Responses of cyanobacteria to herbivorous zooplankton across predator regimes: who mows the bloom?

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Algal Diet of Small-Bodied Crustacean Zooplankton in a Cyanobacteria-Dominated Eutrophic Lake

Small-bodied cladocerans and cyclopoid copepods are becoming increasingly dominant over large crustacean zooplankton in eutrophic waters where they often coexist with cyanobacterial blooms. However, relatively little is known about their algal diet preferences. We studied grazing selectivity of small crustaceans (the cyclopoid copepods Mesocyclops leuckarti, Thermocyclops oithonoides, Cyclops kolensis, and the cladocerans Daphnia cucullata, Chydorus sphaericus, Bosmina spp.) by liquid chromatographic analyses of phytoplankton marker pigments in the shallow, highly eutrophic Lake Võrtsjärv (Estonia) during a seasonal cycle. Copepods (mainly C. kolensis) preferably consumed cryptophytes (identified by the marker pigment alloxanthin in gut contents) during colder periods, while they preferred small non-filamentous diatoms and green algae (identified mainly by diatoxanthin and lutein, respectively) from May to September. All studied cladoceran species showed highest selectivity towards colonial cyanobacteria (identified by canthaxanthin). For small C. sphaericus, commonly occuring in the pelagic zone of eutrophic lakes, colonial cyanobacteria can be their major food source, supporting their coexistence with cyanobacterial blooms. Pigments characteristic of filamentous cyanobacteria and diatoms (zeaxanthin and fucoxanthin, respectively), algae dominating in Võrtsjärv, were also found in the grazers’ diet but were generally avoided by the crustaceans commonly dominating the zooplankton assemblage. Together these results suggest that the co-occurring small-bodied cyclopoid and cladoceran species have markedly different algal diets and that the cladocera represent the main trophic link transferring cyanobacterial carbon to the food web in a highly eutrophic lake.     

A study of the zooplankton community of Billings Reservoir — São Paulo

The composition of the zooplankton of the Billings Reservoir and its variation in an eutrophic environment, subject to frequent blooms of algae (chiefly Cyanophyceae) was studied during one year (from October, 1977 to September, 1978) in two stations in the littoral and in the limnetic zone.The zooplankton community in the limnetic zone was dominated by cyclopoid copepods (Thermocyclops crassus and Metacyclops mendocinus) and by rotifers (Brachionus, Polyarthra and others) which represented, respectively 38.5 and 35.5% of the total zooplankton. At the littoral zone, cyclopoids were the most abundant (42.3%).The cladocerans were the least significant group at both stations, and calanoid copepods were found only at the littoral zone.A higher production of small filtrators, such as rotifers, cyclopoid nauplii and Bosmina sp was observed.     

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.     

The zooplankton-phytoplankton interface in lakes of contrasting trophic status: an experimental comparison

We report here the results of an experimental study designed to compare algal responses to short-term manipulations of zooplankton in three California lakes which encompass a broad range of productivity (ultra-oligotrophic Lake Tahoe, mesotrophic Castle Lake, and strongly eutrophic Clear Lake). To assess the potential strength of grazing in each lake, we evaluated algal responses to a 16-fold range of zooplankton biomass. To better compare algal responses among lakes, we determined algal responses to grazing by a common grazer (Daphnia sp.) over a range ofDaphnia densities from 1 to 16 animals per liter. Effects of both ambient grazers andDaphnia were strong in Castle Lake. However, neither ambient zooplankton norDaphnia had much impact on phytoplankton in Clear Lake. In Lake Tahoe, no grazing impacts could be demonstrated for the ambient zooplankton butDaphnia grazing had dramatic effects. These results indicate weak coupling between phytoplankton and zooplankton in Clear Lake and Lake Tahoe, two lakes which lie near opposite extremes of lake trophic status for most lakes. These observations, along with work reported by other researchers, suggest that linkages between zooplankton and phytoplankton may be weak in lakes with either extremely low or high productivity. Biomanipulation approaches to recover hypereutrophic lakes which aim only to alter zooplankton size structure may be less effective if algal communities are dominated by large, inedible phytoplankton taxa.     

Different Predation Impacts of Two Cyclopoid Species on a Small-sized Zooplankton Community: An Experimental Analysis with Mesocosms

We used mesocosms to analyze predation impacts on the prey populations and prey community structures by two cyclopoid copepod species, the larger Mesocyclops pehpeiensis and the smaller Thermocyclops taihokuensis, who coexist with small-sized herbivorous zooplankton species in a fish-abundant lake. The overall predation impact on the prey populations was stronger for Mesocyclops than for Thermocyclops. Mesocyclops had a strong and less selective impact on the rotifer community but a selective impact on the crustaceans. In contrast, Thermocyclops had a selective predation impact on rotifers but a weak and less selective impact on the crustacean community. As a result, the former predator reduced the diversity of the crustacean community but not the rotifer community, while the latter had an opposite impact on the diversities of the two communities. It has been suggested that fish induce development of a zooplankton community dominated by the small-sized zooplankton species in fish-abundant lakes. Our results demonstrated that cyclopoid copepods altered species composition and diversity of the small-sized zooplankton community in such lakes. Thus, the results have given an important suggestion on the role of the invertebrate predator cyclopoid copepods, which often coexist with fish, that they determine population dynamics and community structures of small-sized zooplankton in fish-abundant lakes.     

The role of light for fish–zooplankton–phytoplankton interactions during winter in shallow lakes – a climate change perspective

1. Variations in the light regime can affect the availability and quality of food for zooplankton grazers as well as their exposure to fish predation. In northern lakes light is particularly low in winter and, with increasing warming, the northern limit of some present-day plankton communities may move further north and the plankton will thus receive less winter light.
2. We followed the changes in the biomass and community structure of zooplankton and phytoplankton in a clear and a turbid shallow lake during winter (November–March) in enclosures both with and without fish and with four different light treatments (100%, 55%, 7% and <1% of incoming light).
3. In both lakes total zooplankton biomass and chlorophyll- a were influenced by light availability and the presence of fish. Presence of fish irrespective of the light level led to low crustacean biomass, high rotifer biomass and changes in the life history of copepods. The strength of the fish effect on zooplankton biomass diminished with declining light and the effect of light was strongest in the presence of fish.
4. When fish were present, reduced light led to a shift from rotifers to calanoid copepods in the clear lake and from rotifers to cyclopoid copepods in the turbid lake. Light affected the phytoplankton biomass and, to a lesser extent, the phytoplankton community composition and size. However, the fish effect on phytoplankton was overall weak.
5. Our results from typical Danish shallow eutrophic lakes suggest that major changes in winter light conditions are needed in order to have a significant effect on the plankton community. The change in light occurring when such plankton communities move northwards in response to global warming will mostly be of modest importance for this lake type, at least for the rest of this century in an IPCC A2 scenario, while stronger effects may be observed in deep lakes.     

Eutrophication mediates rapid clonal evolution in Daphnia pulicaria

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Impact of selective predation by Mesocyclops pehpeiensis on a zooplankton community: experimental analysis using mesocosms

Predation by cyclopoid copepods is an important factor affecting zooplankton communities in freshwater habitats. Experiments provide strong evidence of the role of selective predation by cyclopoid copepods in structuring zooplankton communities. To assess the predation impact of a cyclopoid copepod, Mesocyclops pehpeiensis, we conducted a mesocosm experiment using 20-l polyethylene tanks in which the density of the predator and the food available to herbivorous zooplankton varied. M. pehpeiensis had a notable but selective effect on the zooplankton community. The population of a small cladoceran, Bosmina fatalis was affected negatively, but M. pehpeiensis did not have any apparent impact on the population dynamics of another Bosmina species, B. longirostris. On the other hand, the population of small rotifers responded positively to the presence of M. pehpeiensis, and their densities increased in mesocosms with a high density of M. pehpeiensis. It seems that suppression of B. fatalis by M. pehpeiensis predation indirectly affected rotifers by releasing them from competition with B. fatalis. The results suggest that copepod predation is a powerful factor regulating zooplankton communities directly and indirectly.     

Selective predation on zooplankton by pelagic Arctic charr, Salvelinus alpinus, in six subarctic lakes

Selective predation by planktivore fish appears to be an important regulatory factor of zooplankton communities, potentially causing large changes in species composition and size distributions within populations. In this study, prey preferences and size-selective predation on zooplankton by Arctic charr were examined in six subarctic lakes with Arctic charr as the dominant pelagic fish species. Most of the lakes had a zooplankton community dominated by copepods (Cyclops scutifer and Eudiaptomus graciloides), but the pelagic charr evidently selected cladoceran species (Bythotrephes longimanus, Daphnia sp. and Bosmina sp.), likely because the copepods have a higher mobility and evasiveness than the cladocerans. Furthermore, a strong size selection was also revealed for both Bosmina sp. and Daphnia sp., as individual prey from Arctic charr stomachs were exclusively larger than individuals sampled in the environment. Additionally, visibility due to size, morphology and pigmentation (egg-carrying females) was also a major factor for the selection of zooplankton prey. In conclusion, Arctic charr was found to be highly selective on zooplankton both in respect to species composition and individual size of Bosmina sp. and Daphnia sp.     

Linking Cascading Effects of Fish Predation and Zooplankton Grazing to Reduced Cyanobacterial Biomass and Toxin Levels Following Biomanipulation

Eutrophication has been one of the largest environmental problems in aquatic ecosystems during the past decades, leading to dense, and often toxic, cyanobacterial blooms. In a way to counteract these problems many lakes have been subject to restoration through biomanipulation. Here we combine 13 years of monitoring data with experimental assessment of grazing efficiency of a naturally occurring zooplankton community and a, from a human perspective, desired community of large Daphnia to assess the effects of an altered trophic cascade associated with biomanipulation. Lake monitoring data show that the relative proportion of Daphnia spp. grazers in June has increased following years of biomanipulation and that this increase coincides with a drop in cyanobacterial biomass and lowered microcystin concentrations compared to before the biomanipulation. In June, the proportion of Daphnia spp. (on a biomass basis) went from around 3% in 2005 (the first year of biomanipulation) up to around 58% in 2012. During months when the proportion of Daphnia spp. remained unchanged (July and August) no effect on lower trophic levels was observed. Our field grazing experiment revealed that Daphnia were more efficient in controlling the standing biomass of cyanobacteria, as grazing by the natural zooplankton community never even compensated for the algal growth during the experiment and sometimes even promoted cyanobacterial growth. Furthermore, although the total cyanobacterial toxin levels remained unaffected by both grazer communities in the experimental study, the Daphnia dominated community promoted the transfer of toxins to the extracellular, dissolved phase, likely through feeding on cyanobacteria. Our results show that biomanipulation by fish removal is a useful tool for lake management, leading to a top-down mediated trophic cascade, through alterations in the grazer community, to reduced cyanobacterial biomass and lowered cyanobacterial toxin levels. This improved water quality enhances both the ecological and societal value of lakes as units for ecosystem services.     

Zooplankton interactions in an enclosure experiment: insights from stable isotope analyses

1. Density gradients of cladocerans and copepods were generated in an enclosure experiment to compare the impact on the plankton of a filter feeder (Daphnia hyalina × galeata) with that of more selective feeders (calanoid and cyclopoid copepods). The experiment was conducted in situ over 25 days during spring in a mesotrophic lake, Schöhsee, Germany. 2. The plankton community was monitored regularly. Daphniids were able to graze on the phytoplankton present, which mainly consisted of small (<1000 μm³) species, whereas copepods did not show any impact on algae. 3. At the end of the experiment, Daphnia and remaining cyclopoid copepods were harvested and sorted manually, prior to analyses for stable isotopes of carbon and nitrogen. Daphniids from mesocosms stocked purely with differing densities of Daphnia showed little variability in stable isotope values, whereas those that thrived in enclosure bags together with copepods exhibited lower δ¹³C values. 4. The change in Daphniaδ¹³C indicates a change of food sources, modified by the presence of the copepods: the higher the mean abundance of copepods in the enclosures, the more ¹³C‐depleted the daphniids. Increasing abundance of high nucleic acid (HNA) bacteria in the copepod bags may account for the trend in Daphniaδ¹³C via increased grazing on the bacteria themselves, or via grazing on phytoplankton utilising isotopically light CO₂ from respiratory release. 5. Cyclopoid copepod stable isotope signatures were related to Daphnia and copepod abundances in copepod bags, suggesting that cyclopoids preyed on the available zooplankton.     

An experimental study of the effects of nutrient supply and Chaoborus predation on zooplankton communities of a shallow tropical reservoir (Lake Brobo, Côte d'Ivoire)

1. Based on two mesocosm experiments and 10 in vitro predation experiments, this work aimed to evaluate the impact of nutrient supply and Chaoborus predation on the structure of the zooplankton community in a small reservoir in Côte d'Ivoire. 2. During the first mesocosm experiment (M1), P enrichment had no effect on phytoplankton biomass (chlorophyll a) but significantly increased the biomass of some herbivorous zooplankton species (Filinia sp, Ceriodaphnia affinis). During the second experiment (M2), N and P enrichment greatly increased phytoplankton biomass, rotifers and cladocerans (C. affinis, C. cornuta, Moina micrura and Diaphanosoma excisum). In both experiments, nutrient addition had a negative impact on cyclopoid copepods. 3. Larger zooplankton, such as cladocerans or copepodites and adults of Thermocyclops sp., were significantly reduced in enclosures with Chaoborus in both mesocosm experiments, whereas there was no significant reduction of rotifers and copepod nauplii. This selective predation by Chaoborus shaped the zooplankton community and modified its size structure. In addition, a significant Chaoborus effect on chlorophyll a was shown in both experiments. 4. The preference of Chaoborus for larger prey was confirmed in the predation experiments. Cladocerans D. excisum and M. micrura were the most selected prey. Rotifer abundance was not significantly reduced in any of the 10 experiments performed. 5. In conclusion, both bottom‐up and top‐down factors may exert a structuring control on the zooplankton community. Nutrients favoured more strictly herbivorous taxa and disadvantaged the cyclopoid copepods. Chaoborus predation had a strong direct negative impact on larger crustaceans, favoured small herbivores (rotifer, nauplii) and seemed to cascade down to phytoplankton.     

Feeding convergence in South American and African zooplanktivorous cichlids Geophagus brasiliensis and Tilapia rendalli

Synopsis Acará, Geophagus brasiliensis, and red-breasted bream, Tilapia rendalli, are important planktivorous cichlids in southern Brazilian lakes and reservoirs. In laboratory experiments, I quantified behavior and selectivity of different sizes of these two fish feeding on lake zooplankton. Feeding behavior depended on fish size. Fish < 30 mm were visual feeders. Fish 30–50 mm either visually fed or pump-filter fed depending on zooplankton size. Fish > 70 mm were pump-filter feeders. Replicate 1 h feeding trials revealed that, as the relative proportions of prey changed during an experiment, acará (30–42 mm, standard length) and tilapia (29–42 mm) shifted from visual feeding on large evasive copepods to filter feeding on small cladocerans and rotifers. Electivity and feeding rate increased with prey length, but were distinct for similar-sized cladocerans and copepods. Visual/filter-feeding fish had lowest electivities for small and poorly evasive rotifers and cyclopoid nauplii. They fed non-selectively on cyclopoid copepodites, had intermediate electivities for calanoid nauplii and small cladocerans, and had highest electivities for large cladocerans, cyclopoid adults, and calanoid copepodites and adults. Although belonging to different cichlid genera and native to South America and Africa, respectively, acará and red-breasted bream (= congo tilapia) exhibited similar selectivity for zooplankton. Apparently, few stereotyped feeding behaviors have evolved during the acquisition of microphagy in fish. Shift in feeding modes allows these two species to optimally exploit the variable and dynamic patchy distribution of planktonic resources.     

The dynamics and role of limnetic zooplankton in Loosdrecht lakes (The Netherlands)

The paper summarizes the results of a ten-year (1981–1991) zooplankton research on the Lake Loosdrecht, a highly eutrophic lake. The main cause of the lake's eutrophication and deteriorating water quality was supply up to mid 1984 of water from the River Vecht. This supply was replaced by dephosphorized water from the Amsterdam-Rhine Canal in 1984. The effects of this and other restoration measures on the lake's ecosystem were studied. Despite a reduction in the external P-load from ca. 1.0 g P m^–2 y^–1 to ca. 0.35 g m^–2 y^–1 now, the filamentous prokaryotes, including cyanobacteria and Prochlorothrix, continue to dominate the phytoplankton.Among the crustacean plankton Bosmina spp, Chydorus sp. and three species of cyclopoid copepods and their nauplii are quite common. Though there was no major change in the composition of abundant species, Daphnia cucullata, which is the only daphnid in these lakes, became virtually extinct since 1989. Among about 20 genera and 40 species of rotifers the important ones are: Anuraeopsis fissa, Keratella cochlearis, Filinia longiseta and Polyarthra. The rotifers usually peak in mid-summer following the crustacean peak in spring. The mean annual densities of crustaceans decreased during 1988–1991. Whereas seston (< 150 µm) mean mass in the lake increased since 1983 by 20–60%, zooplankton (> 150 µm) mass decreased by 15–35%.The grazing by crustacean community, which was attributable mainly to Bosmina, had mean rates between 10 and 25% d^–1. Between 42 and 47% of the food ingested was assimilated. In spring and early summer when both rotifers and crustaceans have their maximal densities the clearance rates of the rotifers were much higher. Based on C/P ratios, the zooplankton (> 150 µm) mass contained 2.5 times more phosphorus than seston (< 150 µm) mass so that the zooplankton comprised 12.5 % of the total-P in total particulate matter in the open water, compared with only 4.5% of the total particulate C. The mean excretion rates of P by zooplankton varied narrowly between 1.5 and 1.8 µg P 1^– d^–1, which equalled between 14 and 28% d^–1 of the P needed for phytoplankton production.The lack of response to restoration measures cannot be ascribed to one single factor. Apparently, the external P-loading is still not low enough and internal P-loading, though low, may be still high enough to sustain high seston levels. Intensive predation by bream is perhaps more important than food quality (high concentrations of filamentous cyanobacteria) in depressing the development of large-bodied zooplankton grazers, e.g. Daphnia. This may also contribute to resistance of the lake's ecosystem to respond to rehabilitation measures.     

Beneficial and detrimental interactive effects of dissolved organic matter and ultraviolet radiation on Zooplankton in a transparent lake

While changes in dissolved organic matter (DOM) concentrations are expected to affect zooplankton species through attenuation of potentially damaging ultraviolet (UV) radiation, generation of potentially beneficial or harmful photoproducts, pH alteration, and microbial food web stimulation, the combined effects of such changes on zooplankton community structure have not been studied previously. Our purpose was to determine how an increase in allochthonous DOM and associated changes in pH in an initially transparent lake may affect zooplankton community structure, and how exposure to solar UV may alter these DOM and pH effects. We ran microcosm experiments manipulating UV, DOM, and pH near the surface of Lake Giles in northeastern Pennsylvania. We found that when DOM was added in the presence of ambient UV, Daphnia and copepod UV-mortality was reduced by approximately three and two times compared to UV exposure without extra DOM. When DOM was added in the absence of UV, adult Daphnia and copepods were reduced compared to no DOM addition in the absence of UV. Daphnia and cyclopoid egg production and rotifer abundance were generally higher in the presence of DOM, regardless of UV treatment. The lower abundance yet high egg production in the presence of DOM and absence of UV may be explained by higher abundance of egg-bearing adults compared to non-egg-bearers. We conclude that allochthonous DOM benefits some zooplankton in a high-UV environment, but may be detrimental under low-UV conditions. Overall, Daphnia abundance and egg production were higher than that of calanoid copepods in the DOM additions, indicating that in some lakes an increase in allochthonous DOM may lead to a zooplankton community shift favoring Daphnia over calanoid copepods.     

Impact of fish predation on cladoceran body weight distribution and zooplankton grazing in lakes during winter

1. It is well accepted that fish, if abundant, can have a major impact on the zooplankton community structure during summer, which, particularly in eutrophic lakes, may cascade to phytoplankton and ultimately influence water clarity. Fish predation affects mean size of cladocerans and the zooplankton grazing pressure on phytoplankton. Little is, however, known about the role of fish during winter. 2. We analysed data from 34 lakes studied for 8–9 years divided into three seasons: summer, autumn/spring and winter, and four lake classes: all lakes, shallow lakes without submerged plants, shallow lakes with submerged plants and deep lakes. We recorded how body weight of Daphnia and then cladocerans varied among the three seasons. For all lake types there was a significant positive correlation in the mean body weight of Daphnia and all cladocerans between the different seasons, and only in lakes with macrophytes did the slope differ significantly from one (winter versus summer for Daphnia). 3. These results suggest that the fish predation pressure during autumn/spring and winter is as high as during summer, and maybe even higher during winter in macrophyte‐rich lakes. It could be argued that the winter zooplankton community structure resembles that of the summer community because of low specimen turnover during winter mediated by low fecundity, which, in turn, reflects food shortage, low temperatures and low winter hatching from resting eggs. However, we found frequent major changes in mean body weight of Daphnia and cladocerans in three fish‐biomanipulated lakes during the winter season. 4. The seasonal pattern of zooplankton : phytoplankton biomass ratio showed no correlation between summer and winter for shallow lakes with abundant vegetation or for deep lakes. For the shallow lakes, the ratio was substantially higher during summer than in winter and autumn/spring, suggesting a higher zooplankton grazing potential during summer, while the ratio was often higher in winter in deep lakes. Direct and indirect effects of macrophytes, and internal P loading and mixing, all varying over the season, might weaken the fish signal on this ratio. 5. Overall, our data indicate that release of fish predation may have strong cascading effects on zooplankton grazing on phytoplankton and water clarity in temperate, coastal situated eutrophic lakes, not only during summer but also during winter.     

Microcystin mcyA and mcyE Gene Abundances Are Not Appropriate Indicators of Microcystin Concentrations in Lakes

Cyanobacterial harmful algal blooms (cyanoHABs) are a primary source of water quality degradation in eutrophic lakes. The occurrence of cyanoHABs is ubiquitous and expected to increase with current climate and land use change scenarios. However, it is currently unknown what environmental parameters are important for indicating the presence of cyanoHAB toxins making them difficult to predict or even monitor on time-scales relevant to protecting public health. Using qPCR, we aimed to quantify genes within the microcystin operon (mcy) to determine which cyanobacterial taxa, and what percentage of the total cyanobacterial community, were responsible for microcystin production in four eutrophic lakes. We targeted Microcystis-16S, mcyA, and Microcystis, Planktothrix, and Anabaena-specific mcyE genes. We also measured microcystins and several biological, chemical, and physical parameters—such as temperature, lake stability, nutrients, pigments and cyanobacterial community composition (CCC)—to search for possible correlations to gene copy abundance and MC production. All four lakes contained Microcystis-mcyE genes and high percentages of toxic Microcystis, suggesting Microcystis was the dominant microcystin producer. However, all genes were highly variable temporally, and in few cases, correlated with increased temperature and nutrients as the summer progressed. Interestingly, toxin gene abundances (and biomass indicators) were anti-correlated with microcystin in all lakes except the largest lake, Lake Mendota. Similarly, gene abundance and microcystins differentially correlated to CCC in all lakes. Thus, we conclude that the presence of microcystin genes are not a useful tool for eliciting an ecological role for toxins in the environment, nor are microcystin genes (e.g. DNA) a good indicator of toxins in the environment.     

Zooplankton community structure and environmental conditions in a set of interconnected ponds

We studied the zooplankton community structure in a set of 33 interconnected shallow ponds that are restricted to a relatively small area (`De Maten', Genk, Belgium, 200 ha). As the ponds share the same water source, geology and history, and as the ponds are interconnected (reducing chance effects of dispersal with colonisation), differences in zooplankton community structure can be attributed to local biotic and abiotic interactions. We studied zooplankton community, biotic (phytoplankton, macrophyte cover, fish densities, macroinvertebrate densities), abiotic (turbidity, nutrient concentrations, pH, conductivity, iron concentration) and morphometric (depth, area, perimeter) characteristics of the different ponds. Our results indicate that the ponds differ substantially in their zooplankton community structure, and that these differences are strongly related to differences in trophic structure and biotic interactions, in concordance with the theory of alternative equilibria. Ponds in the clear-water state are characterised by large Daphnia species and species associated with the littoral zone, low chlorophyll-a concentrations, low fish densities and high macroinvertebrate densities. Ponds in the turbid-water state are characterised by high abundances of rotifers, cyclopoid copepods and the opposite environmental conditions. Some ponds show an intermediate pattern, with a dominance of small Daphnia species. Our results show that interconnected ponds may differ strongly in zooplankton community composition, and that these differences are related to differences in predation intensity (top-down) and habitat diversity (macrophyte cover).     

Only the small survive: monitoring long-term changes in the zooplankton community of an Alpine lake after fish introduction

The zooplankton community of Alpine lake Seehornsee (1,779 m a.s.l.) was studied over a period of 13 years. In 1994, a typical high-altitude zooplankton community, consisting of two calanoid copepods (Mixodiaptomus laciniatus, Arctodiaptomus alpinus), one cladoceran (Daphnia rosea), and two rotifers (Keratella quadrata, Synchaeta pectinata) coexisted with infertile charr hybrids, which had been introduced in 1969 and again in 1974. When the aged fish were removed by intensive gill netting, they had fed predominantly on aquatic insects. After a fish-free period of 4 years, 2000 fertile juvenile Alpine charr (Salvelinus umbla) were stocked in 1998 and again in 1999. They preyed on benthic (chydorids, ostracods, cyclopoid copepods, chironomid larvae and pupae) and planktonic prey (diaptomid copepods, Daphnia). Between 2004 and 2006 charr successfully reproduced. Nine years after stocking of fertile charr, the two calanoids had virtually disappeared, and Daphnia rosea had notably declined in abundance. In concordance with the size efficiency hypothesis (Brooks and Dodson 1965), the newly appearing and smaller cladoceran Ceriodaphnia pulchella, together with the two resident, and two emerging species of rotifers (Polyarthra luminosa, Gastropus stylifer) dominated the zooplankton community.