SEASONAL SUCCESSION OF ALGAE IN A EUTROPHIC STREAM IN SOUTHERN ENGLAND

J. W. MOORE'

School of Biological Sciences, University of Bath, Bath, BA2 7AY .

' Present Address : Environmental Protection Service, Yellowknife, Northwest Territories, Canada . XOE 1 HO .Received February 4, 1976

Keywords : Epipelic, epilithic, episammic, epiphytic, planktonic, light, nutrients, temperature, grazing, flooding, attachment quality .

Abstract

The epipelic and epilithic algal communities in a small eutrophicstream situated in southern England expanded rapidly duringMarch of both 1973 and 1974 primarily in response to changinglight conditions. Although numbers varied greatly during thesummer, these fluctuations were probably not due to nutrient,temperature or light conditions . High rates of disappearance ofalgae from the substrate were correlated with flooding, a deterio-ration of attachment characteristics and high metabolic rates .The episammic algal community consisted of only a few species,all of which showed maximum development during the sum-mer. Although the well developed attachment mechanism andsmall size of the species undoubtedly aid in their ability to colo-nize sand grains, each species must be able to withstand frequentburial in the bottom deposits . Although temperature was proba-bly an important factor controlling the number of epiphytesassociated with Cladophora glomerata, light seemed to be ofrelatively less importance . Large numbers of isopods, amphi-pods and copepods occurred in the stream but their grazingseemed to have had little effect of the standing crop of the algae .

Introduction

Although algae frequently play an important role in theenergy cycle of flowing waters (Hynes, 1970), scarcely anydata are available concerning the factors which controlthe seasonal abundance of the different species . Thepurpose of this study was therefore to describe changes inthe composition of the algal assemblages occurring in theplankton and in those associated with the sediments (epi-pelic), sand grains (episammic), rocks (epilithic) andhigher plants (epiphytic) in a small eutrophic stream . Anattempt was then made to correlate these findings withchanges in environmental parameters .

Dr. W. Junk h . A, . Publishers - The Hague, The Netherlands

Materials and methods

This investigation was conducted between March 1973and March 1975 on a tributary stream of the River Wylye(Lat. 5i°o9; Long 2°11) in the vicinity of LongbridgeDeverill in southern England. The stream, measuringabout 1 km in length and 3 m in width, supports a largecommercial growth of water cress (Nasturtium officinaleR. Br.) near its source . Immediately downstream fromthis area, the water passes through a short concrete sluice-way into a stream about 120 m in length which dischargesinto the Wylye .

Each algal assemblage was sampled at least every twoweeks near midday . The epipelic flora was collected fromthe uppermost 2 cm of sediment using an aspirator fromone site . This area, measuring 30 cm', was situated nearthe bank immediately downstream from the sluice-way .Sampling of episammic algae also took place in the lowerpart of the tributary but towards mid-stream where theorganic content of the substrate remained comparativelylow (see below) . The method of collection of this assem-blage was identical to that outlined for the epipelic flora .Five stones measuring 5-15 cm in diameter were pickedfrom each of four sites in the sluice-way . One site wassituated immediately adjacent to the water cress bedswhile the others were located at to m intervals below thisarea.The stones were returned immediately to the labo-ratory where the attached epilithic algae were carefullyscraped onto a microscope slide with a scalpel blade(Moore, 1975) . Collections of Cladophora glomerata (L.)Kz. were also taken from these rocks for analysis of theassociated epiphytic flora . Duplicate samples of potamo-plankton/ plankton were collected in 4 1 plastic containersjust before the point where the tributary entered theWylye. Each of the above collections was made in areas

Hydrobiologia vol . 53, 2, pag. 181-192, 1977

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where there was no shading by terrestrial plants .The relative abundance of the species in each sample

was estimated from a count of 300 plants . Determina-tions of the total number of cells in the epipelic and epi-sammic samples were made by diluting the material indistilled water to a volume of 100 ml and shaking vigor-ously for 30 sec. Three 0.01 ml aliquots were taken and allthe cells in each aliquot enumerated, this then enabling anestimate of densities on the substrate to be made . Pre-cisely the same method was employed using the scrapingsof epilithic algae . In the case of the potamoplankton/plankton, however, 0.5-1 .0 1 of river water were filteredthrough a Sartorius membrane (pore size 1 .2 µm). Thematerial was washed into a graduated cylinder and thevolume made up to 1o ml. Three 0.01 ml aliquots weretaken which enabled an estimate of standing crop to bemade. The sample of Cladophora glomerata was driedby sublimation to constant weight and oxidized in aknown volume of nitric acid . The number of cells in three0.01 ml aliquots was estimated, thus permitting a deter-mination of density in terms of the dry weight of Cla-

dophora to be made.The cell volume of predominant diatoms was deter-

mined by obtaining camera lucida drawings of about 20specimens of each species and estimating the area using aHayashi Denko automatic area meter (Type AAM-5) .The average depth of the actual cells was then determinedat a magnification of iooo x, thus enabling an estimate tobe made of volume. In the case of algae other than dia-toms, the average cell volume was estimated from directmeasurements of length, breadth and depth .

Temperatures were collected at midday and currentvelocity over the collection sites was estimated about 5cm off bottom using an Ott meter. Discharge values weredetermined in the sluice, which possessed a more or lessregular rectangular shape . The silica, orthophosphate,nitrate and total alkalinity content of the water in theplankton samples was determined monthly using hetero-poly blue (APHA, 1971), molybdenum blue (Vogel,1964), phenoldsulfonic acid (Welcher, 1963) and titration(APHA, 1971) methods respectively . Estimates of theorganic content of the upper 2 cm of sediment were madeat monthly intervals by ashing at 550°C for 24 h . Dailyrates of growth and disappearance of algae were deter-mined from changes in standing crop values between twoconsecutive sampling periods .

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Results

Physico-chemical AnalysesWaters temperatures increased from 3-4°C in March to17.5°C in June but then fell inconsistently to 3°C by No-vember (Fig . 1) . Thereafter a gradual increase occurreduntil a value of 17'C was attained near the end of August .During the following winter temperatures remained rela-tively high, i.e. 5-7°C. Discharge stayed comparativelystable throughout the study averaging 0 .044 m'/ sec withmaximum and minimum values of 0 .06 and 0.025 m 3 /sec respectively (Fig . 1) . Little variation was exhibited inthe water velocity over the epipelic, epilithic and epi-sammic collection sites with respective average values of8, 35 and 25 cm/sec. Water depth at the three sites alsofailed to show large variations, the respective mean valuesbeing 5, 11 and 20 cm. The pH and nutrient content of thestream water remained high (Fig . 1) and the organic con-tent of the sediments used for the collection of epipelicalgae averaged 20% with extreme values of 16 and 28% . Inthe case of the episammic collection site, however, theaverage value for the organic content of the substrate was6.3% (range 3 .5-9 .8%) .

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Fig . i . Seasonal changes in some physico-chemical parametersof the study stream .

Epipelic CommunitiesThe epipelic community was diverse, comprising a totalof 176 different species, varieties and forms of algae . TheBacillariophyta predominated with 148 taxa, followed bythe Chlorophyta with 13, the Cyanophyta with 12 and theEuglenophyta with 3 . The most common members ofthe first group of algae were Achnanthes lanceolata(Breb) Grun ., A. minutissima v . cryptocephala Grun .,Melosira varians Ag ., Nitzschia linearis W . Sm. and N.palea (Kz .) W. Sm. and varieties (Fig . 2) while the Chloro-phyta were represented mainly by Chamydomonasmonadina Stein, Closterium acerosum (Schr.) Ehr . andC. lunula (O.F . Mull .) Nitz . Oscillatoria limosa (Roth)Ag. and O. brevis Kz . were the most abundant of theCyanophyta while the Euglenophyta were represented bythe occasional specimen of Euglena spp .

Most of the algal species in the epipelic assemblageexhibited well defined and similarly recurring patterns ofsuccession (Fig . 2) . Nitzschia linearis, for example,

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Fragilaria capucina

Achnanthes minutissima

Achnanthes lanceolata

reached its greatest abundance towards the middle ofMay in both years, with the standing crop of the species atthis time being 2 .1 x 10 6 cells/cm 2 in 1973 and o.9 x 106cells/cm`' in 1974 . Precisely the same sort of pattern wasexhibited by Achnanthes lanceolata, A . minutissima v .

cryptocephala and Nitzschia palea. On the other hand,Synedra ulna (Nitz .) Ehr . and varieties occurred muchmore abundantly in 1973 than in 1974 and considerablevariation existed in the periods of greatest density. Thedensity of the entire community increased sharply duringApril 1973 from 1 .8 x 10 6 to 8 .1 x lo b cells/cm 2 (Fig . 3),the numbers subsequently fluctuating a great deal . Aconsiderable fall in density occurred during Septemberand it was not until the beginning of April 1974 that asignificant recovery was apparent, at which time the den-sity reached 2 .9 x ,o 6 cells/cm'. The pattern of changeduring the remainder of 1974 was characterized by a grad-ual fall in numbers . Changes in the standing crop in termsof cell volume during both 1973 and 1974 roughly paral-

Fig. 2 . Seasonal changes in the standing crop of common epipelic algae .

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Fig . 8 . (continued)

Opephora martyi

Fragilaria intermedia

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leled those outlined for numbers with a maximum den-sity of 14 .1 x 109µm3/cm 2 in the former year and 5 .0 x109 µm'/cm' in 1974 . The fastest daily rate of expansionof the epipelic community during 1973, 3 .1 x 1o 5 cells/cm 2 , occurred in April while by volume the greatest value,6.6 x to 8µm 3 /cm2 , was observed in August (Fig . 3) . Dur-ing 1974 maximum daily rates of 1 .4 x 10 5 cells/cm 2(April) and 1 .5 x 10 8 µm3/cm 2 (March) were recorded .Algae disappeared most rapidly from the sediments inthe spring and early summer of 1973 when a value of 3 .0 x105 cells/ cm 2 was recorded, while in 1974 the fastest dailyrate was 0 .4 x 10 5 cells/cm 2 .

Episammic CommunitiesThe episammic community in the tributary was highlyrestricted with only 13 taxa being found . Diatoms ac-counted for 12 species, the remaining taxon belongingto the Cyanophyta . The most common species, Ampho-ra ovalis v . pediculus Kz ., occurred abundantly, i .e . up to

4.7 x 10 5 cells/cm 2 , between June and September 1973but during the summer of 1974 was comparatively res-tricted, a pattern virtually opposite to that exhibited byOpephora martyi Herib . (Fig. 4) . Although a difference of1 .5 months existed in the phasing of the seasonal peaks,the abundance of Fragilaria construens (Ehr.) Grun . wassimilar in both years .The total standing crop of the episammic community

failed to exhibit a spring bloom in 1973 but did increaselater in the summer (Fig. 5) only to fall to low overwin-tering levels . A well defined spring bloom did occur inMarch of 1974 with values subsequently rising in May butthen falling gradually again until the winter . As withnumbers the maximum standing crop in terms of vol-ume was also observed in September 1973 and May 1974(Fig . 5) . The fastest daily growth rate recorded during thestudy, 1 .7 x 104 cells/cm 2 , took place during September1973 while in 1974 the most rapid rate, 0 .7 x 10 4 cells/ cm 2 ,was observed during the spring bloom . In terms of cell

volume, the highest daily rate, 7 .0 x 10 6 µm 3/cmz , wasrecorded in July 1973 with the second fastest rate, 4 .5 x106 µm3/cmz, occurring during both September and Mayof 1973 and 1974 . The episammic algae disappeared from

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Fig . 3 . Seasonal changes in the standing crop of the epipelic assemblage .

Fig . 4 . Seasonal changes in the standing crop of common episammic algae .

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the sand at the relatively fast rate of 2 .1 x 10 4 cells/cm z /day during October 1973 but at other times, such asin June 1974, lower values, 2.0 x 104 cells /cm z , were re-corded .

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1974Fig. 5 . Seasonal changes in the standing crop of the episammic assemblage .

Epilithic Community

diatoms predominated with 117 taxa followed by theThe epilithic community remained comparatively di-

Cyanophyta with 20, the Chlorophyta with 8 and the Eu-verse throughout the study with a total of 147 different

glenophyta with 2 . The most common diatoms werespecies, varieties and forms being observed . As usual

Nitzschia palea, Achnanthes minutissima v. crypto-

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Fig . 6 . Seasonal changes in the standing crop of common epilithic algae.

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cephala, Nitzschia linearis and Melosira varians whileOscillatoria brevis and Phormidium foveolarum (Mont .)Gom. were the predominant cyanophytes . Both the chlo-rophytes and euglenophytes remained comparativelyrare, the predominant species being Scenedesmus qua-dricauda (Turp.) Breb., S. obliquus (Turp.) Kz . and Eu-glena sp .

The standing crop of the epilithic algae fell continuous-ly as the distance from the water cress beds increasedwith values, for example, at the uppermost collectionarea usually being 150-200% greater than those in thedownstream site . Since, however, the proportion of thedifferent taxa remained more or less homogeneous overthe entire area, the data for the 4 sites were pooled . Thus,although the abundance of the different species generallyexhibited a recurring pattern of seasonal abundance, con-siderable differences existed in the actual numbers ofthese algae in the two sampling years . For example, Nitz-schia palea, N. linearis, Melosira varians and Synedraacus exhibited a high degree of development during April1974 compared with 1973 whereas the opposite patternwas observed in the case of Achnanthes minutissima v .cryptocephala (Fig. 6) . Of the to most common taxa in

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the community, only Oscillatoria brevis showed a highlyconsistent pattern of seasonal abundance. During 1973,this species reached peak abundance, 6 .75 x 106 filaments/cm 2 , near the middle of May, virtually the same time asthe peak, 9 .o x 10 6 filaments/ cm', was attained in 1974 .

The total assemblage began to develop markedly inApril when the standing crop initially averaged 6 x io bcells/cm' (Fig . 7) . By the beginning of May the densityhad increased to 44 x 10 6 cells/ em' after which the num-bers fell sharply . The community remained relativelywell developed during the winter with a precipitous up-surge in numbers taking place in April . Thereafter thecommunity waned dramatically but with relativelyconstant numbers being maintained from August untilFebruary 1975 . Seasonal changes in the standing crop ofthe flora in terms of cell volume paralleled those outlinedabove (Fig . 7) . The daily growth rate of the communitywas much faster than that exhibited by either the epipelicor episammic flora (Fig. 7). For example, during 1973maximum values of 1 .47 x io6 cells/ cm z and 2.5 x 109µm 3 /cm z were observed in May while in 1974 at the be-ginning of April even higher values, i .e . 2 .4 x 10 6 cells and3.6 x io9 Am'/ cm', were found .

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Fig. 7 . Seasonal changes in the standing crop of the epilithic assemblage .

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Potamoplankton and the Planktonic CommunitiesThe assemblage of algae found floating in the tributaryremained diverse throughout the study with approxi-mately 18o different species and varieties of diatomsbeing observed while 21 additional taxa belonged to theChlorophyta, 12 to the Cyanophyta and 4 to the Euglen-ophyta. The most frequently encountered members ofthe first group of algae included Achnanthes lanceolata,A. minutissima v. cryptocephala, Nitzschia linearis, N .palea and Melosira varians while Chlamydomonasmonadina and Scenedesmus obliquus were the predomi-nant chlorophytes . The other Divisions were normallypoorly represented, the most common species beingTrachelomonas volvocina Ehr ., Phormidium foveola-rum and Dinobryon sertularia Ehr .

In terms of numbers the true planktonic species repre-sented from 5 to 40% of the algae floating in the water .The most common species in this group, Chlamydo-

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monas monadina, showed some develoment, 3-5% of theflora, during November and December of 1973, but thehighest proportions, 15-33%, occurred between June andSeptember 1974. Much the same pattern of successionwas exhibited by another unidentified species of Chlamy-domonas but the relative abundance of this taxon neverexceeded 10.5% and was usually much less . In contrast tothe above patterns, Scenedesmus spp. showed maxi-mum development during September 1973 when theyaccounted for about 15% of the total algal flora but atother times remained comparatively restricted . Plank-tonic diatoms, mainly Cyclotella comta A . Cl. and Step-hanodiscus hantzschii Grun., seldom accounted for morethan 3% of the algae.The standing crop of the entire assemblage varied

tremendously during the two years of the investigation .For example, in the warmer months of 1973, the numberof cells ranged from 0.2 x 10 6 to 1 .9 X 10 6 / 1(0.15-1 .3 x 10 9

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Fig . 8 . Seasonal changes in the standing crop of common algae epiphytic upon Cladophora glomerata .

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µm'/ i) while in 1974 the corresponding value was 0.1 -4.5x 10` cells/1 (0.06-2 .2 x lo'pm 3 / i) . As discharge in bothsummers was similar, these differences were not appa-rently related to fluctuations in the volume of river water .

Epiphytic CommunityThe diversity of the flora found in association with Clado-phora glomerata was, as usual, high . Diatoms once againpredominated with 178 species followed by the Chloro-phyta with 22, the Cyanophyta with 13 and the Eugleno-phyta with 5. The most common members of the firstgroup of algae were Achnanthes lanceolata, A . minutissi-ma v. cryptocephala, Diatoma vulgare Bory, Gompho-ma olivaceum (Lyn.) Ehr . and Meridian circulare (Grey .)Ag. while Scenedesmus obliquus and S. quadricaudawere the most important chlorophytes . The cyanophytawere most frequently represented by Chamaesiphon in-

Fig . 2 . (continued)

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crustans Grun. and Oscillatoria limosa while Euglena sp .was the dominant euglenoid .

The predominant species generally failed to show well-defined, recurring seasonal patterns in abundance . Ach-nanthes lanceolata and most other species showed at leastsome development throughout the study but attained re-latively high peaks in abundance at only one point dur-ing the investigation (Fig . 8) . Achnanthes minutissima v .cryptocephala on the other hand occurred abundantlymost of the time but on several brief occasions exhibiteda sharp drop in numbers . Gomphonema olivaceum andvarieties were the only taxa to show a clear recurring sea-sonal pattern in peak densities (Fig . 8) .

A well-defined spring upsurge in the numbers of theentire assemblage failed to occur during both 1973 and1974 with the standing crop in the former year fluctuatinga great deal, e .g . 6 x i06 cells/g in May to 39 x 106 cells/g

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1974Fig . 9 . Seasonal changes in the standing crop of the algal assemblage epiphytic upon

Cladophora glomerata.

in Juli . A sharp drop in numbers occurred during the fallbut thereafter the amount of algae increased erratically to48 x 10 6 cells/g by August. The standing crop then fellsharply leveling off at 10-20 x 10 6 cells/g for the remain-der of the investigation . Seasonal changes in the amountof algae in terms of cell volume almost exactly paralleledthose outlined for numbers (Fig . 9) . The maximum dailyrate of development, 1 .3 x 10 6 cells/g, took place duringJune 1973 while in 1974 the fastest rate, 0 .4 x 10 6 cells/g,occured in March . In terms of volume maximum rates,1 .5 x Io 9 and 0.95 x Io 9 /Am'/ g, were observed during Mayand July of 1974 respectively. Algae disappeared fromCladophora at a maximum daily rate of 2 .1 x 106 cells/gduring August 1974 followed by a value of o .9 x 106 cells/ gtowards the beginning of November 1973 .

Discussion

Initial expansion of the epipelic community took placeduring March, following a period of increasing day lengthand coincident with similar development in other areasof the British Isles (Round, 1960, 1961 ; Moss & Round,1967), indicating that light is probably the most impor-

tant factor initiating rapid algal growth during the spring .This view is supported by the fact that there was no up-surge in nutrients during the spring and, although a consi-derable rise in temperature did take place, this factor wasclearly not responsible for increased growth during thespring in several nearby rivers were temperatures wereconstant (Moore, In prep .) . Numbers varied greatly dur-ing the warmer months in the tributary, a feature alsonoted in other areas (e .g . Round, 1960, 1961). Since nu-trients, temperature and light conditions in the tributaryremained comparatively favourable throughout thesummer, these fluctuations may be due to a decrease inthe quality of the environment brought about by factorssuch as the production of growth inhibiting metabolites .Algae disappeared from the sediments at exceptionallyhigh rates during the warmer months of 1973 . Since thefirst of these occasions was in May and June, coincidentwith flooding, it can be safely assumed to be correlatedwith the effects of high discharge rates . By contrast thesecond drop in numbers occured in September after aperiod of high standing crop and low water levels, sugges-ting that in this case the large scale detachment might bedue to a high level of metabolic activity (Blum, 1954 ;Muller-Haeckel, 1966) .

The species composition of the episammic communi-ty was similar to that described for other such commu-nities from diverse bodies of water (Moss & Round, 1967 ;Hickman, 1974), a homogeneity that is perhaps surpri-sing in view of the vast differences in environmental con-ditions among these studies . This suggests that sandgrains tend always to provide a similar environment,while the limited number of species present indicates thatonly a few algae are well adapted to colonizing this type ofsubstrate . Since sand grains can be easily disturbed byeither current or wave action, poorly attached specieswould tend to be scoured away . This probably accountsfor the relatively high proportion of Amphora ovalis v .

pediculus, Opephora martyi and Fragilaria construensfound in episammic communities, each of these specieshaving a well developed attachment mechanism . At thesame time it should be noted that since Cocconeis placen-

tula and Rhoicosphenia curvata, species also possessingconsiderable adhesive ability, are not present in any num-bers, there must be some other factor limiting their abili-ty to colonize this type of environment . Since many of thepredominant species attached to sand grains are relativelysmall, this characteristic may enable them to find protec-tion in crevices and hollows, a view previously suggestedby Hickman (1974) . At least one of the predominant taxain this investigation, Fragilaria construens, formed, how-ever, comparatively long filaments which extruded fromthe substrate surface . In any consideration of episammiccommunities, it should be remembered that periods ofextreme turbulence would tend to result in the burial ofmany members of the community . A characteristiccommon to all successful episammic species may there-fore be their ability to withstand such conditions, a viewsupported by the presence in a lake investigated by Hick-man (1974) of large quantities of viable episammic floraat depths of up to to cm from the substrate surface .

The absence of a well defined spring bloom in the spi-sammic community and the presence of the highest algaldensities in the summer parallels the results of Hickman(1974) for a community in the Canadian subarctic. Thegenerally low standing crop and slow growth rates exhibi-ted by the assemblage in the present study indicate eithera slow photosynthetic rate or a high and continuous de-gree of detachment. The latter suggestion seems morelikely as much of the episammic flora was frequentlyfound in the potamoplankton. Algae disappeared, how-ever, from the community at a slow rate compared withthat exhibited by Hickman (1974), a feature which hefound (pers. comm.) was related to heavy wave action .

The standing crop of the epilithon remained muchhigher than the values given in other studies (Gumtow,1955; Douglas, 1958; Dillard, 1971 ; Evans & Stockner,1973). This feature can almost certainely be related to theaddition of fertilizers to the water cress beds upstreamand also possibly to the production of growth inducingmetabolites by the dense population of these macro-phytes . This view is also consistent with the great abun-dance of Nitzschia palea in the tributary since this speciesis known to occur in large numbers in mesosaprobic areas(Butcher, 1947) . The only epilithic species showing a con-sistent pattern of seasonal abundance in the two years wasOscillatoria brevis. This alga displayed peak numbers inthe spring, a pattern which contrasted with to late sum-mer and fall peak recorded by Butcher et al. (1937) in ariver in northern England .

The very high rates of disappearance of algae from therocks during the periods of high standing crop in May1973 can undoubtedly be attributed to the differences inthe overall pattern of attachment by the algae withvarying density . At most times of the year, when the algaeformed a relatively thin layer over the substrate, the com-munity can largely withstand normal water flows . Duringperiods of increasing density, however, when the com-munity produces a much thicker mass, the water flowwill tend to dislodge algae more easily since many will beattached to each other rather than to the stone substrateitself. This process may be more exaggerated through thefact that the cells located at the base may tend to becomesenescent in dense populations (Moore, 1974) thus furtherreducing the general attachment properties of the com-munity as a whole. In addition, the large quantities of gasbubbles often found trapped among the mass of algaeincrease its buoyancy and further add to its susceptibilityto the effects of water flow .

The absence of any significant development in theplankton during the spring of both years can be correlatedwith the fact that since the community consisted mostlyof detached diatoms at this time, their numbers in thewater depend largely on discharge rate and disturbancesupstream rather than on an improvement in environ-mental conditions. As the standing crop during 1973 farexceeded that recorded during 1974, a pattern parallelingthat exhibited by the epipelic communities but not by theepilithic flora, it seems likely that most of the potamo-plankton was derived from the former of these assem-blages .

Since the standing crop of the assemblage on Cladop-hora dropped from high to negligible levels during

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November 1973 but thereafter increased sharply, parallel-

ing changes in temperature, it appears that the shift in

algal numbers was related to this factor . It is also im-

portant to note that the epiphytes associated with water

cress in the main Wylye exhibited a comparable changein numbers at the same time (unpublished data), thus

further supporting the above suggestion . An interesting

feature of this investigation is the high numbers of

Achnanthes minutissima v . cryptocephala maintained

throughout the year on Cladophora, a pattern also noted

by Douglas (1958) and Dillard (1969) for other areas .

Since Achnanthes minutissima is, when it occurs as an

epiphyte, normally rare in both fast flowing and still

water (Godward, 1934 ; Whitton, 1970), the rate of water

flow is clearly of great importance in governing its distri-

bution . However, in other habitats (rocks and sediments),

flow appears to be of less significance (Douglas, 1958 ;

Stockner & Armstrong, 1971 ; Moore, 1972) as it also is,

but to a lesser extent, in other epiphytic diatoms (Whit-

ton, 1970) .

Although large numbers of isopods and amphipods

occurred in the tributary, their grazing probably had little

effect on the standing crop of the epilithic algae (Moore,

1975) . Since only a few representatives of these species

occurred in the sediments, they would also likewise have

had little effect on the epipelic community . Large num-

bers of copepods, mainly CIII-CV stages of Cyclopsstrenuus (Fischer), occurred in the plankton but, in all

instances, algae were absent from the gut .

Summary

The purpose of this study was to determine which environ-

mental factors effected the seasonal succession of epipelic,

episammic, epilithic, epiphytic and planktonic algae in asmall eutrophic stream in southern England . Although

changing light conditions seemed to effect most com-

munities, nutrient levels and herbivorous grazing proba-

bly never limited development . Large variations in

numbers during the summer were correlated with flood-

ing, a deterioration of algal attachment characteristics

and high metabolic rates .

References

American Public Health Association . 1971 . Standard Methodsfor the Examination of Water and Wastewater . N.Y., 13th ed .

1 92

Blum, J . L . 1954. Evidence for a diurnal pulse in stream phyto-plankton . Science 119 : 732-734.

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