Cladoceran Biomass Research Articles

Is Recovery of Large-Bodied Zooplankton after Nutrient Loading Reduction Hampered by Climate Warming? A Long-Term Study of Shallow Hypertrophic Lake Søbygaard, Denmark

Nutrient fluctuations and climate warming can synergistically affect trophic dynamics in lakes, resulting in enhanced symptoms of eutrophication, thereby potentially counteracting restoration measures. We performed a long-term study (23 years) of zooplankton in Danish Lake Søbygaard, which is in recovery after nutrient loading reduction, but now faces the effects of climate warming. We hypothesized that the recovery of large-bodied zooplankton after nutrient loading reduction would be hampered by climate warming through indirect effects on fish size structure. We found a shift in macrozooplankton from initial dominance of Daphnia spp. towards Bosmina spp. as well as a decline in the body size of copepods and an increase in the abundance of nauplii. These changes coincided with the increase in small sized fish as a result of rising water temperature. Despite a reduction in body size, the total biomass of cladocerans increased coinciding with a diminished fish catch per unit effort (CPUE), and likely then an overall reduction in the predation on zooplankton. A cascading effect to phytoplankton was evidenced by enhanced zooplankton:phytoplankton and cladoceran:phytoplankton ratios and a decrease in Chl-a:TP and Chl-a:TN ratios. Our results indicate that climate warming, through changes in the size structure of fish community, has major effects on zooplankton size structure. In Lake Søbygaard, the decline in zooplankton size did not prevent, but modulated, the positive cascading effect on phytoplankton through an expected diminished fish CPUE related to nutrient loading reduction.

Can microcystins affect zooplankton structure community in tropical eutrophic reservoirs?

The aim of our study was to assess whether cyanotoxins (microcystins) can affect the composition of the zooplankton community, leading to domination of microzooplankton forms (protozoans and rotifers). Temporal variations in concentrations of microcystins and zooplankton biomass were analyzed in three eutrophic reservoirs in the semi-arid northeast region of Brazil. The concentration of microcystins in water proved to be correlated with the cyanobacterial biovolume, indicating the contributions from colonial forms such as Microcystis in the production of cyanotoxins. At the community level, the total biomass of zooplankton was not correlated with the concentration of microcystin (r2 = 0.00; P > 0.001), but in a population-level analysis, the biomass of rotifers and cladocerans showed a weak positive correlation. Cyclopoid copepods, which are considered to be relatively inefficient in ingesting cyanobacteria, were negatively correlated (r2 = - 0.01; P > 0.01) with the concentration of cyanotoxins. Surprisingly, the biomass of calanoid copepods was positively correlated with the microcystin concentration (r2 = 0.44; P > 0.001). The results indicate that allelopathic control mechanisms (negative effects of microcystin on zooplankton biomass) do not seem to substantially affect the composition of mesozooplankton, which showed a constant and high biomass compared to the microzooplankton (rotifers). These results may be important to better understand the trophic interactions between zooplankton and cyanobacteria and the potential effects of allelopathic compounds on zooplankton.

Changes in the structural parameters of the summer zooplankton in the pelagic zone of Lake Sevan (Armenia) during an increase in fish abundance and biomass

Features of the composition and structure of the summer zooplankton in the pelagic zone of Lake Sevan in years that were characterized by different abundances and biomasses of fish are described. It is shown that the species diversity of the community and the specific number of species of zooplankteurs, the quantity of Copepods in the total abundance and biomass of zooplankton, as well as the value of the Shannon index and the trophic coefficient, increased upon an increase in the fish abundance. At the same time, the total biomass of zooplankton and biomass of cladocerans decreased, among which the density and biomass of the larger and more colored Daphnia (Ctenodaphnia) magna Straus decreased, but the density and biomass of D. (Daphnia) hyalina Leydig and Diaphаnosoma brachyurum Lievin increased.

Experimental effects of large-bodied Daphnia, fish and zebra mussels on cladoceran community and size structure

Understanding the factors that regulate the abundance, size structure and community structure of cladocerans is an important goal of aquatic ecologists. While both top-down and bottom-up factors help to structure cladoceran communities, there may be interactions between these factors. We conducted a mesocosm study to determine how alien large-bodied Daphnia, zebra mussels and fish affected cladoceran community and size structure. We found that large-bodied Daphnia reduced algal resources and the fecundity of smaller bodied cladocerans. Fish removed the large-bodied Daphniamagna from the mesocosms and shifted the cladoceran community to a smaller body size. Fish also appeared to promote increases in cladoceran diversity through the coexistence of several smaller bodied taxa. In contrast, zebra mussels increased cyanobacteria and helped to promote the success of the alien Daphnia, but reduced the biomass of small-bodied cladocerans. Zebra mussels reduced the carbon (C):phosphorus (P) ratio of the phytoplankton in the mesocosms which may have favored the relatively P-limited Daphnia. Combined, our results highlight the complex interactions of multiple factors that help to regulate cladoceran community and size structure.

Trophic status may influence top–down effects of anadromous alewife Alosa pseudoharengus (Actinopterygii, Clupeidae) in lakes

We investigated the seasonal influence of an effective size-selective planktivore, the anadromous alewife (Alosa pseudoharengus Wilson), on summer zooplankton and phytoplankton communities in a series of lakes in Maine, USA (4 with and 4 without alewife) that ranged from oligotrophic to eutrophic to determine the role of lake trophic state in influencing the relative strength of top–down forces on phytoplankton biomass. Predation by young of the year alewife reduced the mean body length of cladoceran and copepod biomass from spring to summer in alewife lakes, while mean body length remained unchanged in nonalewife lakes. Cladoceran biomass decreased substantially from spring to summer in 3 of 4 alewife lakes, increasing only in the most eutrophic lake. Conversely, cladoceran biomass increased from spring to summer in 3 of 4 nonalewife lakes. Predation by alewife on zooplankton did not have consistent cascading effects on phytoplankton biomass. Analysis of planktonic biomass ratios (phytoplankton biomass: zooplankton biomass) suggests that cascading effects were stronger in oligotrophic systems and weakened with increasing trophic status, as ratios in alewife and nonalewife lakes converged at higher total phosphorous levels. Our results suggest that lake trophic status may influence the relative importance of top–down control of both zooplankton and phytoplankton biomass. The mechanisms that explain this pattern remain elusive and likely require additional efforts to estimate alewife densities, rates of zooplankton and phytoplankton production relative to consumption, and the influence of other physical lake features.

Impacts of large-bodied crustaceans on the microbial loop

We conducted a mesocosm experiment to assess the impacts of large-bodied crustaceans on microbial communities. Three alien crustacean species (Daphnia pulex, Simocephalus vetulus and Macrocyclops albidus) were collected from the regional species pool and added to mesocosms that were filled with water from a eutrophic lake (Masurian Lake District, Poland). We then analysed chemical (total phosphorus and nitrogen concentrations) and biological (algae, bacteria, nanoflagellates, ciliates, rotifers, crustaceans) parameters over the course of the 40 day experiment. Alien crustacean species constituted 59–88% of the total crustacean biomass throughout the experiment. The final biomass of bacteria and copepods were not affected by the addition of alien species. However, rotifer and native cladoceran biomass tended to be lower while nanoflagellate and ciliate biomass were higher in mesocosms with alien species. Our research suggests that the large-bodied crustaceans altered the structure of the microbial loop. In the control, nanoflagellates were likely the main consumers of bacteria and thus constituted the main link between bacteria and higher trophic levels. In the mesocosms with large-bodied crustaceans, protists were likely not important as bacterial grazers because of strong top-down control of nanoflagellates by crustaceans. Combined, our results provide evidence that alien large-bodied crustaceans can significantly impact the microbial loop.

Predicting impacts of an invading copepod by ecological assessment in the animal’s native range

We assembled data from 11 years of observational research on zooplankton in subtropical Florida, USA, lakes to determine potential impacts of the copepod Arctodiaptomus dorsalis on biodiversity and biomass of co-occurring crustacean zooplankton. This synthesis provided insight into how the species might impact plankton in limnologically similar lakes that it invades outside of its native range. A. dorsalis recently was found in the Philippines, and it was suggested that it could invade lakes in Asia and reduce biodiversity. In 7 shallow eutrophic lakes in Florida, we found no relationship between relative biomass of A. dorsalis and number of species of crustacean zooplankton, and we were equally likely to find a similar-sized copepod Diaptomus floridanus in lakes with or without A. dorsalis. In Lake Okeechobee, where a 7-year dataset existed for crustacean zooplankton, we found no evidence that A. dorsalis impacts the biomass of cladocerans or other dominant copepods. Gut analysis and grazing experiments documented that A. dorsalis is capable of eating bacteria and nearly all phytoplankton taxa, including cyanobacteria. This ability to exploit a broad range of resources and to execute rapid escape maneuvers to avoid fish predation may explain why A. dorsalis often is dominant in shallow eutrophic lakes with high densities of omnivorous fish. We conclude that A. dorsalis will not negatively affect the biodiversity of plankton in similar lakes of Asia; instead, it may fill a vacant niche in lakes with high fish predation where other copepods cannot survive. Further research is needed to determine how A. dorsalis will influence the plankton in less enriched lakes where food resources may be limiting.

Red tides in Masan Bay, Korea, in 2004–2005: III. Daily variations in the abundance of mesozooplankton and their grazing impacts on red-tide organisms

To investigate the role of mesozooplankton in the dynamics of red tides in Masan Bay, Korea, we measured the abundance of mesozooplankton in daily samples collected from June 1, 2004 to May 31, 2005. Mesozooplankton were abundant in the winter, but rare in the summer, and had a range of abundance of 3–52,843ind.m−3. Similarly, both copepods and cladocerans were abundant in the winter, but rare in the summer, and had ranges of abundance of 0–48,817ind.m−3 and 0–10,951ind.m−3, respectively. Invertebrate larvae were abundant in the fall but not in other seasons. The biomass of copepods was significantly positively correlated with salinity, dissolved oxygen, the biomass of the phototrophic dinoflagellates Heterocapsa triquetra and Prorocentrum minimum, and the biomass of the heterotrophic dinoflagellate Gyrodinium glaucum, but negatively correlated with water temperature and the biomass of heterotrophic bacteria and small algae. In addition, the biomass of cladocerans was significantly positively correlated with salinity and the biomass of euglenophytes and G. glaucum, but negatively correlated with water temperature. The biomass of invertebrate larvae was significantly positively correlated with water temperature, but negatively correlated with dissolved oxygen. These observations suggest that copepods and cladocerans may increase their populations by feeding on large phytoplankton in cold water, whereas invertebrate larvae may prefer warm water. The grazing coefficients for the copepods Acartia spp. on co-occurring Pfiesteria-like dinoflagellates (PLDs), P. minimum, Skeletonema costatum, H. triquetra, Heterosigma akashiwo, and Scrippsiella trochoidea were 0.104, 0.083, 0.042, 0.034, 0.033, and 0.030d−1, respectively. These results suggest that grazing by Acartia populations in Masan Bay can have a considerable impact on the populations of PLDs and P. minimum, but only a moderate impact on S. costatum, H. triquetra, S. trochoidea, and H. akashiwo.

Length-weight relationships and biomass of the main microcrustacean species of two large tropical reservoirs in Brazil

Length-weight equations were determined for the main microcrustacean species of Três Marias and Furnas reservoirs (state of Minas Gerais, Brazil), and evaluated the fluctuations of their biomass. We examined the following species: Thermocyclops minutus, Bosminopsis deitersi, Bosmina hagmanni, Ceriodaphnia cornuta, and Moina minuta (Três Marias Reservoir), and Notodiaptomus henseni, Daphnia ambigua, Ceriodaphnia silvestrii, Diaphanosoma spinulosum, D. fluviatile, and Bosmina freyi (Furnas Reservoir). Dry weight was obtained in a microbalance for each size class (Cladocera) or developmental stage (Copepoda). Microcrustacean mean biomass varied from 5.76 mg DW.m(-3) and 20.36 mg DW.m(-3) (Furnas Reservoir) and from 3.75 mg DW.m(-3) and 18.14 mg DW.m(-3) (Três Marias Reservoir). Significant differences (p < 0.000) between seasons were registered with higher biomass during the rainy seasons. Thermocyclops minutus was the most important species in Três Marias, whereas in Furnas, N. henseni contributed in equal proportion. In Furnas, the higher cladoceran biomass was explained by the contribution of larger-sized species, such as D. ambigua, C. silvestrii, D. spinulosum, and D. fluviatile. Even though both reservoirs are considered oligotrophic, there were significant differences in the estimated microcrustacean biomass, which reinforces the importance of this parameter for the description of the real contribution of each species in the community.

Effects of Microcystis blooms on the crustacean plankton community: enclosure experiments in a subtropical lake

An enclosure experiment was conducted in July–September 2001 in subtropical eutrophic Lake Donghu (China) to test a hypothesis that a moderate cyanobacterial biomass would have a positive effect on small-sized cladocerans. Eight enclosures (12.5 m3) were arranged with different nutrient concentrations using the lake water, tap water, and sediment from Lake Donghu. Microcystis blooms appeared in enclosures with higher nutrient concentrations and the average fresh weight biomass of Microcystis spp. ranged from 4.6 to 30.4 mg l−1 during the bloom period. Three cladocerans (Moina micrura, Diaphanosoma brachyurum, and Ceriodaphniacornuta) and two cyclopoids (Mesocyclops dissimilis and Thermocyclops taihokuensis) dominated the crustacean plankton community during the experimental period. The C. cornuta biomass constituted the greatest percentage (55.9–90.0%) of cladoceran biomass in the Microcystis bloom treatments. When the Microcystis biomass increased, the average biomass of C. cornuta increased and the biomass of M. micrura and D. brachyurum decreased, whereas the cyclopoid biomass did not change significantly. The total biomass of cladoceran and crustacean plankton were significantly positively correlated with the Microcystis biomass. Our results indicate that a moderate biomass of Microcystis spp. can favor crustacean plankton to some extent and, furthermore, may impact food web structures in a eutrophic lake.

Effects of temperature and resource abundance on small- and large-bodied cladocerans: Community stability and species replacement

Understanding when small- or large-bodied cladocerans dominate zooplankton communities has received considerable debate over the past 50 years. While a large body of research has proposed that large-bodied species are superior competitors over small-bodied species, other studies have shown that small-bodied species can dominate at least under some environmental conditions. We tested the hypothesis that dominance by small- and large-bodied cladocerans varied in response to the coupled effects of food supply and temperature. Laboratory experiments with poly- and monocultures of small- and large-bodied cladocerans were performed at three temperatures (16°C, 22°C and 27°C) and with varying amounts of food supply. The results of the experiments showed that the small-bodied species (Ceriodaphnia quadrangula) dominated at low food supply and higher temperature, while the large-bodied species (Daphnia magna and Daphnia pulex) in contrast dominated at lower temperature and higher food supply. Furthermore, although there were variations in the relative biomass of the small- and large-bodied cladocerans in the polycultures, C. quandrangula replaced the two larger Daphnia species when they declined in biomass at low food supply. Species replacement in response to temperature and food supply helped to maintain the relatively constant level of total cladoceran biomass in the polycultures which was the most pronounced at the intermediate temperature. We suggest that the observed changes in dominance were similar to facilitative replacement rather than competitive exclusion. Physiological processes such as clearance rates can help to promote the succession of large- and small- bodied populations within a community along gradients of temperature and food availability.

Effects of silver carp (Hypophthalmichthys molitrix) and nutrients on the plankton community of a deep, tropical reservoir: an enclosure experiment

Summary1. An in situ enclosure experiment was conducted in a deep reservoir of southern China to examine (i) the effects of a low biomass (4 g wet weight m−3) of silver carp (Hypophthalmichthys molitrix) and nutrients on the plankton community and (ii) the response of Daphnia to eutrophication.2. In the absence of fish, Daphnia galeata dominated the zooplankton community, whereas calanoids were dominant in the fish treatments, followed by D. galeata. Silver carp stocking significantly reduced total zooplankton biomass, and that of D. galeata and Leptodorarichardi, but markedly increased the biomass of smaller cladocerans, copepod nauplii and rotifers. In contrast, nutrient enrichment had no significant effect on the plankton community except for cyclopoids.3. Chlorophyta, Cryptophyta and Bacillariophyta were dominant phytoplankton groups during the experiment. Chlorophyta with high growth rates (mainly Chlorella vulgaris in the fish enclosures and Ankyra sp. in the fishless enclosures) eventually dominated the phytoplankton community. Total phytoplankton biomass and the biomass of edible phytoplankton [greatest axial linear dimension (GALD) &lt; 30 μm], Chlorophyta, Cryptophyta, Bacillariophyta and Cyanobacteria showed positive responses to fish stocking, while inedible phytoplankton (GALD ≥ 30 μm) was significantly reduced in the fish enclosures. However, there was no significant effect on the plankton community from the interaction of fish and nutrients.4. Overall, the impact of fish on the plankton community was much greater than that of nutrients. High total phosphorus concentrations in the control treatment and relatively low temperatures may reduce the importance of nutrient enrichment. These results suggest it is not appropriate to use a low biomass of silver carp to control phytoplankton biomass in warmer, eutrophic fresh waters containing large herbivorous cladocerans.

Grazing rates of crustacean zooplankton communities on intact phytoplankton communities in Canadian Subarctic lakes and ponds

Zooplankton grazing can potentially affect the biomass and composition of phytoplankton communities directly and indirectly. Low chlorophyll a concentration for a given TP concentration and simplified fishless food webs lead to the expectation that zooplankton community grazing rates are high in Subarctic regions; however, zooplankton community grazing rates have not been determined for Subarctic lakes/ponds. We estimated zooplankton community grazing rates on phytoplankton in 12 lakes and ponds in Wapusk National Park, Canada using a microcosm grazing experiment. Lakes and ponds differed in zooplankton taxonomic composition, Chl-a concentration, and zooplankton biomass. We found that the grazing rates on the total chlorophyll a (GRTotal) ranged 0–13.7% grazed per day and the grazing rates on the edible (<30 μm, GR<30) chlorophyll a was 0 to 16.7% per day. GRTotal increased with lake Daphnia and cladoceran biomass, as did GR<30, which also had a negative relationship with the total in-lake Chl-a. The calculated zooplankton grazing rates were within the range found for larger, temperate lakes.

Copepod dominance contributes to phytoplankton nitrogen deficiency in lakes during periods of low precipitation

We investigated meso-zooplankton nutrient recycling as a driver of nitrogen deficiency for phytoplankton production in Arctic Alaskan lakes. Copepods and cladocerans differentially recycle inorganic nutrients based on nutrient demands for new tissue. Copepods typically sequester more nitrogen in the tissue, and can lead to nitrogen, relative to phosphorus, deficient conditions for primary production. Cladocerans sequester more phosphorus relative to nitrogen, potentially resulting in phosphorus deficiency for phytoplankton growth. We observed a positive relationship between meso-zooplankton communities with higher copepod relative to cladoceran biomass and levels of water column nitrogen deficiency. We also observed negative relationships between total precipitation over the 10 days prior to lake sampling and metrics of both nitrogen and phosphorus deficiency. These results support our predictions that relatively high watershed nutrient loading can overwhelm internal nutrient sources and sinks. However, when nutrient loading from the catchment is minimal meso-zooplankton community composition can affect nitrogen relative to phosphorus deficiency for primary production. Relationships between copepod biomass and nitrogen deficiency provide a potential mechanism for the high frequency of nitrogen limitation in Arctic Alaskan lakes.

Transfer of bacterial production based on labile carbon to higher trophic levels in an oligotrophic pelagic system

Additions of labile organic carbon (C) enhanced bacterial production (BP) and were associated with increases in crustacean zooplankton and planktivorous fish biomasses. This was shown in a mesocosm experiment where we traced the contribution of BP to zooplankton and fish using stable isotopes and labile glucose-C as a biomarker. BP increased with glucose-C addition, and all zooplankton and fish incorporated some glucose-C. However, the effect of labile-C addition on zooplankton was taxa-dependant, as although cladocerans incorporated the most labile-C, increased BP did not affect cladoceran biomass. Instead, calanoid copepod biomass increased with glucose addition. This suggests that the ability to selectively graze on high quality food, such as bacterial grazing protists capable of trophic upgrading, had a stronger positive effect on calanoid copepods biomass than unselective grazing on bacteria and protists had on cladoceran biomass. Higher BP was associated with increased survival and population growth of young-of-the-year perch ( Perca fluviatilis ) when stocked at high densities, which suggested that BP had a density-dependant positive effect on fish growth.

Bythotrephes invasion elevates trophic position of zooplankton and fish: implications for contaminant biomagnification

We estimated the effects of Bythotrephes longimanus invasion on the trophic position (TP) of zooplankton communities and lake herring, Coregonus artedi. Temporal changes in lacustrine zooplankton communities following Bythotrephes invasion were contrasted with non-invaded reference lakes, and along with published information on zooplankton and herring diets, formed the basis of estimated changes in TP. The TP of zooplankton communities and lake herring increased significantly following the invasion of Bythotrephes, whereas TP in reference lakes decreased (zooplankton) or did not change significantly (lake herring) over a similar time frame. Elevated TP following Bythotrephes invasion was most prominent in lakes that also supported the glacial relict, Mysis diluvania, suggesting a possible synergistic interaction between these two species on zooplankton community composition. Our analysis indicated that elevated TPs of zooplankton communities and lake herring are not simply due to the presence of Bythotrephes, but rather reflect changes in the zooplankton community induced by Bythotrephes; namely, a major reduction in the proportion of herbivorous cladoceran biomass and a concomitant increase in the proportion of omnivorous and/or predatory copepod biomass in invaded lakes. We demonstrated that increases in TP of the magnitude reported here can lead to substantial increases in fish contaminant concentrations. In light of these results, we discuss potential mechanisms that may be responsible for the disconnect between empirical and theoretical evidence that mid-trophic level species invasions (e.g., Bythotrephes) elevate contaminant burdens of consumer species, and provide testable hypotheses to evaluate these mechanisms.

Temperature effects of interspecies competition between cladoceran species in experimental conditions

In experiments with mono- and polycultures the hypothesis that small-size cladoceran species (Diaphanosoma brachyurum Lievin and Ceriodaphnia reticulata Jurine) are more successful at higher temperatures whereas large-size species (Daphnia magna Straus and Simocephalus vetulus O.F. Muller) under low temperatures was tested. The biomass of Simocephalus vetulus exceeded the biomass of the other species both in mono- and polycultures. An increase of temperature from 18 up to 25°C led to reduction of the biomass of all species. The largest species, D. magna, responded to higher temperature by increased mortality. We assume that the reduction of the biomass of the large species allowed the small C. reticulata to develop successfully in mixed cultures at higher temperature; however it was completely eliminated at lower temperature. Apparently, the survival strategy of large cladocerans involves delayed juvenile development at lower temperatures. However, at higher temperatures, juvenile development of large species accelerates, and their mortality increases. Individual strategies of the studied species to survive at different temperatures are analyzed. On the whole the increase of temperature can result in decrease of the biomass of cladocerans and prevailence of small-size species over large-size ones.

The Lake Ontario zooplankton community before (1987–1991) and after (2001–2005) invasion-induced ecosystem change

We assessed changes in Lake Ontario zooplankton biomass, production, and community composition before (1987–1991) and after (2001–2005) invasion-induced ecosystem changes. The ecosystem changes were associated with establishment of invasive dreissenid mussels and invasive predatory cladocerans (Bythotrephes and Cercopagis). Whole-lake total epilimnetic plus metalimnetic zooplankton production declined by approximately half from 42.45 (g dry wt∙m−2∙year−1) during 1987–1991 to 21.91 (g dry wt∙m−2∙year−1) in 2003 and averaged 21.01 (g dry wt∙m−2∙year−1) during 2001–2005. Analysis of two independent data sets indicates that the mean biomass and biomass proportion of cyclopoid copepods declined while the same measures increased for the invasive predatory cladocerans. Changes in means and proportions of all other zooplankton groups were not consistent between the data sets. Cyclopoid copepod biomass and production declined by factors ranging from 3.6 to 5.7. Invasive predatory cladoceran biomass averaged from 5.0% to 8.0% of the total zooplankton biomass. The zooplankton community was otherwise resilient to the invasion-induced disruption as zooplankton species richness and diversity were unaffected. Zooplankton production was likely reduced by declines in primary productivity but may have declined further due to increased predation by alewives and invasive predatory cladocerans. Shifts in zooplankton community structure were consistent with increased predation pressure on cyclopoid copepods by alewives and invasive predatory cladocerans. Predicted declines in the proportion of small cladocerans were not evident. This study represents the first direct comparison of changes in Lake Ontario zooplankton production before and after the invasion-induced disruption and will be important to food web-scale investigations of invasion effects.

Zooplankton of lakes in the spurs of the Putorana Plateau and adjacent territories (North of Krasnoyarsk Krai)

Species composition and parameters of biomass of net plankton were analyzed for four large and 35 small lakes from 10 areas located between 69.5° and 67.5°N and 87° and 92°E that were examined in July–August, 2001 and 2003–2004. In the composition of the first (dominant) and/or second (subdominant) species in the ranged row of the average biomass, the lakes were copepod in 78.8% of cases, in 67.3%, calanoid. The share of biomass of cladocerans in the total biomass of zooplankton exceeded 50% only in 31% of cases. By the frequency of occurrence as dominant or subdominant by biomass the first place is occupied by copepodites calanoid (34.6% of cases), the second, by sexually mature Heterocope appendiculata (26.9%), the third, by copepodites cyclopoid (17.3%), the fourth, by Polyphemus pediculus (15.4%), the fifth, by Acanthodiaptomus denticornis (13.5%), the sixth, by Bosmina cf. longispina (11.5%), the seventh, by Heterocope borealis (9.6%), the eighth, by Daphnia cf. longispina (7.7% of cases). For the first time in the region 14 species of crustacean zooplankton are given. The known range of some species and subspecies (for example, Chydorus biovatus, Camptocercus fennicus, Ophyoxus gracilis kolymensis, Arctodiaptomus acutilobatus, Acanthrodiaptomus tibetanus) is widened owing to the findings in the studied region. The groups of the areas are more similar in terms of zooplankton north to south than east to west; the number of the cladoceran species and the number of cladoceran lakes grew north to south and east to west, which corresponded to the general change of climate towards more mild north to south for similar altitudes and east to west for different altitudes from mountain areas to the low lands. The comparison of the species lists from different northern regions showed that the species diversity (total number of species) of cladocerans and copepods does not change significantly west to east. Distribution of species and changes in the zooplankton structure are discussed from the point of view of possible influence of the global warming.

Effects of omnivorous filter‐feeding fish and nutrient enrichment on the plankton community and water transparency of a tropical reservoir

Summary1. The major aim of this study was to test the hypothesis that nutrient enrichment and the introduction of the Nile tilapia (Oreochromis niloticus), an exotic omnivorous filter‐feeding fish, operate interdependently to regulate plankton communities and water transparency of a tropical reservoir in the semi‐arid northeastern Brazil.2. A field experiment was performed for 5 weeks in 20 enclosures (9.8 m3) to which four treatments were randomly allocated: tilapia addition (F), nutrient addition (N), tilapia and nutrient addition (F + N) and a control treatment with no tilapia or nutrient addition (C). A two‐way repeated measures anova was undertaken to test for time, tilapia and nutrient effects and their interactions on water transparency, total phosphorus and total nitrogen concentrations, phytoplankton biovolume and zooplankton biomass.3. Nutrient addition had no effect except on rotifer biomass, but there were significant fish effects on the biomass of total zooplankton, copepod nauplii, rotifers, cladocerans and calanoid copepods and on the biovolume of total phytoplankton, large algae (GALD ≥ 50 μm), Bacillariophyta and Zygnemaphyceae and on Secchi depth. In addition, we found significant interaction effects between tilapia and nutrients on Secchi depth and rotifers. Overall, tilapia decreased the biomass of most zooplankton taxa and large algae (diatoms) and decreased water transparency, while nutrient enrichment increased the biomass of rotifers, but only in the absence of tilapia.4. In conclusion, the influence of fish on the reservoir plankton community and water transparency was significant and even greater than that of nutrient loading. This suggests that biomanipulation of filter‐feeding tilapias may be of importance for water quality management of eutrophic reservoirs in tropical semi‐arid regions.