Cyanophytes and chlorophytes versus raphidophytes in humic lakes with different catchment basins

The aim of the study was to evaluate the effects of coniferous forest cover in the catchment basin and relative catchment area (catchment area to lake volume ratio) on phytoplankton composition in humic lakes. The study was carried out in 11 small and shallow lakes situated in the West Polesie region (Eastern Poland). The lakes were divided with respect to forest cover in their catchment basins into two groups: high forest cover — HFC (more than 60%) and low forest cover — LFC (less than 60%). The study showed that both, land use in the catchments (proportion of forests) and the relative catchment area determined physicochemical and biological parameters in the lakes. The high relative catchment area affects their high productivity expressed by high chlorophyll a concentration and low water visibility. The lakes of the LFC group had low water colour as well as high concentration of total phosphorus (P_tot), reaction (pH), and conductivity of water and a large number of cyanophytes and chlorophytes. The dominant species, e.g., Planktolyngbya limnetica, Limnothrix planctonica, Planktothrix agardhii, Coenococcus planctonicus, were characteristic of high trophic status. In the lakes of the HFC group, P_tot, pH, conductivity of water and the contribution of cyanophytes and chlorophytes was considerably lower, whereas the water colour and the number of raphidophytes represented by Gonyostomum semen was high. The large number of raphidophytes and the small amount of chlorophytes and cyanophytes in the lakes of the HFC group indicated the lake naturalness.     

The meticulous phycological investigation of Lake Rudträsket, Central Sweden, 1947–1949, by Heinrichs Skuja

In 1947–1949, studies of water chemistry, aquatic macrophytes, bryophytes and algae were performed in a small lake, L. Rudträsket, near Stockholm, but only a few results concerning water chemistry and macrophytes have so far been published. Heinrichs Skuja (1892–1972) left 62 hand-written pages with the results of his analyses from the lake: a list of taxa, some measurements and several algal sketches. The main part of this material is presented here. Skuja identified about 640 taxa with a majority for desmids, ca. 290, followed by ca. 100 chlorophyceans, 65 euglenophytes and 60 cyanophytes. The species composition gives a fairly reflection of the character of the lake: small, shallow, rich in humus, acid and surrounded by swampy areas with Sphagnum. Dimensions of algae as well as drawings from L. Rudträsket have partly been published in publications by Skuja. In this paper seven plates with about 115 figures have been drawn in Indian ink and printed. Today, the lake has a lower pH value, pH 4.6, than in 1947–1949, pH 5.3–6.1, and would be a stimulating object for comparative studies.     

Mesocosm experiments on the interaction of sediment influence, fish predation and aquatic plants with the structure of phytoplankton and zooplankton communities

1. Little Mere, U.K., received large quantities of sewage effluent until 1991, when the effluent was diverted. Experiments, carried out in mesocosms in 1992 and 1993, were designed to predict the effects of: (i) reduced external nutrient loading; (ii) reduced internal loading from the sediment; and (iii) recolonization by fish of the better aerated water. Treatments included isolation of the water from the underlying sediment or exposure to the sediment (which lacked plants in 1992, but was covered by Potamogeton berchtoldii in 1993) crossed with different population densities of Rutilus rutilus in 1992 and of Perca fluviatilis in 1993.
2. Exposure to sediment (as opposed to isolation from it) resulted in no net change in the biovolumes of most major algal groups, but this masked major complementary effects on individual species. The experiments showed a decreasing influence of the sediment, between 1992 and 1993, in determining water chemistry, and an increasing pH between years but no increase in cyanophyte dominance. This had been anticipated because a lake upstream provides abundant inocula, and conditions in Little Mere after diversion of effluent were expected to favour cyanophytes.
3. Roach and perch additions to the mesocosms resulted in reductions in Daphnia populations but increases in numbers of small Cladocera and copepods. Plant-associated Cladocera were unaffected by fish. The presence of submerged plants to some extent reduced fish predation effects on Daphnia hyalina .
4. The experimental results in general accurately predicted subsequent events in the open lake.     

Mesocosm experiments on the interaction of sediment influence, fish predation and aquatic plants with the structure of phytoplankton and zooplankton communities

1. Little Mere, U.K., received large quantities of sewage effluent until 1991, when the effluent was diverted. Experiments, carried out in mesocosms in 1992 and 1993, were designed to predict the effects of: (i) reduced external nutrient loading; (ii) reduced internal loading from the sediment; and (iii) recolonization by fish of the better aerated water. Treatments included isolation of the water from the underlying sediment or exposure to the sediment (which lacked plants in 1992, but was covered by Potamogeton berchtoldii in 1993) crossed with different population densities of Rutilus rutilus in 1992 and of Perca fluviatilis in 1993.
2. Exposure to sediment (as opposed to isolation from it) resulted in no net change in the biovolumes of most major algal groups, but this masked major complementary effects on individual species. The experiments showed a decreasing influence of the sediment, between 1992 and 1993, in determining water chemistry, and an increasing pH between years but no increase in cyanophyte dominance. This had been anticipated because a lake upstream provides abundant inocula, and conditions in Little Mere after diversion of effluent were expected to favour cyanophytes.
3. Roach and perch additions to the mesocosms resulted in reductions in Daphnia populations but increases in numbers of small Cladocera and copepods. Plant-associated Cladocera were unaffected by fish. The presence of submerged plants to some extent reduced fish predation effects on Daphnia hyalina .
4. The experimental results in general accurately predicted subsequent events in the open lake.     

The structure and dynamics of the phytoplankton assemblages in Lake Kinneret, Israel

The phytoplankton of Lake Kinneret, a warm monomictic lake,is dominated by a Pyrrhophyta-Chlorophyta assemblage. Four stagesof succession of planktonic algae occur in the lake, startingwith thermal and chemical destratification and ending with stratification. The index of diversity of the phytoplankton communities is highduring the destratification and mixed periods. The index reachesminimal values during late summer, when the ecosystem is subjectto strong physical, chemical and biological stresses. The diversityin Lake Kinneret increases with the increase in nutrients andnot with the increase in temperature. During most of the year, the nanoplanktonic forms are in greaternumbers than the netplankton species. This fact is correlatedwith the amounts of available nutrients in the lake. The annual averages of the wet autotrophic biomass in Lake Kinneretare very high in comparison with other warm lakes. The contributionof the nanoplanktonic species to the total algal biomass isvery small during the Peridinium bloom, but represents approximatelyhalf of the total algal biomass during the rest of the year. The concentration of nutrients in the water, together with theadverse competitive effect of Peridinium on other algae, areto a large extent responsible for the composition, successionand abundance of the phytoplankton assemblages in Lake Kinneret.     

Factors controlling pelagic populations of ciliates and heliozoans-- late summer investigations in an acidic lake before and after liming

Ciliates and heliozoans were studied in a small oligotrophicSwedish lake with the objective of investigating what controlledthe community structure and population dynamics. Enclosure experimentswere performed with five levels of inorganic nutrient concentrationsand the presence or absence of crustacean zooplankton. The experimentswere performed during the summers immediately preceding andfollowing liming of the lake. The responses of protozooplanktonto these manipulations were investigated and compared with theresponses of bacterioplankton, phytoplankton and metazooplankton.In contrast to most studies on protozoan zooplankton, ciliateswere determined to the lowest taxon possible, and biomassesand abundances were calculated for the different taxa. The protozooplanktonbiomass of the lake, expressed as summer average, did not changebetween the years. However, the community composition changedfrom smaller to larger species as the small prostomatid ciliatesUrotricha and Balanion were largely replaced by oligotrichsof the genera Strombidium and Strobilidium. The experimentssuggested that the protozooplankton community in the lake wascontrolled by metazooplankton in the acidic environment, whilefood limitation was the most important controlling factor afterliming. Before liming, Urotricha and Balanion were unaffectedby nutrient additions as well as by metazooplankton, while otherciliates and Heliozoa showed a significantly negative responseto metazooplankton. After liming, Urotricha and Balanion showeda significant positive response to nutrient addition as wellas to bacterioplankton biomass. A possible explanation for thisresponse in these algivore genera is that the response was indirectand mediated through small chrysoflagellates.     

Are bacteria an important food source for rotifers in eutrophic lakes?

In situ grazing measurements using fluorescent particles of0.5, 2.4 and 6.3 µm diameter in eutrophic Lake Loosdrecht(The Netherlands) showed that Anuraeopsis fissa, a small rotifer,filtered the smallest, bacteria-sized particles as efficientlyor more efficiently than the larger particles. In contrast,three other rotifer species (Brachionus angularis, Filinia longisetaand Pompholyx sulcata) filtered the bacteria-sized particlesless efficiently than the larger particles. Both Keratella cochlearisand Conochilus unicornis only ingested the bacteria-sized particles.Anuraeopsis fissa had a higher uptake of fluorescent bacteria-sizedparticles than K.cochlearis, both in 1 µm filtrate oflake water and in lake water. Within both species, uptake didnot differ between juveniles and adults. When cultured on threedifferent size fractions of lake water (1, 3 and 15 µmfiltrate) in July, all rotifer species declined in numbers onthe 1 and 3 µm filtrates, while A.fissa and B.angularisincreased in numbers on the 15 µm filtrate. The high abundanceof small bacteria in the lake water could not support rotiferpopulations. It is concluded that bacteria are not a suitablefood source of high quality for A.fissa because its populationdoes not grow even though the bacterial concentration was higherthan its estimated threshold food concentration. In August,when individually cultured, the mortality was high for all species,but especially for F.longiseta. The lifespan of K.cochleariswas reduced in the 1 and 3 µm filtrates of lake water,compared with in the 15 µm filtrate. The lifespan of A.fissawas similar in all filtrates, but reproduction was reduced inthe 1 and 3 µm filtrates, as in Keratella. On the 15 µmfiltrate, their ages at first reproduction and growth ratesdid not differ. Individuals of A.fissa older than 4 days showeda higher survival in the 15 µm filtrate than in the othertwo filtrates, as did K.cochlearis throughout its life. Hence,bacteria seem to be a more important food source for youngerindividuals of A.fissa than of K.cochlearis.     

The calanoid copepod Acartia italica Steuer, phenomenon in the small saline Lake Rogoznica (Eastern Adriatic coast)

Lake Rogoznica, near ibenik, Croatia, is a salt coastal lakewith a maximum depth of about 15 m. During most of the year,this small, naturally eutrophied lake is highly stratified,with hypoxia/anoxia occurring in the bottom layer. Total anoxia,with the presence of hydrogen sulphide, was recorded on September27, 1997. At that moment, massive mortality of all planktonicand benthic organisms was observed. After the anoxia, Acartiaitalica, the only planktonic copepod species of the lake, re-establishedquickly. Before anoxia, the population dynamics of the A.italicapopulation depended primarily on predator/prey relationships.However, in the post-anoxic period, nutrient–phytoplankton–copepodrelationships became more important, as there was no longerany predation pressure. From all accounts, it would appear thatonly A.italica is adapted to the extreme conditions which appearfrom time to time in the lake, and that this species has animportant role in the functioning of the lake ecosystem. Acartiaitalica was very important for the gradual normalization oflife in the lake after total anoxia.     

The synergistic effect of carbon concentration and high temperature on lipid peroxidation in Peridinium gatunense

Lipid peroxidation in Peridinium samples taken from two differentdepths in Lake Kinneret fluctuated throughout the spring withan overall increasing trend. Samples from 0.5 and 5 m showeda similar peroxidation pattern, which was maximal after thefall off in algal biomass. The rapid decline in Peridinium biomasscoincided with ambient lake temperatures of 21–23C. Fattyacid composition profiles were similar at both depths, althoughafter the peak of the bloom, a significant increase in polyunsaturatedfatty acids and oleic acid was only found at 0.5 m, togetherwith a decrease in the percentage of polyunsaturated fatty acids.These effects were related to ambient light stress rather thana result of lipid peroxidation. Lake samples taken at differentperiods of the bloom and incubated at various temperatures showeddifferential peroxidation. Higher temperatures caused increasedlipid peroxidation, but this appeared to be dependent on thesampling period. Samples withdrawn from the lake at the beginningof the bloom showed little peroxidation after a 5 day incubationat 14C, room temperature (25C) or ambient lake temperature(16C) compared to mid-bloom samples in which there was a significantincrease in peroxidation when they were incubated at room temperature(25C) or ambient lake temperature (22C). Incubation at 14Cinhibited peroxidation; however, samples from mid-bloom againshowed enhanced peroxidation compared with those from the beginningof the bloom. These in situ results suggested a relationshipbetween temperature, another environmental variable during thebloom and lipid peroxidation in Peridinium. As total dissolvedinorganic carbon (DIC) concentrations fall significantly duringthe progress of the bloom and represent an important sourceof environmental stress, laboratory experiments were establishedto investigate the synergistic effect of temperature and carbonnutrition on lipid peroxidation in Peridinium cultures. Increasedtemperature alone caused a slight increase in lipid peroxidation,but this was greatly augmented by carbon limitation. Althoughcarbon limitation induced increased catalase activity, at highertemperatures activity declined after 48 h, allowing for thesubstantial increase in lipid peroxidation.     

Regulation of the Stoichiometry of Thylakoid Components in the Photosynthetic System of Cyanophytes: Model Experiments Showing that Control of the Synthesis or Supply of Chl a can Change the Stoichiometric Relationship between the Two Photosystems

Regulation of the assembly of the photosystem I (PS I) complexin response to the light regime in the photosynthetic systemof cyanophytes was studied in Synechocystis PCC 6714. The relationshipbetween the assembly of the PS I complex and synthesis of Chla was examined by model experiments in which synthesis of Chla was controlled by two inhibitors, gabaculine (GAB) and 2,2'-dipyridyl(DP). Both inhibitors caused a change to a lower ratio of PSI to PS II even under light that normally induces a high ratioof PS I to PS II. The change in stoichiometry induced by theseinhibitors was suppressed when protein synthesis was inhibitedby chloram-phenicol, similarly to the change in the stoichiometryinduced by light that excites mainly PS I (PS I light). Comparisonof the levels of PS I, PS II and Cyt b₆-f complexes per cellindicated that a selective suppression of the assembly of thePS I complex was induced by the inhibitors: the stoichiometricrelationship among PS I, PS II and Cyt b₆-f complexes was identicalto that induced by PS I light or white light of high intensity.GAB induced a decrease in size of the phycobilisome also, whileDP did not, similarly to PS I light. The results indicate thatthe ratio of PS I to PS II can be changed by the control ofsynthesis of Chl a. They also suggest that control of the synthesisor supply of Chl a probably exerted at site(s) in or after theprocess of the Mg-protoporphyrin branch, is involved in themechanism of regulation of the assembly of the PS I complexin cyanophytes. (Received September 7, 1989; Accepted November 20, 1989)     

Phosphorus utilization by Asterionella formosa Hass

Observations have been made on the relationship in Lake Windermerebetween the growth of Asterionella formosa and the concentrationof dissolved phosphate. Asterionellaformosa has also been grownin culture and the amount of phosphorus required by this organismhas been determined. These experiments have shown that: (1) Asterionella can take up and store in reserve phosphorusfrom concentrations below those found in phosphorus-poor lakes(i.e. below 1 µg.P/l.). (2) Growth can continue in phosphorus-deficient media by makinguse of such reserves, cell phosphorus being steadily reduced. (3) The limiting requirement per cell of phosphorus is veryminute—about 0—06µg P/10⁶ cells/1, so thatinitial concentrations as low as rog.P/l. can theoreticallyproduce a population of some i6 x io6 cells/l. before limitationby phosphorus deficiency. This has been realized in culture.The behaviour of Asterionella formosa growing in nature hasbeen found to conform with that found in culture. It is concludedfrom such observations that phosphorus deficiency is unlikelyto provide a limit to growth of Asterionella in Windermere,despite the very low initial concentration of dissolved phosphate.Further experiments have shown that Asterionella cells low inphosphorus can rapidly take up added phosphate from lake waterbut not from distilled water. Some factors which affect therate of phosphate uptake of depleted cells are investigated,and attempts have ben made to throw some light on the natureof the apparent difference between lake water on the one handand distilled water and a number of artificial lake waters onthe other. No conclusion is reached on the reason why Asterionellacells behave differently in lake water and in artificial media.     

Prevalence and inferred effects of microsporidia of Holopedium gibberum (Crustacea:Cladocera) in a Canadian Shield lake

Two species of microsporidial parasites are reported from Holopediungibberum in a small Canadian Shield lake. Seasonal changes inprevalence of the parasites were not attributable to changesin water temperature or to changes in size distribution of hostsover the season. Seasonal changes in resistance of hosts toparasitism, related to food stress may account for the patternin prevalence but it was more likely a consequence of host andparasite population dynamics and interplay. An examination ofH.gibberum fecundity and size structure indicated that microsporidiosisreduced birth rates and may have decreased survival of Holopedium.     

Phytoplankton community succession in a lake subjected to artificial circulation

East Sidney Lake, a small, eutrophic bottom release impoundment in NY, has undergone artificial circulation for three seasons. The artificial circulation system resulted in an overall reduction in the physical stability of the water column, making the lake subject to alternating periods of weak chemical stratification and mixing. Phytoplankton community succession exhibited a high degree of regularity from year to year, culminating in mid summer dominance by heterocystous cyanophytes in all years. Changes in the physical structure of the water column, with attendant changes in Z _eu :Z _mix , were not important determinants of phytoplankton community makeup in East Sidney Lake. Seasonal patterns and community characteristics were not affected by artificially induced alterations in stability, but instead were most sensitive to surface temperatures, flushing rate and TN:TP. The timing of cyanophyte blooms was not affected by artificial circulation, nor was maximum seasonal phytoplankton biomass reduced.     

Planktic cyanophytes and their ecology in the shallow Lake Mikri Prespa, Greece

Cyanophytes dominated the phytoplankton of shallow Lake Mikri Prespa during the period May 1990 to September 1992 (76.5 and 52.0% of the total phytoplankton biomass in 1990–1991 and 1991–1992, respectively). Biomass peaks were observed in autumn (from 5.2 to 34.5 g m^-3) when low dissolved inorganic nitrogen, high phosphate phosphorus and low Z_cu to Z_mix ratio prevailed. The dominant species were Microcystis aeruginosa and M. wesenbergii. These represented 64 and 86% of the cyanophyte biomass in 1990–1991 and 1991–1992, respectively and revealed similar patterns of seasonality forming biomass peaks in late summer - autumn. Small chroococcalean cyanophytes (< 2 μm) showed also similar temporal distributions. Of the filamentous cyanophytes, the most important species was Anabaena lemmermannii var. minor which formed peaks in late summer and autumn (2.6 and 1.1 g m^-3 in 1990 and 1992, respectively). Rainfall and the N:P ratio were probably the main factors influencing the seasonality of all of the filamentous cyanophytes in the lake. Cyanophytes, tended to increase at temperatures higher than 16 °C and at inorganic nitrogen concentrations lower than 100 μg 1^--¹. The frequent mixing of the water column did not seem to prohibit the substantial increase of the group.     

The allometry of algal respiration

For 30 years, study after study has shown that respiration ratesincrease as {small tilde}0.75 of body size for organisms rangingfrom protozoans to mammals. However, a number of studies suggestedthat the respiration-size relationship for algae may be a rareexception to this general rule. Algal respiration may be almostproportional to cell size, such that the slope of the respiration-sizerelationship is closer to unity. The present study examinedthe effect of cell size and taxon on phytoplankton respiration,using data collected from the literature. To this end, we collecteda data set of 178 observations of algal respiration and cellsize representing six divisions-chlorophytes. chrysophytes,cyanophytes, euglenophytes, pyrrophytes and rhodophytes. Therelationship between respiration (R, in p1 O₂ cell^–1 h^–1)and cell carbon content (C, in pg C cell^–1) is describedas R = 0.030C^{0 93} and the exponent is significantly >3/4.When we expressed cell size in terms of volume, the exponentdecreased to 0.88 but this is still significantly >3/4. Amongthe six divisions studied, chlorophytes, euglenophytes and rhodophytesseemed to differ significantly in their respiration-size relationshipfrom other taxa. However, euglenophytes and rhodophytes havesuch small size ranges that no meaningful relationships canbe developed for those groups alone. The chlorophyte respiration-sizerelationship has obvious patterns in its residuals which mayindicate that significant sources of error were not controlledin these heterogeneous data. Thus, for the present, the generalmodel seems most appropriate for the prediction of respirationrates of phytoplankton.     

Diurnal residence of the larger stages of the calanoid copepod Acartia tonsa in the anoxic monimolimnion of a tropical meromictic lake in New Guinea

We studied diel vertical migration of the calanoid copepod Acartiatonsa in a coastal meromiclic lake (Lake Nagada, Papua New Guinea).During the day, the animals migrate several metres below theoxycline to just above or into the sediments, and remain inanoxic water. Vertical migration patterns differ between ontogeneticstages; adults and larger copepodites do not occur in appreciablenumbers in the water samples during the day (deepest sampletaken at {small tilde}0.5 m above the sediment). Around sunset,the larger copepodites gradually appear in the water column,reaching the near-surface food layer (0–2 m) only wellafter sunset. Our results suggest that the evening ascent ofthe copepods in this tropical lake is related to relative changesin light intensity, and that there are age- and sex-relateddifferences in the responses to these stimuli.     

Comparison of nets and pump sampling gears to assess zooplankton vertical distribution in stratified lakes

The efficiency of a cantilevered bridle net was tested in comparisonwith a Wisconsin net and a pumping system to sample zooplanktonorganisms in three water layers (epi-, meta- and hypolimnion)of three Canadian Shield lakes. Variations among samplers werecompared to variations due to within-lake vertical distributionof zooplankters and among lake variations. For each lake andwater layer, we also assessed the efficiency of the three methodsaccording to the catches of zooplanktonic taxa. The highestpercentages of variation were generally due to lake or water-layereffects; interaction between sampling gears and water layerswas above 50% for most taxa, except cladoceran. Sampling methodsexplained more variation than the lake effect for some zooplanktontaxa, indicating that using different sampling devices couldpotentially alter the among-lake variation interpretation ofzooplankton abundance. The pumping system captured higher densitiesof animals per taxa than the cantilever and the Wisconsin nets.The cantilever net generally captured mobile taxa more efficiently(Polyarthra vulgaris, copepods, Daphnia sp., Diaphanosoma brachyurumand chaoborids) than the Wisconsin net and the pumping system,but its efficiency varied among water layers.     

Demography of a natural population of Daphnia retrocurva in a lake with low food quality

The hypothesis was tested that Daphnia exhibit low populationrecruitment in a lake with phosphorus (P)-deficient food, whereDaphnia populations failed to increase in response to greatlylowered minnow density. Egg ratio analysis was performed onDaphnia retrocurva from L110 of the Experimental Lakes Area,Ontario, in 1994. L110 has high food abundance (seston carbon(C) 100 µM C), but the seston is P deficient (C:P atomic- 600). Chrysophytes dominated the algal community. The fishcommunity in this lake was manipulated in 1993 and 1994 by theaddition of northern pike, Esox lucius. Zooplanktivorous minnowswere greatly reduced after the piscivore addition, but the expectedincrease in abundance of the Daphnia to this altered predationregime was not observed, If food quality constrains the responseof the Daphnia to changed predation, there should be low recruitment in the relatively sparse Daphnia population in this lake.Birth rates estimated by standard egg ratio analysis were <0.1day^–1 for most of the sample period. In comparison toannual averages from the literature, recruitment in L110 wasmoderately, but not dramatically, low. Daphnia exhibited pronounced‘normal’ vertical migration (up at night, down duringthe day) all year, and a strongly declining size at first reproductionthrough the season. Population density and birth rates seemedto show much less short-term change than previously observedin other lakes. Although the demo graphic predictions for foodquality constraints seemed clear, in situ demography unfortunatelyneither supported nor strongly rejected the overall hypothesisof food quality limitation in this lake with very high C:P ratios.Possibly, direct demographic effects of low food quality willbe difficult to observe in nature.     

Fish-induced sediment resuspension: effects on phytoplankton biomass and community structure in a shallow hypereutrophic lake

An in situ mesocosm experiment was performed at Old Woman CreekEstuary, OH, to assess the importance of fish-induced sedimentresuspension in regulating phytoplankton biomass and communitystructure Six polyethylene tubes (1 m diameter x 2 m long) wereplaced into the lake, enclosing portions of the water columnand sediments Three duplicated treatments were established:(i) control, no fish, (ii) fish, stocked with small fish fromthe lake; and (iii) fish/net, stocked with fish, but into tubeswith coarse nets suspended above the sediments to prevent resuspension.Total P concentrations and algal biomass in the fish tubes becamemarkedly higher than the fish/net and control tubes. Centricdiatoms and small cryptomonads were the initial dominants. Inthe lake and fish tubes, this community was replaced by nanochlorophytes.In the fish/ net and control tubes, a very different successionoccurred, where large cryptomonads became dominant These resultsindicate that sediment nutrient resuspensions by fish activitiescan maintain a phytoplankton community in an immature state,with small r-selected dominants. When sediment nutrient resuspensionwas prevented (in the fish/net and control tubes), larger algalspecies increased in relative biovolume, regardless of whetherfish were present.     

A multivariate analysis of phytoplankton and food web changes in a shallow biomanipulated lake

1. Phytoplankton dynamics, food chain changes and resilience in Lake Zwemlust, a shallow lake in The Netherlands, are described for the period 1986–94.
2. After biomanipulation in 1987, the lake moved through two alternative states, while the external nutrient loadings were maintained. A clear-water phase, mostly dominated by macrophytes, persisted from 1987 to 1991, and a rather turbid state, dominated by algae, occurred in the summers of 1992–94, after several consecutive and sustained perturbations affecting different parts of the food web in the lake. These two periods were characterized by different community structures.
3. The phytoplankton assemblage gradually changed in a pattern that reverted in later years towards that of the pre-biomanipulation stage, although the same species composition was not regained. This agrees with some mathematical models. During the clear-water phase, nutrient shortage, light climate and zooplankton feeding selected in favour of small, high surface : volume ratio and rapidly reproducing algae. However, in mid-summer of 1992–94, nutrient availability and cladoceran grazing on edible algae favoured cyanophytes.
4. Nutrients were transferred to higher trophic levels or lost from the system at relatively high rates when the lake was in a piscivore–macrophyte-dominated state, while they tended to accumulate in the algae in a planktivore-dominated chain without macrophytes. The role of weed beds was central for nutrient competition (mostly nitrogen) with algae, as well as a refuge and a base for alternative food sources to grazers. Weed beds seemed to have a strong effect in increasing connectedness, resilience and stability of the lake community.
5. The complete return of Zwemlust to a turbid state dominated by phytoplankton seems to have depended upon turnover of the limiting nutrient, which was retarded by macrophytes and stimulated by planktivorous fish and waterfowl.