Top-down control of natural phyto- and bacterioplankton prey communities by Daphnia magna and by the natural zooplankton community of the hypertrophic Lake Blankaart

Biomanipulation, through the reduction of fish abundance resulting in an increase of large filter feeders and a stronger top-down control on algae, is commonly used as a lake restoration tool in eutrophic lakes. However, cyanobacteria, often found in eutrophic ponds, can influence the grazing capacity of filter feeding zooplankton. We performed grazing experiments in hypertrophic Lake Blankaart during two consecutive summers (1998, with and 1999, without cyanobacteria) to elucidate the influence of cyanobacteria on the grazing pressure of zooplankton communities. We compared the grazing pressure of the natural macrozooplankton community (mainly small to medium-sized cladocerans and copepods) with that of large Daphnia magna on the natural bacterioplankton and phytoplankton prey communities. Our results showed that in the absence of cyanobacteria, Daphnia magna grazing pressure on bacteria was higher compared to the grazing pressure of the natural zooplankton community. However, Daphnia grazing rates on phytoplankton were not significantly different compared to the grazing rates of the natural zooplankton community. When cyanobacteria were abundant, grazing pressure of Daphnia magnaseemed to be inhibited, and the grazing pressure on bacteria and phytoplankton was similar to that of the natural macrozooplankton community. Our results suggest that biomanipulation may not always result in a more effective top-down control of the algal biomass.     

Plankton Succession in the Temporary Lake Koronia after Intermittent Dry‐Out

This study examines the plankton succession in a polluted temporary lake after intermittent dry‐out. The initial stage after flooding was heterotrophic (zooplankton/phytoplankton carbon biomass ratio > 1). Phytoplankton species richness increased exponentially within a few months after inundation. The chlorophyte Koliella cf. longiseta was the pioneer colonist which was replaced by Oocystis sp. reaching 300 340 ind mL^–1. The initial conditions favored rotifer and cladoceran colonists, not previously recorded, to successfully establish populations. The species that finally became dominant hatched from the lake's sedimentary egg bank with Daphnia magna being prominent. Nevertheless, the zooplankton community was unable to control the high biomass of chlorophytes (zooplankton/phytoplankton carbon biomass ratio < 0.4). Plankton succession in this temporary lake was mostly determined by the past phytoplankton – zooplankton species pool rather than by the established new colonists. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nutrient removal from polluted stream water by artificial aquatic food web system

For the removal of nutrients from eutrophic stream water polluted by non-point sources, an artificial aquatic food web (AAFW) system comprising processes of phytoplankton growth and Daphnia magna grazing was developed. The AAFW system was a continuous-flow system constructed with one storage basin of 3 m³ capacity, one phytoplankton tank of 3 m³ capacity, and one zooplankton growth chamber of 1.5 m³ capacity. The system was optimized by setting hydraulic retention time of phytoplankton tank as 3 days and D. magna density as 740–1000 individual l⁻¹. When the system was operated on eutrophic stream water that was delivering 471 g of total nitrogen (TN) and 29 g of total phosphorus (TP) loadings for 45 days, 250 g (53%) of TN and 16 g (54%) of TP were removed from the water during its passage through the phytoplankton tank. In addition, 64 g (14%) of TN and 4 g (13%) of TP were removed from the water by harvesting zooplankton biomass in the zooplankton growth chamber, resulting in significant overall removal rates of TN (69%), nitrate (78%), TP (73%), and dissolved inorganic phosphorus (94%). While the removal efficiency of the AAFW system is comparable to those of other ecotechnologies such as constructed wetlands, its operation is less limited by the availability of space or seasonal shift of temperature. Therefore, it was concluded that AAFW system is a highly efficient, flexible system for reducing nutrient levels in tributary streams and hence nutrient loading to large aquatic systems receiving the stream water. Handling editor: J. Padisak     

The effect of Daphnia on filament length of blue-green algae

Water from a hypertrophic lake rich in filamentous blue-green algae was passed through a continuous-flow system of aquaria containing Daphnia magna, and a control system without Daphnia. Daphnia caused a significant decrease in the blue-green algal density, and a two-fold reduction in filament length. It is suggested that feeding activity of Daphnia may result in an increase in the availability of blue-green filaments to filter-feeding cladocerans.     

Comparison of the roles of Ostracods and Cladocerans in regulating community structure and metabolism in freshwater microcosms

Laboratory microcosms were used to compare the effects of the littoral ostracod Cypridopsis vidua and the planktonic cladoceran Daphnia magna on community structure and metabolism. Filter-feeding by cladocerans, both in the presence and absence of ostracods, greatly reduced the abundance of planktonic algae when D. magna reached peak density around day 50; rotifers and euglenids were then limited to flocculent matter on the container bottom. Both net production and community respiration rates decreased as community composition changed. Microcosms containing ostracods as the only microcrustacean showed little reduction in total algal numbers but the otherwise dominant alga, Scenedesmus spp., was replaced by Ankistrodesmus spp. when peak ostracod density was reached around day 100. Rotifers were completely eliminated but euglenids were able to coexist with ostracods. Ostracods impacted community metabolism less than cladocerans, but depressed respiration slightly more than net production.     

Growth of Daphnia longispina L. in a polyhumic lake under various availabilities of algal,bacterial and detrital food

The availability and importance of food sources for growth of Daphnia longispina L. from a highly coloured fishless lake with anoxic hypolimnion were assessed by combining in situ and laboratory experiments. In in situ experiments populations were enclosed in tubes with natural temperature stratification and with or without anoxic hypolimnion. In the laboratory experiments the importance of food source (littoral zone vs pelagic epilimnion) was assessed by enclosing moss thalli and a natural zooplankton population in a large-scale flow-through system supplying food for experimental Daphnia. Growth of juveniles of Daphnia in epilimnetic water was determined in batch culture experiments and the importance of increasing concentrations of bacteria and algae for their growth and development was investigated with a small-scale flow-through system. Access to the anoxic hypolimnion enhanced the growth of Daphnia by 23–24%. Growth rates in the tubes with anoxic hypolimnion were 0.36 and 0.16 d^–1 in July and August respectively. In tubes without anoxia the corresponding values were 0.29 and 0.13. In batch-cultures the highest growth rate determined was 0.16 and the overall rates were lower than in in situ experiments. In batch culture Daphnia was able to grow in darkness for 10 days with a rate of 0.16. In the large-scale flow-through system Daphnia population fed with littoral water reproduced well despite the low concentration of algae and increased its number by a factor of c. 32 in 10 days. However, the animals were small and net production of Daphnia population thus lower under the littoral influence than in the control treatment. Population could survive and grew slowly on pelagial water processed by a natural zooplankton community and with very little algae left. It is thus possible that bacteria serve as a life-support system enabling the population survival over periods of algal shortage. Small-scale flow-through experiments revealed that Daphnia longispina is able to mature and reproduce on a bacterial diet if the food concentration is high enough and Daphnia on bacterial food could achieve growth rates similar to those on an algal diet. The threshold food level for Daphnia longispina was estimated to be c. 18.5 g C 1^–1. Detrital material is of limited value in nutrition of Daphnia even in a lake where more than 75% of carbon is bound in particulate detritus.     

Nutrient removal from sewage by an artificial food web system composed of phytoplankton and Daphnia magna

In order to remove nutrients from sewage, ecotechnology with an artificial food web composed of phytoplankton and Daphnia magna was used. To optimise performance of the system, phytoplankton growth, zooplankton growth, and a continuous-flow system were used. For phytoplankton growth, stirring was 6.7 times faster than the settling in growth rate of Scenedesmus. Zooplankton growth was not influenced by phytoplankton succession, and the specific production coefficient of D. magna was 110.4 mg Daphnia dry weight (DW) per mg chlorophyll a (Chl a). Results indicated that removal of nutrients was better in a long hydraulic residence time (HRT) system than in a system with short HRT. The optimum retention time was found to be 3 days for the phytoplankton chamber and 1.5 days for the subsequent D. magna chamber, respectively, with total retention time of the combined chambers being kept at 4.5 days. When a pilot plant was operated under these conditions, the removal rates of total nitrogen (TN) and total phosphorus (TP) were 68 and 56%, respectively. In the material budget of TN, 32% of inputs passed on to effluent, 39% to sludge, 27% to air and 2% to harvested Daphnia. For TP, 44% of inputs passed on to effluent, 51% to sludge and 4% to Daphnia.     

Biomanipulation by introduction of herbivorous zooplankton. A helpful shock for eutrophic lakes?

In 1988 and 1989Daphnia magna were introduced into the Gewerbepark Pond and into the University Pond, and also into a limnocorral (10 m diameter, 5 m deep) in Postfelden Reservoir. Limnological parameters were regularly monitored in all the three water bodies over three months or longer. The objective of the experiments was to induce a clear-water period in turbid eutrophic waters by direct manipulation of the zooplankton community. Introduction of four million individuals ofD. magna (10 ind. 1⁻¹) caused a long lasting clear-water period in the limnocorral. While Secchi-disc transparency in the reservoir decreased from 1.3 m to<1 m during summer, transparency increased to a stable level of about 4 m in the corral. Despite very low phytoplankton biomass,D. magna was still abundant. The experiment was terminated after three months because of oxygen depletion in the corral. In the Gewerbepark Pond 3.3 million individuals ofD. magna (0.6 ind. 1⁻¹) were introduced at the beginning of August. The daphnids soon increased 60-fold in their density causing a persistently high water transparency and low phytoplankton biomass up to November. Ammonia concentration rose steeply, whereas that of nitrate and oxygen decreased. Subsequently in summer, concentrations of chlorophyll as well as of inorganic phosphorus and nitrogen were extremely low. Instead of the phytoplankton,Elodea canadensis grew enormously and covered 12% of the pond surface. The biomanipulation experiment in University Pond failed, probably due to predation by carp.     

Production ecology of phyto- and zooplankton in a eutrophic pond dominated byChaoborus flavicans (Diptera: Chaobolidae)

Primary production of phytoplankton and secondary production of a daphnid and a chaoborid were studied in a small eutrophic pond. The gross primary production of phytoplankton was 290 gC m⁻² per 9 months during April–December. Regression analysis showed that the gross primary production was related to the incident solar radiation and the chlorophylla concentration and not to either total phosphorus or total inorganic nitrogen concentration. The mean chlorophylla concentration (14.2 mg m⁻³), however, was about half the expected value upon phosphorus loading of this pond. The mean zooplankton biomass was 1.60 g dry weight m⁻², of whichDaphnia rosea and cyclopoid copepods amounted to 0.69 g dry weight m⁻² and 0.61 g dry weight m⁻², respectively. The production ofD. rosea was high during May–July and October and the level for the whole 9 months was 22.6 g dry weight m⁻².Chaoborus flavicans produced 10 complete and one incomplete cohorts per year. Two consecutive cohorts overlapped during the growing season. The maximum density, the mean biomass, and the production were 19,100 m⁻², 0.81 g dry weight m⁻², and 11.7 g dry weight m⁻²yr⁻¹, respectively. As no fish was present in this pond, the emerging biomass amounted to 69% of larval production. The production ofC. flavicans larvae was high in comparison with zooplankton production during August–September, when the larvae possibly fed not only on zooplankton but also algae.     

P-load,phytoplankton, zooplankton and fish stock in Loosdrecht Lake and Tjeukemeer: confounding effects of predation and food availability

The fish community in the Loosdrecht lakes is dominated by bream, pikeperch and smelt and is characteristic of shallow eutrophic lakes in The Netherlands. The biomasses of the respective fish species amount to ca. 250, 25 and 10 kg ha^–1 and correspond to those in Tjeukemeer, another lake in The Netherlands. The average size of bream, however, is much smaller in the Loosdrecht lakes as a consequence of poorer feeding conditions. The zooplankton community in the Loosdrecht lakes is predominantly composed of relatively small species such as Daphnia cucullata, Bosmina coregoni and cyclopoid copepods, whereas in Tjeukemeer, Daphnia hyalina is permanently present in relatively high densities and the other species show a larger mean length. In the Loosdrecht lakes, the absence of D. hyalina and the smaller sizes of the other zooplankton species could be the consequence of a higher predation pressure, in combination with unfavourable feeding conditions for the zooplankton including the low density of green algae and the high density of filamentous cyanobacteria. A biomanipulation experiment in Lake Breukeleveen, one of the Loosdrecht lakes, indicated that feeding conditions were too unfavourable for large zooplankton to develop in spring, when the reduced fish biomass was not yet supplemented by natural recruitment and immigration.     

The relationship between temperature,competition and the potential for colonization of a subtropical pond by Daphnia magna

The role that temperature plays in excluding large daphnid species from subtropical and tropical ponds, and competitive relationships between Daphnia laevis and Daphnia magna, are explored. D. magna, a large temperate species is probably not excluded from subtropical or tropical systems by elevated temperatures. However, D. magna was a poorer competitor, under a restricted set of laboratory conditions, than D. laevis, the only pond dwelling daphnid in subtropical Florida. Competition is proposed as one mechanism that may limit the number of daphnid species in subtropical and tropical ponds and lakes. Reduced environmental fluctuations in subtropical and tropical systems (compared with temperate systems) may allow zooplankton populations to reach an environmental carrying capacity where competition limits the number of similar, coexisting species in a habitat.     

Zooplankton induced decrease in inorganic phosphorus uptake by plankton in an oligotrophic lake

Experiments conducted on samples collected from a large oligotrophic lake revealed the following: (1) excretion rates of PO _inf4 ^sup3– by single Daphnia thorata were below detection (5 pmol animal^–1 min^–1) in 20 ml of oligotrophic lake water over a period of 10 min, (2) experimental addition of D. thorata to 20 ml aliquots of lake water decreased community-wide microbial uptake of PO _inf4 ^sup3– on two occasions (as measured by ³²PO _inf4 ^sup3– incorporation), and (3) the presence of D. thorata increased uptake by organisms smaller than 1µm, and decreased uptake by large phytoplankton. The specific mechanism for these responses remains unclear, but the results imply that when phytoplankton larger than 1µm encounter cm scale patches of water recently occupied by Daphnia they may experience decreased PO _inf4 ^sup3– availability rather than elevated concentrations of PO _inf4 ^sup3– caused by excretion. We show that ³²P uptake experiments using natural plankton assemblages can be influenced by the presence or absence of large zooplankton, and that neither grazing, turbulence, nor PO _inf4 ^sup3– excretion can account for this influence.     

Disturbance events affecting phytoplankton biomass,composition and species diversity in a shallow,eutrophic, temperate lake

The seasonal changes in phytoplankton biomass and species diversity in a shallow, eutrophic Danish lake are described and related to different disturbance events acting on the phytoplankton community.Both the spring diatom maximum and the summer bloom of the filamentous blue-green alga, Aphanizomenon flos-aquae (L.) Ralfs, coincided with low values of phytoplankton species diversity and equitability. Diatom collapse was mainly due to internal modifications as nutrient depletion (Si, P) caused by rapid growth of phytoplankton, and increased grazing activity from zooplankton. A large population of Daphnia longispina O.F. Müller in June effectively removed smaller algal competitors, thus favouring the development of a huge summer bloom (140 mm³ l^–1) of Aphanizomenon flos-aquae. Heavy rainfall and storms in late July increased the loss of Apahnizomenon by out-flow and disturbed the stratification of the lake. These events caused a marked decline in phytoplankton biomass but had no effect on species diversity. A second storm period in late August circulated the lake completely and was followed by a rapid increase in phytoplankton diversity, and a change in the phytoplankton community structure from dominance of large, slow-growing K-selected species (Aphanizomenon) to small, fast-growing r-selected species (cryptomonads).     

Impact of resuspended sediment on zooplankton feeding in Lake Waihola, New Zealand

1. Wind‐induced sediment resuspension in shallow lakes affects many physical and biological processes, including food gathering by zooplankton. The effects of suspended sediment on clearance rate were determined for a dominant cladoceran, Daphnia carinata, and calanoid copepod, Boeckella hamata, in Lake Waihola, New Zealand. 2. Animals were incubated at multiple densities for 4 days in lake water containing different amounts of suspended lake sediment. Rates of harvest of major food organisms were determined for each sediment level (turbidity) from changes in net growth rate with grazer density. 3. Daphnia cleared all food organisms 7–40 μm in length at similar rates, but was less efficient in its removal of free bacteria, phytoplankton <7 μm, and large cyanobacterial filaments. Elevation of sediment turbidity from 2 to 10 nephelometric turbidity units (NTU) (63 mg DW L^?1 added sediment) reduced Daphnia clearance of phytoplankton, heterotrophic flagellates and ciliates by 72–100%, and of amoebae and attached bacteria by 21–44%. Further inhibition occurred at higher turbidity. 4. Boeckella hamata removed microzooplankton primarily, rather than phytoplankton. The rate at which it cleared rotifers was reduced by 56% when turbidity was increased from 2.5 to 100 NTU. 5. In the absence of macrozooplankton, algal growth increased with sediment turbidity, suggesting that sediment also inhibits rotifer grazing. 6. As mid‐day turbidity in Lake Waihola is ≥10 NTU about 40% of the time, sediment resuspension may play a major role in moderating energy flow and structuring pelagic communities in this lake.     

Top-down control of phytoplankton in a shallow hypertrophic lake: Little Mere (England)

Top-down control of phytoplankton by zooplankton is possible through reductions in density of zooplanktivorous fish. Little Mere is a shallow lake where the effects of sewage effluent caused such a reduction. This allowed the large-bodied cladoceran, Daphnia magna Straus, to develop huge populations, preventing potentially large algal crops from developing.Subsequent diversion of the effluent is anticipated to lead to recovery of the fish community, reduced numbers of large-bodied grazers, and increased phytoplankton biomass. Whether the aquatic plant community, present in Little Mere, is resilient to such changes may depend upon whether cyanophytes are favoured, or not.     

Migrations of haemoglobin-rich Daphnia longispina in a small,steeply stratified,humic lake with an anoxic hypolimnion

Migrations of Daphnia longispina were studied in a small humic lake with an exceptionally shallow oxic epilimnion. Horizontal distributions showed clear avoidance of the shoreline, which might be explained by the lower density of predators (Chaoborus sp. and Notonecta sp.) in the central parts of the lake. In early summer all size classes of D. longispina exhibited upward nocturnal vertical migration, descending to the upper hypolimnion in daytime. Later in summer, when the nocturnally migrating Chaoborus sp. had grown large enough to graze on small Daphnia, the latter seemed to shift towards twilight migration. However, large Daphnia individuals showed no synchronized migration; rather their bimodal vertical distributions suggested asynchronous vertical migration. Large individuals showed a particular tendency to concentrate near to the oxycline, close to the dense phytoplankton and bacteria populations in the upper part of the anoxic hypolimnion. According to vertical trap experiments, large D. longispina visited the anoxic hypolimnion and might harvest its abundant food resources. The high haemoglobin content of large individuals seems a specific adaptation to allow access to low oxygen water and hence to maximize grazing potential, in both epi- and hypolimnion, and minimize predation pressure. By staying predominantly in cooler water near the oxycline, Daphnia might also minimize its energy consumption to adjust to low food availability while sustaining a sufficiently high population density to exploit those unpredictable short periods with abundant food which are common in small headwater lakes. It is suggested that migrations of zooplankton are a complex behavioural adaptation which may not be explained by any single factor. In humic lakes with shallow stratification, vertical migrations seem to offer particularly high potential advantages, because of the short distances between dramatically different environments in the water column. In further studies more emphasis should be placed on migrations of individuals rather than populations, and migrations should be considered as a dynamic part of the structure and function of the whole planktonic ecosystem.     

Ecological implications of parasites in natural <Emphasis Type="Italic">Daphnia</Emphasis> populations

In natural host populations, parasitism is considered to be omnipresent and to play an important role in shaping host life history and population dynamics. Here, we study parasitism in natural populations of the zooplankton host Daphnia magna investigating their individual and population level effects during a 2-year field study. Our results revealed a rich and highly prevalent community of parasites, with eight endoparasite species (four microsporidia, one amoeba, two bacteria and one nematode) and six epibionts (belonging to five different taxa: Chlorophyta, Bacillariophyceae, Ciliata, Fungi and Rotifera). Several of the endoparasites were associated with a severe overall fecundity reduction of the hosts, while such effects were not seen for epibionts. In particular, infections by Pasteuria ramosa, White Fat Cell Disease and Flabelliforma magnivora were strongly associated with a reduction in overall D. magna fecundity. Across the sampling period, average population fecundity of D. magna was negatively associated with overall infection intensity and total endoparasite richness. Population density of D. magna was negatively correlated to overall endoparasite prevalence and positively correlated with epibiont richness. Finally, the reduction in host fecundity caused by different parasite species was negatively correlated to both parasite prevalence and the length of the time period during which the parasite persisted in the host population. Consistent with epidemiological models, these results indicate that parasite mediated host damages influence the population dynamics of both hosts and parasites.     

Hypolimnetic anoxia hampers top–down food-web manipulation in a eutrophic lake

SUMMARY 1. A biomanipulation experiment was carried out in a small (10 ha), but relatively deep (17 m) and highly eutrophic lake in northern Poland. The lake had been stocked in 1996, 1997 and 1998 with a variety of piscivorous fish (pike, catfish, trout and pikeperch), in order to reduce numbers of cyprinid planktivores.
2. Piscivore stocking was associated with a threefold decrease in the offshore fish density (night echosounding). Despite this reduction, the large planktonic cladoceran, Daphnia hyalina , remained scarce, whereas the density of small-sized zooplankton increased greatly.
3. The lack of demographic response in D. hyalina was probably due to the anoxia in the hypolimnetic refuge of this vertically migrating species. The anoxic hypolimnion, below 3–4 m depth, was inhabited day and night by numerous Chaoborus flavicans larvae.
4. Changes in zooplankton were associated with shifts in the taxonomic composition (from single-cell green algae to filamentous cyanobacteria), size structure (from nano- to net phytoplankton) and seasonal dynamics of phytoplankton, but not in the average biomass of planktonic algae. A clear-water phase, which was absent in the prestocking years, developed in spring, with Secchi depth reaching 2.5 m, a value which had never been recorded in the 20 years preceding the biomanipulation. In general, the lake's status was switched from hypertrophic to eutrophic.
5. Deteriorating food conditions, resulting from qualitative changes in the phytoplankton community, combined with predation pressure by the remaining fish and Chaoborus larvae were associated with the ultimate elimination of D. hyalina from the lake.     

Grazer-induced changes in the desmid Staurastrum

In aquatic environments, predator kairomones have been shown to affect morphology of prey species. Past work on the interaction between zooplankton and phytoplankton was based mainly on the Daphnia–Scenedesmus model. Algae of the genus Staurastrum can produce mucilage, causing cell clumping and settling out of the water column. These clumps are too large to be eaten by daphniids. Thus, we hypothesised that this may be a grazer defence. We investigated whether Daphnia magna induces the formation of mucus globules in Staurastrum, how this occurs, and if the formation of clumps inhibits growth in juvenile Daphnia. Eight strains of Staurastrum were used to check whether mucus extrusion is induced by the presence of Daphniaor possibly by a chemical excreted by Daphnia magna. None of the strains reacted to the presence of Daphnia water alone, animals had to be present to induce clumping. Mechanical action (gentle stirring) caused the same strains to clump. The ecological relevance of clumping was then investigated. The different Staurastrum strains were used as food in a growth experiment with ecologically relevant densities of neonates of Daphnia hyalina. These small daphniids did not cause the same clumping observed for Daphnia magna when present in experiments at high densities. We observed that juvenile daphniids grew less well on strains with larger cell size.     

Plankton community of a polyhumic lake with and without Daphnia longispina (Cladocera)

The impact of Daphnia longispina (Cladocera) on the plankton food web was studied in a polyhumic lake where this species comprised almost all zooplankton biomass. Plastic enclosures (volume 7 m³) were inserted into the lake retaining the initial water stratification except that in one enclosure zooplankton was removed. After the removal of Daphniaa rotifer, Keratella cochlearis, ciliates and heterotrophic nanoflagellates increased markedly and the density and biomass of bacteria decreased. Edible algal species, Cryptomonas rostratiformisand three small chrysophytes,Ochromonas, Pedinella and Spinifermonas, took advantage of the removal of Daphnia, while more grazing-resistant species declined. In spite of the changes in the species composition of phytoplankton, the removal of Daphnia did not affect the biomass, primary production or respiration of plankton. The results implied that the density of heterotrophic flagellates and ciliates was controlled by Daphnia, but in its absence the former took its role as the bacterial grazers.