter 1966/67 was attributed to differential erosive water movements.

The seasonal distribution of epipelic popiilations on sediments under water deeper tlian t h a t at stations 2 and 3 (Fig. 7) was markedly different from that at these marginal stations. UiaLoms were abundant at 1-2.5 in. During May/June 1966 a flagellate, T?-uchelonzo7zns voiuorina Ehr., was frequent arid A?-thi~ospii-u Jenne?-i Stiz. and Nitzschia f lexti Schum. formed large interwoven populatioris on seclirrieiils uritler 2-3 m of water. The biology of A r t h ~ o . c p i ~ u Je717ici.i and associated species on these sedinierits iri relation to chemical and physical factors which ap- peared to control this community is discussed elsewhere (22).

It is difficult to corrclate chlorophyll a levels with cell counts at stations 5 antl 6 and 7 and 8 duririg Milay/June 1966 because of tlic errors iritrodiicecl in counting cyanopliytan filaments of variable length. Crops cliiririg tlie rest of the year were sniall antl almost entirely diatomaceous. The Mar./April 1967 growth at 1-2 111 was entirely of diatoms and corre- lated well with observecl c1~lorop11yll cl levels. In calculating mean standing crops per sqiiare meter irrider various water depths in the pond during May 1966-ApriI 1967, station 2 has been taken as being representative of the sediment from the water’s edge to that under 1 ni of water. Table 3 gives relevant data for the calculation of mean total standing crop of epipelic algal communities in the pond, wliicli was 0.27 x 105 nig clilorophyll (1 or 7.3 ing chloropliyll n per inz. C r o p a t station 3 were larger than those at station 2 (mean for I965/66, 13.3 mg/m2; 1966/67, ‘3.0 ing/in2) despite erosion at this station. However, greater availability of light at station 3, owing to lack of overhanging trees, may- have been responsible for the larger crop.

From M a y 1965-April 1966, sampling was carried out only a t stations 2 arid 5. However, assuming that the mean standing crop a t station 2 Iwre a siiiiilar relationship to tlic iiiean total epipelic ;tlgal cliloropliyll N content of tlie pond iri both 196.5166 ant1 1966/67, ;in estimate of tlie iiiean total crop in

i 15

t

7 I3 0 1

1

j ?,,

r hl 4 c 1 J I A S 0 N D Y F M A M J J A ? C U 0 J

I965 1966

F i c . . 8. Seasonal distribution of ccll numlxm ot cliloroph~ll n conlcnl in arca-bascd s;implcs o f epipelic algal populations at Station 2 in Prickly pool.

l963/66 can be inatle. The estimate, 0.28 X ing chlorophyll n pond, or 7.6 mg/m’, is close to tlie value obtairiecl directly in 1’366/67.

Pr iddy pool cpipclir cllgne. Good correspondence bctweeri the coiirits for tube arid area-based samples was obtained, a n d results from tlie latter only are illustrated (Fig. 8).

At station 2 (Fig. 8) diatom poopulation changes correspoiidcd closcly wi tli tliose of cliloropliyll a levels, to which little was contributed by numerically large populations of fine (1-2 p wide) blue-green alga1 filaments in spring antl simmer 1965. Flagel- hles occasiorially forrried moderately large populations at the same tinie as the diatoms. Green algae (mainly Scenedcsniir., and desniids) were not ntmieri- cally prorriiricrit althougli the variety of species was large.

Station 3 was saniplecl over a shorter period than stalion 2, frorri wliicli it differed in the presence of almntlant poplarions of Anabaenu spp. during Sept.-l\’ov. Fluctuations in chlorophyll a content were paralleled b y changes in the diatom poptilations froin J ; i i i . to Aiig., but the bulk of the annual crop was niade iip of Anabclrnu spp. in tlie aiitiiinn.

For ISG Clan.-I)ec.), a iiiean crop (based on data for station 2) of 2210 iiig cliloropliyll N per total sediment in the open water (2.1 nig/ni2) was found. The crop w; is slightly larger in 1966 being 4080 nig (3.8 irigjin2). T h e n i c m crop at station 3 during 1966 was 2.2 mg/m2.

BRIAN MOSS 164

TABLE 4. Mean standing crops as clilorophyll a, of phytoplankton and epipelic algae in Abbot’s pond and Priddy pool.

Abbot’s pond Priddy pool

1965/66 1966/67 1965 1966

Phytoplankton mg/m2 35.8 108.1 3.7 2.5 Epipelic algae mg/m2 7.6 7.3 2.1 3.8

Total mg/mz 43.4 115.4 5.8 6.3 Epipelic algae as % of total 17.7 6.4 36.7 60.7 Concentration of phyto-

plankton, n1g/m3 18.0 53.0 5.8 3.9

_ _ _ _ - _ _

TABLE 5. T h e depths (mi at which surface intensity of red light is reduced to 1, 5, 10, and 50% by increasing levels of phytoplankton standing crop (measured as mean tng chloro-

phyll alms in the upper 2 m) in Abbot’s pond.

% of surface light mg chlorophyll a/m3

0 40 80 120 160 200

50 0.6 0.5 0.45 0.4 0.35 0.3 10 2.1 1.7 1.5 1.3 1.2 1.1 5 2.7 2.3 2.0 1.75 1.6 1.4 1 4.1 3.5 3.0 2.7 2.4 2.2

DISCUSSION

The mean standing crops of epipelic and plank- tonic algal populations of both Abbot’s pond and Priddy pool are summarized in Table 4. Mean total (phytoplankton + epipelic algae) crops in Abbot’s pond were 1 order of magnitude greater than those in Priddy pool. This may reflect the richer nutrient status of Abbot’s pond, but the estimate for Priddy pool is for the open water area only, and competition for nutrients with the extensive bed of Equisetum and its associated algal epiphytes may explain the low crops found. The epiphytic algae on the Equisetum alone had standing crops of 100-300 mg/m2 of habitat in early 1967 (J. G. Brown, personal communication), but it is not yet known how representative these figures might be of the yearly mean. Standing crops of attached algae are not directly comparable with these of free-living algae, since the former are able to resist removal from the habitat by water movements, sinking etc., and can accumulate material over a long period, even though the rate of accumulation (productivity) may be low.

Priddy pool had phytoplankton chlorophyll a levels typical of nutrient-poor lakes, and Abbot’s pond had levels associated with nutrient-rich waters (12,42). Species characteristic of oligotrophy and eutrophy, respectively, were found in the pools (5,13,

Crops in Abbot’s pond (Fig. 3) in 1966/67 sometimes approached the area-based estimate of 300 mg chlorophyll a/m2, suggested (37) as the maximum crop density capable of appreciable photosynthesis throughout the whole crop.

Estimates of area-based standing crops of epipelic algae are rare and insufficient data are yet available to justify generalizations. Odum et al. (27) gave measurements of the chlorophyll a content of algal oozes, which may have been epipelic, but their estimates were made after extraction of surface mud, which almost certainly contained chlorophyll degradation products, for which no correction was made. Epipelic algae appear to be important in the production ecology of intertidal mud flats (29) but information on their role in other ecosystems is very meager. Epipelic algae studied here did not have mean crops

23).

approaching in size those of the phytoplankton in Abbot’s pond, but had crops of similar or greater magnitude in Priddy pool (Table 4). The contrasting light regimes at the sediment surface, occasioned by different phytoplankton crop sizes in the 2 pools, are almost certainly important in determining these crop budgets.

Round (31) attributed the limitation of epipelic algal populations on sediments in the English Lake District to lack of sufficient light. Measurements of light penetration were routinely made in the water column of Abbot’s pond using an underwater pho- tometer and colored filters. From these data (18) the depth at which light was reduced to the approximate compensation point (1% of its surface intensity) was calculated. From an analysis of the relation between chlorophyll a content of the phytoplankton and light penetration on a given occasion, penetration of the least attenuated waveband (red) in the hypothetical absence of phytoplankton was calculated. For red light the 1% (compensation) level lay at a maximum depth of 4.1 m, i.e., at about the depth of the deepest sediments. Any increase in the extinction of red light owing to phytoplankton development must therefore be expected to have reduced the light intensity to below the compensation point in appropriate parts of the pond, with consequent light limitation of epipelic algae. Table 5 shows the depths at which red light was reduced to 1, 5, 10, and 50% of its surface intensity in the presence of different phytoplankton standing crops. Autotrophic epipelic algae on sediment under 2 m of water or more would have been incapable of much net growth during most of the summer, autumn, and winter 1966, when phytoplankton standing crops were high. Fig. 7 shows that very small populations of epipelic algae were recorded during this period at stations in water deeper than 1 m. Populations at these stations were relatively large from May 1966 until mid-July 1966, during which time phytoplankton standing crops were generally below 40 mg chlorophyll aim3, and epipelic algal populations markedly decreased immediately after the time of great phytoplankton growth in mid-July. Similarly, epipelic algal crops at stations in 1-2.5 m of water increased in Mar. 1967, after large winter crops of phytoplankton had disappeared. At this

ALGAL STANDING CROPS 165

time, epipelic algal populations on sediment under 2.5 + in of water did not increase. This may have reflected the smaller inocula available for renewed population growth, as a result of the effects of previous tleoxygenation (21) and of severer light limitation at 2.5+ m stations than at those in 1-2.5 m of water.

The mean epipelic algal standing crops in Abbot's pond decreased with overlying water depth (Table 3) and on most sampling dates regular decrease of standing crops with depth was found. This was owing neither to erosion effects (31), because wave distur- bance was effective only at the extreme margins, nor to chemical differences, at least between the margins antl 2 ni (21). Deoxygenation and release of toxic ions from the sediment during summer 1966 may have partly limited epipelic algal crops at 2.5 in (21).

During June/July 1966 epipelic mat-forming algal crops of A1-throsfiil-a Jenne?.i on sediment under 2.5 + m of water were greater than those at the margins despite the much lower light intensities at 2.5 + in. The high crops at 2.5 + m appear to contradict the hypothesis of light limitation of epipelic algal growth in Abbot's pond. Mat formation, however, is advan- tageous in the maintenance of large standing crops (20), and the existence of large crops of mat-forming algae does not imply greater productivity of these mats relative to that of unicellular epipelic algal populations subject to greater rates of loss of cells. The mean crop sizes at different depths suggest that productivity was greater in shallow water. Only when crops of algae having similar growth forms are being compared can standing crops be directly related to productivity, and mat-forming algae differ sufficiently from populations of unicells for this relationship not to hold.

Since both the phytoplankton and epipelon largely consisted of free-living cells or colonies, they may be expected to have had broadly similar spectra of growth and turnover rates. There are no comparative data for this proposition, but there is no intrinsic reason to expect otherwise. They are also both poten- tially exposed to comparable risks of mechanical loss from the habitat by sinking into the mud and of loss by grazing. Grazing risks may vary greatly between individual species, since they depend on size and chemical palatability. However similar spectra might again be expected within each of the 2 communities. Their mean standing crops might then be expected to approximately parallel their respective rates of production (productivities), according to tlie relation:

Production = R/Iean standing crop x Turnover rate.

(mass per (during unit time, (per unit time, unit time, M ) T-I) '\I r-1)

Productivity of epipelic algae was probably then

TABLE G . Xelationslrip of epipelic algal crops to dissolrled ioriir leur/ .~ in Inkes. Al l data are hosed on rstii?iolions o w i . lirriorls

of .several montlts, miizg siitzilnr l e r l i n iq i i es (6,17).

Concentration Epipelic a l ~ a l standing crop mg C'phyll a / m > crf weak

acid salts, ineq/liter niean niax Location Reference

Mlungusi dam Moss, iii prep'ti. 0.09-0.1 1 2.2 10.2 (Malawi)

Pritldy pool Prcsent tlata 0.05-0.4 2.1 (1965) 15'' 3.8 (1966)

Shear I\'ater (21) 1.6 7 . 3 5 0 ( U . K . )

Abbot's pond PI-cscnt data 2.5-3.5 7.6 (19F5/66) 47.2,' i . 3 (1966/67)

Lakc Cliilwa (24) X- 15-200 Mi 1 229 (Malawi) M o s s , unpLtb'd.

- (' Station 3 data.

comparable to that of the phytoplankton in the opcn water of Pridtly pool, but of somewhat lesser magnitude in Abbot's pond (Table 4). T h e productivities of the 2 communities are at present being measured by oxygen evolution and 14C uptake estimates (M. Hickinan, personal communication) as a n extension of the present work.

Because phytoplankters irom different lakes have growth rates of about the same order of magnitude, arid are subject to the same factors responsible for removing cells froin the habitat, a general direct correspondence between nutrient-status of the water, standing crop (15) , and productivity (42) has been shown. Available data for epipelic algal communities have been compiled (Table 6). T h e concentration of weak acid salts (mainly bicarbonate deter- mined by titration of the water to p H 4.5 with standard acid) lias been taken as an index of nutrient status (15). The levels of important minor ions, such as nitrate, phosphate, and silicate, although showing greater fluctuations than those of weak acid salts, showed a similar trend.

Table 6 shows a general increase in epipelic algal crops with increasing nutrient levels, similar to that found for the phytoplankton. Productivity data are not available but i t is reasonable to infer that the same trend would be followed.

This investigation has given information on the relative importance of different sized cells in the composition of standing crops. Lurid (16) studied tlie periodicity of algae whose cells were 15 or less along every diameter in tlie English Lake District. Cells in this category, e.g., C1z~ysocorrzc.s dinplirinirs, were important contributors to the crops in Abbot's pond, but considering the period as a whole, and previous data (13), larger algae accounted for the bulk of tlie crops. Lurid (16) found dry weights in the range 3-20 pg/106 cells for species 15 p or less, antl 30-5000 g/ 10" cells for species larger than this. Overall, the crops of larger algae in the English Lake

166 BRIAN MOSS

District tended to be about 10 times the weight of those of the smaller algae. I n the published discussion following ref. 16 the turnover rates of small cells were believed not to be significantly greater than those of large cells. Comparison of growth rates for Chrysococcus diaphanus arid for the larger colonial Pandorina morurn in Abbot's pond under conditions when animal grazing of both species was small or absent gives somewhat higher values for the latter species, but this may reflect environmental conditions rather than intrinsic properties of the species. The < 5 p forms tended to reach numerically very large populations very quickly. Their rate of synthesis of cell material, however, which is the more important measure for production ecology, would be consid- erably smaller.

The < 5 p category of phytoplankters used here corresponds roughly with the ultraplankton (11) although the latter places the upper size limit somewhat lower. Subsequent work on physiology of different sized organisms must determine if there is any value in retaining size categories for phytoplankton. The distinction between nanoplankton and micro- plankton is certainly very artificial. Thomasson (38) has noted the tendency for these categories to be demarcated at a decreasing cell size as plankton nets of finer mesh become available, and both lie and Bourrelly (1) now propose 10 p as the upper size limit for nanoplankton. However, since the concept of micro- and nanoplankton seems to be largely an artefact of net manufacture, there seems little point in retaining the terms, particularly if mean cell volumes of individual species are presented in sets of seasonal data.

The limitations imposed by considering only cell counts in ecological investigations can be readily shown from data obtained on favorable occasions. For example, in early April 1967, cell counts and chlorophyll a estimates from samples at various depths in the water column were compared. There were 3 predominant species, Asterionella formosa, evenly distributed down the column, Chrysococcus diaphanus, which reached a numerical peak at 2-2.5 m, and a small chlamydomonad-type species, 2.4 p in diameter, hereafter referred to as microflagellate present mainly in the upper 2 m. Occasional cells of other diatoms and Chlorococcales have been treated together as "other species." Stratification of different species in the water column occurred at other times in Abbot's pond and is considered elsewhere (18,22).

Assuming that the chlorophyll a content per cell of a given species is constant with depth, such that each cell of Asterionella had w mg chlorophyll a, of Chrysococcus x mg, of the microflagellate y mg, and of other species, z mg, it is possible to set up simul- taneous equations, one for each depth sampled. If A is the total amount of chlorophyll a in mg/m3 at that depth:

CHLOROPHYLL a No o t CELLS

E

f a a, n

percentage representation

FIG. 9. The percentage composition by cell numbers and by chlorophyll a content of species of phytoplankton at 8 depths in the water column of Abbot's pond on April 17, 1967.

A = U * W +box +cay + d * z

where a, b, c, and d are the numbers per cubic meter at that depth of Asterionella, Chrysococcus, microflagellate, and other species, respectively.

Since there are only 4 unknowns, w, x, y, and z, only 4 equations are required for complete solution. In practice, since data from 8 depths were obtained, groups of results from 2 or 3 depths were added together to minimize the effects of errors in estimation. This approach to the calculation of chlorophyll contents of given species is practicable only when a few predominant species are present, and when stratification of cells occurs, resulting in suitably different equations for different groups of depths. For the data in question, the counts have been expressed as percentages of the total count at each depth (Fig. 9). The chlorophyll a contents per cell for each species were calculated by solution of suitable equations, and the % contribution of each species to the total chlorophyll a content at each depth was found, and also plotted in Fig. 9, which shows the very different pic- tures obtained by considering chlorophyll a content or cell numbers as indices of the standing crop of mixed populations. The importance of microflagel- lates is greatly overemphasized numerically, and that of Chrysococcus is underemphasized. Asterionella, although homogeneously distributed down the profile, is proportionately much less frequent where the microflagellate is numerous and vice versa. Plotting Fig. 9 in terms of cell volumes rather than chlorophyll a content would have resulted in similar con- clusions, since the trend in cell volumes of the major species concerned is generally related to that of their cellular chlorophyll a contents (Table 7).

Rawson (30) gave a similar example for Great Slave Lake netplankton, where Asterionella constituted 60% of the numerical total, but only 18% by volume;

AILAL 5TANI)ING CROPS I ti7

T A I ~ . E 7. Cliloropliyll a contents and itrean cell 7~ol1i~nc.s of Asterionella formosa, Chrysococcus diaphanus, and a species of

in icrof loge 1 In t P .

Microflagellate Asterionella Chrysococcos

C:hlorophyll a, mg/ccll 1.9 x IO-’” 3 X 10-o 1.75 X lo-’ Cell volume. B’ 7.23 723 I770

and Tnbcllcii-in, although only 8% by numbers, constituted 497; by volume. Cell volumes were not row tiiiely measured for all species in this investigation. It is expected tliat total cell volume would have shown a closer relationship with chlorophyll n content than was obtained with cell numbers.

ACKNO1VI.F.DGMENT

I wish to acknowledge the cncouragemcnt gi\cn to me at all limes by- Dr. F. E. Round during this work, which forms part of the 1 1 . K . contrihution to the Productivity of Freshwaters section of the International Biological Programme. Maintenance was provided h v an award from the National Euvironmcntal Re- search Couiicil ( U . K . ) .

I .

2.

3.

,4

7

6

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8 .

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10.

1 1 .

12.

13.

14.

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Iii t ? o i l rtrl io ir to L r i k c N io logy (in d 111 e Id i i i i ri oplnn k toil , Wile!, S . Y . Ickiizrrn.t, S. 1956. On the standing crop and productix c sti-ucture of phytoplankton community in some lakes o r central Japan. not. Mag . Tokyo 69:7-16. KARIM, .I. C;. A. 1965. Ecological and electron micro- scopical studies on the phytoplankton of two pools, Pl1.D. thesis. Kiii\. Rristol. LUNI), J. TV. G. 1942. Tlie marginal algae of certain

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15. __ 1957. Cliemical analysis in ecology illastratctl from Lake District tarns and lakcs. 2. Algal tliffcrences. Proc. Iinn. Soc. L o i i r l . 167:165-7l.

16. __- 1961. The periodicity of p-algae in tlri-cc Euglisli lakcs. T’PTII. Z ? r I r i n . Yewin Tllrol-. Angew I , i ? l z ? 7 0 / . 14:147- 51.

17. M o s s , 1%. 1967. .-\ note on the estimation of cliIoroph\ll n in fresliwatcr alg;il communitirs. Liiirnol. Ocr~nfrogi - . 12: 340-2,

18. -~ 1967. 1.imnological and ;ilgologic;il inYestigatiolls on two Somersetshirc pools, 1’h.T). thesis, 1Tniv. Bristol.

19. __ 1968. Studies on t l i c : degradation of chloroph\ll n and carotenoitls in freshwaters.

20. -~ 1968. T h e clilol-oph>ll n content of S C J ~ I C I)cntliic. algal communities. Arcli. Hytli-ohiol. 655-62 .

21. __ 1969. Vertical heterogeneity in thc watcI columi i of .\bl)ot’s pond. 1. Tlie distribution of tcmpci’atui-r : ~ r l t l

tlissolretl oxygen. I . Erol. (in press). 22. 1969. Vertical heterogeneity in the wiitcr colrinln

i n Ihlmt’s pond. TI. Ttic influence of ph3sicnl and chcmi- cal conditions on the spatial and temporal tlistl-il,ution of tlie pli) toplankton and of a communitr of epipelic algae. /. Erol. (in press).

23. __- K- K A R I M , .\. (;. .\, 1969. Ph!toplankton associa- lions in twn pools. and theit- I-elationships w i t h associatc4 IJcntliic flora. Hyrlrobiologio (in pi-ess).

24. Mosc. B. Ez Moss, J. 1969. Aspects of the lirnliologv of a n cntlni-lieic tropical lake ( I . . Chilwa. Malawi). Ecology (in press).

25. h l o s . 1%. Ez Roum. F. E. 1967. Ohservatiotls on st;i!i(liiig crops ti: epipclic and cpipsammic algal popnliitions i l l

Shear IVater, T\’ilts. 26. mot,^.. E. T. Ez Mzsoy, D. 1957. S t u d y of di:~tom pop11-

latioris on sand and mud tlats in tlir T\’ootls Holc :iIea. f l i o i . Bitll. 113:331.

27. OD[ > I , H. T., McCo\stt.t., W., Ez .41moIr, \V. I!).%. Tlre cliloropli>ll ‘,\’ of communities. Pi ib . f m f . A f m . Sr i . ‘f‘(,vrl\ 5:6.5-97.

28. P.\RSOXS, T. R. Rc S IHICKI. \ \ I ) , J . 1). H. 1963. 1)isriissio:l of the spectrophotometric determination o f marine plant pigments with revised equations for ascertaining chltrloph~lls and carotcnoids. 1. Afar . Res. 21:155-63.

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of Ckorgia. Tiitinol. Oceairop. 43386-98. 30. R A ~ W S , 1). S . 1956. Algal inclicatoi-s nf tropliic lake

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some lakes of tlie Knglish Lakc District. V I . ‘l’hc~ effect of deptli on the cpipelic algal communit!. 1. Etol . 49245-34.

.1L. 1964. The ecology of benthic iilgac. 117 Jarksoii, I). [Ed./ , llgric (ti1rl .Ifon., Plenuiii Press, N.Y. , 138-84.

3 3 . -~ 1965. Tlre Il iology of f l te d/gor<. Etlwa~-tl :\rnolcl. Imidon.

34. ~ 1 965, T h e cpips;imnion ; a re la t i c el y 11 t lkll owl freslirvatcr algal association.

.%5. Siii \ i .4t)t , H. M. 1958. 1)iirrnal tluctu;ition in pliotos\n- thetic rate a r i d chlorophyll n content of phytoplanktorl from Eastern Pacific waters.

36. SPI X C E R , C . P. 1964. Tlie cstimation o f ~ ~ l i ~ t o p l a i i k t ~ r n pigments.

E. 1955. Tlie chloroph~ll crnitent and light utilisation in commuuitics o f pla~ikton algae and tci-rcsti-ial higher plants. Pliy~iol . PkrJit. 10:1009-3-21.

58. TErozricsox, I<. 196i. Pliytoplanktoii from some lahcs on Mt. TVillielm, East S c ~ v (;uine;i.

39. ~ V I X ~ E L , R. C;. 11964. A comparati\e s tud ) of 11i(. prim;ny producti\it) o f Iiigher aquatic plants, pcriphytoll a n d pli)-

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168 ALBERT COLLIER, W. B. WILSON, AND MARILYNN BORKOWSKI

toplankton in a large shallow lake. Int . Rev. Ges Hydro- biol. Hydrogr. 49:141.

40. ~ 1965. Techniques and problems of primary productivity measurements in higher aquatic plants and peri- phyton.

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J. Phycol. 5, 168-172 (1969)

RESPONSES OF GYMNODZNIUM BREVE DAVIS T O NATURAL WATERS OF DIVERSE ORIGIN1~2

A l b e r t Col l ier , W. B. Wilson,” a n d M a r i l y n n Borkowsk i4 Department of Biological Science, Florida State University, Tallahassee, Florida 32306

SUMMARY

T h e effect o n the growth of Gymnodinium breve of river waters and seawater .samples collected in different seasons and locations was investigated wi th and without differential enrichment.^

Growth i n natural seawater was most clearly en- hanced by addi t ion of EDTA-Fe, sulfide, AT and P.

T h e seasonal variation i n growth-promoting properties of seawater and river watel- dominates val-ia- tions due to differences i n location.

Gymnod in ium breve Davis, the causative organism of infrequent but catastrophic fish kills along the west coast of the Florida peninsula, is an extremely fastidious organism. I t is more difficult to maintain in the laboratory than other dinoflagellates, with which we have had experience. All attempts to estab- lish it in isolated cultures were unsuccessful until Wilson & Collier (7) applied Collier’s earlier findings (unpublished) that the addition of sulfide to the medium would stimulate growth.

I t is one thing to grow such an organisin under controlled conditions and quite another to extend the findings to field conditions. In this case it is particularly hazardous to do so because of the extreme sensitivity of the organism to unfavorable conditions (4). This sensitivity may be operative under natural conditions, thus contributing to the sporadic nature of the fish kills due to this organism.

In 1956 two of us began a series of studies designed to assay natural waters for their compatibility with the growth of G. breue. These were continued by

We gratefully acknowledge partial financial support by the Florida Board of Conservation.

2 Received September 9,1968; revised March 15,1969. 3 Present address: Marine Laboratory, Texas A & M Univer-

Present address: Bureau of Commercial Fisheries, Miami, sity, Galveston, Texas 77550.

Florida 33100.

Wilson in 1964 and the 2 series were reported by Wilson (6). This work is an extension of that report. This type of study is being continued by Collier in multifactorial designs with the goal of making laboratory studies more useful in understanding the ecology of plankton blooms in general, and of G. breve spe- cifically.

Much has been written on the role of phosphorus in “red tide” outbreaks; Wilson (6) considered the phosphorus question in great detail and concluded: “some factor or combination of factors other than phosphorus or phosphorus plus salinity probably limits G. breve populations along the west coast of South Florida most of the time.”

With respect to inorganic and total PO,-P, nitrate and nitrite N, copper, alkalinity, silicon, calcium, total organic N, and NH,-N, Rounsefell & Drago- vitch (5) found no significant correlations with the growth of G. breve, although “a multiple curvilinear correlation showed that 61% of the monthly variabil- ity in abundance of G. breve was associated with variations in salinity, temperature, onshore winds of over 7 kts. and abundance the previous month.” (quoted from abstract).

The optimum salinity range for G. breve blooms lies between 270/,0 and 67%, ( I ) .

The possibility that river discharge may be a factor in the generation of large blooms was postulated by Collier (3). This possibility is still considered as significant and for this reason river water samples were included in the bioassay studies.

MATERIALS AND METHODS

Experimental design. As mentioned above the sensitivity of this organism makes it very difficult to handle; all technical procedures have been influenced by this and many desirable refinements simply cannot be applied. These experimental difficulties will be mentioned in the appropriate places.

algae of two somersetshire pools standing crops of phytoplankton and epipelic algae as measured by...

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