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The influence of filter-feeding bivalves on plankton communities, nutrients, and water quality in a given aquatic ecosystem is so profound that they can be considered ecosystem engineers. In a 70-day mesocosm experiment, we tested the hypothesis that Corbicula fluminea would change plankton community structure by reducing small zooplankton and large phytoplankton and improve water quality by reducing nutrients. We monitored levels of nitrogen and phosphorus, organic suspended solids (OSS), and light at the sediment surface. Within the plankton, phytoplankton biomass (as Chl a, &gt, 0.45 μm), the biomass of microphytoplankton (&gt, 20 μm), nanophytoplankton (2–20 μm), picophytoplankton (0.2–2 μm), and zooplankton were determined. Compared with the controls, C. fluminea reduced the abundance of rotifers and the biomass of phytoplankton, and picophytoplankton, thereby modifying the plankton community structure. We did not observe reductions in TN and TP concentration, but OSS concentrations were reduced, and light intensity at the sediment surface was increased as a result of the improved water transparency. Our research shows that colonization by C. fluminea may modify plankton community structure and improve water quality of eutrophic shallow lakes, shedding further light on the ecological roles of filter-feeding bivalves in aquatic ecosystems.

The purpose of this study was to determine how zebra mussels affected cladoceran community structure under eutrophic conditions. We conducted a mesocosm study where we manipulated the presence of zebra mussels and the presence of large-bodied Daphnia (Daphnia magna and Daphnia pulicaria). We also conducted a complimentary life-table experiment to determine how water from the zebra mussel treatment affected the life history characteristics of the cladoceran species. We anticipated that small- and large-bodied cladoceran species would respond differently to changes in algal quality and quantity under the effects of zebra mussels. Large-bodied Daphnia successfully established in the zebra mussel treatment but failed to grow in the control. We did not observe positive relationships between food concentrations and cladoceran abundances. However, the phosphorus content in the seston indicated that food quality was below the threshold level for large-bodied cladocerans at the beginning of the experiment. We believe that zebra mussels quickly enhanced the phosphorus content in the seston due to the excretion of inorganic phosphorus, thus facilitating the development of large-bodied Daphnia. In conclusion, our results suggest that zebra mussels can alter the phosphorus content of seston in lakes and this can affect the dynamics of crustacean zooplankton.

Fish, being an important consumer in aquatic ecosystems, plays a significant role by affecting the key processes of aquatic ecosystems. Omnivorous fish consume a variety of food both from pelagic and benthic habitats and may directly or indirectly affect the plankton community as well as the lake trophic state. We conducted a 72-day outdoor experiment in mesocosms with and without Prussian carp (Carassius auratus) to evaluate the effect of this often-stocked omnivorous fish on the plankton community and water quality. We found that the presence of fish increased the biomass of planktonic algae, total and inorganic suspended solids, leading to decreased light intensity in the water and a lower biomass of benthic algae. Fish also prevented development of submerged macrophytes and the establishment of large-bodied zooplankton. However, the fish did not increase nitrogen concentrations and even was lowered total phosphorus levels, in part due to nutrient storage in the fish. We conclude that stocking of Prussian carp should be avoided, or removed where stocked and abundant, to obtain good ecological quality of shallow lakes, characterized by clear water and high abundance of macrophytes.

The impacts of fish and nutrient enrichment on the biomass of small and large cladoceran species were studied in mesocosms under eutrophic conditions. Fish shifted the ratio between small and large species to favour small cladocerans compared to control mesocosms, while nutrient enrichment favoured large cladocerans. Both fish vital activity and nutrient enrichment increased total and blue-green algae concentrations. While chlorophyll concentrations increased in response to fish and nutrient enrichment, there were no significant effects of fish on total cladoceran biomass and the relative proportion of large species due to high variability among replicates within treatments. However, fish and nutrient enrichment affected average total cladoceran biomass and the biomass of large species compared to control conditions.

Dreissena polymorpha is one of the bivalve species having a great impact on the environment, composition, and structure of aquatic communities, thereby acting as an efficient driver of changes in the ecological state of water bodies. We studied five Belarusian lakes in order to compare the water quality between lakes inhabited by and devoid of D. polymorpha, as well as to reveal the reasons for the observed differences. Lake Obsterno with a natural thermal regime and cooling reservoir Lake Lukolmskoe were invaded by this mollusk. Lakes Gorushka and Nobisto with a natural thermal regime, as well as cooling reservoir Lake Beloe, were devoid of D. polymorpha. The water quality classes were determined using the indices based on the physical and chemical parameters and characteristics of phytoplankton and Cladocera communities in line with the requirements of the EU Water Framework Directive and the saprobity index calculated by the Pantle–Buck method in Sladechek’s modification. It is commonly accepted that transparency is a good indicator of the ecological state of lakes. According to the data on transparency, Obsterno and Lukolmskoe lakes have the highest ecological status. However, the estimates obtained with the help of biotic indices did not differ as significantly as transparency in these lakes. The calculations demonstrate that the discrepancies in transparency of lakes inhabited by and devoid of D. polymorpha were not in accordance with those found with the saprobity index (mismatch above 70%) and other biotic indices (above 50%). This allows us to conclude that introduction of D. polymorpha does not improve the ecological water quality through changes in the planktonic communities, regardless of higher transparency values and reduction of the phytoplankton biomass.

Common carp (Cyprinus carpio) have been introduced into aquatic systems across the world, where their benthivorous feeding behavior has resulted in serious water quality problems. A 12-week mesocosm experiment was set up to test the hypotheses that common carp increase water column nutrient levels and decrease water clarity in aquatic ecosystems dominated by submerged plants. Further, we tested whether the effect of common carp on macrophytes depended on the species of plants. Relative to the controls, the presence of carp decreased water clarity by increasing total suspended solids (TSS) and light attenuation. However, levels of total nitrogen (TN) and total phosphorus (TP) in the water column were reduced. No significant change in phytoplankton biomass (measured as chlorophylla) and the biomass ofHydrilla verticillatawas observed between common carp treatment mesocosms and controls, but the common carp did reduce the biomass of the submerged macrophyteVallisneria denseserrulata. We conclude that removal of common carp is likely to improve water clarity in aquatic ecosystems dominated by submerged plants primarily by decreasing TSS and that the effect of common carp on macrophytes is stronger for the meadow formingVallisneriathan for the canopy formingHydrilla.

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.

In 11 lakes from cold and warm regions we studied the content of highly unsaturated fatty acids (HUFA) in seston (

Mathematical modelling and laboratory experiments were used to study how exploitative competition and predation influence the species structure in cladoceran community. For five species of Cladocera (Sida crystallina, Daphnia magna, Simocephalus vetulus, Daphnia longispina, and Diaphanosoma brachyurum), representing a gradient of body size, population characteristics were described as functions of food concentration. Abundance dynamics were simulated in mixed species cultures and invasion experiments under different levels of food supply corresponding to oligo-, meso-, and eutrophic conditions. Separate simulations were also run including and excluding (fish) predation. The competitive ability of each species was estimated as the values of the population equilibrium food concentration. Simulation results showed that for the no-predator scenario, increases in the level of food supply promoted species coexistence while under lower food concentrations only one species remained at the end of the simulation runs. When predation was allowed, the number of species that coexisted at the end of the simulations increased up to four species, indicating that predator pressure facilitated species coexistence because it shortened periods of food depletion. Simulation results were verified in laboratory experiments which suggested that population equilibrium food concentration can be used as an estimate of competitive ability. Finally, species structure and relative abundance in Lake Naroch (Belarus) during the summer of 2004 was found to be consistent with our results from computer simulation and laboratory experiments with regard to competition and predation impacts on zooplankton community.

The birth rate of natural cladoceran populations can change rapidly (during 2–3 days), reflecting rapid changes in their environment. If the egg ratio is calculated on the basis of egg age distribution, the birth rate can be estimated at short sampling intervals (shorter than egg stage duration) by modified Paloheimo's (1974) formula. When female size structure and age of eggs in clutches at the beginning and the end of a sampling interval are known, death rates of ovigerous females and eggs in separate size classes can be determined and incorporated in birth rate estimates. All these methods have been employed using the data on the population of Diaphanosoma brahyurum from the lake Obsterno (North-Western Belarus) in July–August, 1992. The birth rate values computed by the proposed methods and Poloheimo's formula differed significantly in many cases. The accuracy of birth rate estimations from various calculation methods was tested using a computer simulation. The model contains the essential features of cladoceran life history: distinct egg, juvenile and adult stages, development of eggs and reproduction. The population was divided into 25 age classes, each of 1 day duration. Durations of the egg, juvenile and adult stages were set at 3, 6 and 20 days, respectively. The embryogenesis was divided into three egg stages, each of 1 day duration. Survivorship was set from 0.2 up to 1.0 for each age class. The survivorship and brood size were changed through each of five time intervals (days) that allowed to simulate an increase or reduction of population density. Fecundity, survivorship and egg stage duration remained constant during each of 5 days that assumed stability of an environment (this does not occur in nature). Nevertheless, the egg ratio, proportion of juveniles and birth rates were variable even under these circumstances. Computer simulations showed that Poloheimo's formula evaluates birth rate with the relative error of 62% and usually overestimates its values. We propose methods to decrease errors of birth rate estimations by 3.5–5.5 times.

391 The role of topical factors, such as depth, bottom type, and the presence of water vegetation, in the functioning of plankton populations in relatively small shallow water bodies is not clear. In addition, the effect of macrophytes on plankton may be not only topical, but also metabolic (see, e.g., review [1]). The influence of macrophytes on littoral zooplankton may be studied with the use of the recently developed method of direct determination of the mortality in crustacean plankton unrelated to predation [2‐5]. The mortality due to aging, starvation, diseases, parasites, and negative physical and chemical factors is evaluated on the basis of calculation of dead individuals in samples stained with a special dye (e.g., [6]) and measurements of the sedimentation rate of dead bodies with the use of sedimentation traps. It was shown [3, 7] that the relative number of dead zooplankters is proportional to the specific mortality and, hence, is a reliable quantitative index. In this work, we measured the percentage of dead individuals in the populations of the dominant zooplankton species (Cladocera) in three biotopes of a freshwater lake—the pelagial biotope and two littoral biotopes, grown with bul rush and bare (the macrophyte-free littoral)—to determine the possible topical difference in the mortality. The work was performed in July of 2006 in the meso-eutrophic Lake Obsterno belonging to the Perebrodskaya group of lakes (northwestern Belarus). The lake area is 990 ha, the average depth is 5.3 m, and the maximum depth is 12 m. Reed and bul rush are the predominant species among littoral emergent plants. The main sediment type in the lake littoral is sand. For a detailed description of the lake morphometry, see [8, 9]. Samples of zooplankton were collected at 11 a.m. and 7 p.m. on July 25 and 26 using Apstein’s plankton net (the area of the inlet hole was 0.05 m 2 ; the mesh size was 70 µ m). In the pelagials and bul rush littorals, the net was towed from the bottom to the surface (4‐0 and 1‐ 0 m, respectively); the horizontal tow in the macrophyte-free littoral was performed over the space of 4 m; two samples were collected in each biotope (the replications and all the necessary characteristics were determined separately for each sample). The depth of sampling stations was 5.5 m in the pelagial and 1.3 m in the

The North American gastropod Physella acuta (Draparnaud, 1805) has been recorded for the first time in the Neman River basin, Belarus during a biological survey carried out on 23 August 2007. One specimen was found in a shallow water microhabitat (depth 0.2 m) amongst sand and mud. Most likely, this alien gastropod was introduced either by local aquarium keepers, or from Poland via the Augustov canal.