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Daphnia longispina and other large cladocerans are active filterers, consuming algae and other suspended matter. Undigested particles sink to the bottom and are further involved in the biotic cycle. The absence of planktivorous fish favors the development of large zooplanktonic species showing high filtration activity. Owing to filtration activity, zooplankton can clarify water masses. [ABSTRACT FROM AUTHOR]

Cyanobacteria blooms occurred in the pond in early August; Microcystis aeruginosa was constantly present in the plankton. Its biomass reached its peak of 24.1–28.6 mg/L in mid-August. The proportion of individual cells (4 µm in size) and colonies up to 50 µm in size in the M. aeruginosa biomass increased to 68–85%. In late August, the proportion of 50–100-µm fraction did not exceed 5–7%. The proportion of a larger fraction >100 µm decreased from 24 to 8%. Rotifers, represented by 14 taxonomic units recorded in the plankton, dominated in the water body. The abundance of Rotifers reached 51–96% of that of zooplankton. Dominating in abundance was Keratella cochlearis cochlearis. It accounted for 44 to 86% of rotifer population, and its abundance varied from 500 to 2000 ind./L. The rotifer abundance during the M. Aeruginosa bloom period reached its highest of 4.5 thous. ind./L and at the end of this period decreased to 580 ind./L. In the high-trophicity pond, rotifers play the leading role in the water body trophodynamics. [ABSTRACT FROM AUTHOR]

Large zooplankton can efficiently graze on algae and thereby improve water quality. However, zooplankton body size is considered to decrease with decreasing latitude because of the high fish predation and warm temperatures at lower latitudes. To explore how fish stocking has destabilized the trophic cascade and influenced water quality along a latitudinal gradient of reservoirs, we compared zooplankton body size and trophic cascades to water quality indicators in the subtropical Lake Qiandaohu and in tropical Guangdong reservoirs. The results showed that the body length and total biomass of the dominant zooplankton in Lake Qiandaohu were much larger than those of zooplankton in the Guangdong reservoirs. Moreover, fish predation was the key factor influencing the changes in zooplankton body size and total biomass in Lake Qiandaohu. In the Guangdong reservoirs, water temperature and total phosphorus were the pivotal drivers of zooplankton body size and biomass, respectively. In addition, structural equation models showed that the decreasing zooplankton body size and biomass under fish pressure weakened the grazing pressure on phytoplankton and indirectly contributed to the low water clarity in Lake Qiandaohu. However, fish had little influence on the inefficient algal grazing of zooplankton in the Guangdong reservoirs. Overall, zooplankton can function as an indicator of water quality in fish-controlled subtropical reservoirs but not in nutrient-controlled tropical reservoirs. [ABSTRACT FROM AUTHOR]

Zooplankton play a key role in energy transfer within lake food webs, but we have a poor knowledge concerning their role as phytoplankton grazers in shallow subtropical lakes. In this study, we aimed to determine how zooplankton grazing upon phytoplankton is altered in different scenarios of fish predation and turbidity, and we explored the relevance of grazing compared to other environmental variables, to explain phytoplankton biomass changes. A mesocosm experiment was conducted by including the following treatments: fish, turbidity, fish + turbidity, and a control (without fish or varying turbidity). The experiment lasted 21 days, and samples were taken four times. Zooplankton grazing was only effective for the microphagous group upon Cryptophyceae, while large Chlorophyceae and small pennate Bacillariophyceae biomass were benefited in the presence of copepods and cladocerans, being negatively affected by depletions in nitrogen availability. In the turbidity treatment, a reduction in phytoplankton biomass was obtained, artificially increasing zooplankton grazing on phytoplankton, while fish presence inhibited grazing of adult copepods and cladocerans. The other groups of phytoplankton were only influenced by the environment. This experiment suggests that phytoplankton biomass variations would be more affected by the environment than by zooplankton grazing in shallow lakes from the Paraná River. [ABSTRACT FROM AUTHOR]

Planktivorous fishes are known to consume coral larvae due to their high nutritional value that can benefit both the individual and their progeny. However, how the consumption of coral larvae varies among coral and fish taxa, between day and night and with the density of coral larvae is not well understood. Here, we used a series of laboratory feeding assays to determine how the consumption of coral larvae by five planktivorous damselfish species (Abudefduf sexfasciatus, Amblyglyphidodon curacao, Dascyllus trimaculatus, D. reticulatus, and Chromis viridis) differed (1) between larvae of the broadcast-spawning scleractinian coral Acropora tenuis and the brooding non-scleractinian coral Heliopora coerulea when offered at a constant density (100 larvae/50 L), (2) between day and night (A. tenuis only), and (3) among larval densities (A. tenuis: 5–500 larvae/50 L; H. coerulea: 5–250 larvae/50 L). When coral larvae were offered at 100 larvae/50 L, all five fish species examined consumed A. tenuis (78–97 larvae h−1) at a greater rate than H. coerulea (23–57 larvae h−1), with these differences likely related to the behavior and/or pigmentation of the larvae. Consumption also varied among the fishes examined, though the differences were small when compared to differences between coral species. Consumption of A. tenuis was twofold higher at day than night likely reflecting the diurnally active feeding behavior of the fishes examined. When the density of coral larvae offered to the fishes was varied, three of five species exhibited a type II functional response (decelerating intake rate) for A. tenuis, while four out of five damselfish species exhibited a type III (sigmoidal intake rate) or type I (linear intake rate) for H. coerulea. This study demonstrates that predation by fishes contributes significantly to coral larvae mortality and that differences in consumption rates among coral species may affect the composition of coral recruits and ultimately coral assemblages. [ABSTRACT FROM AUTHOR]

Climate change projections indicate increased precipitation in northern Europe, leading to increased inflow of allochthonous organic matter to aquatic systems. The food web responses are poorly known, and may differ depending on the trophic structure. We performed an experimental mesocosm study where effects of labile dissolved organic carbon (DOC) on two different pelagic food webs were investigated, one having zooplankton as highest trophic level and the other with planktivorous fish as top consumer. In both food webs, DOC caused higher bacterial production and lower food web efficiency, i.e., energy transfer efficiency from the base to the top of the food web. However, the top-level response to DOC addition differed in the zooplankton and the fish systems. The zooplankton production increased due to efficient channeling of energy via both the bacterial and the phytoplankton pathway, while the fish production decreased due to channeling of energy mainly via the longer and less efficient bacterial pathway. We conclude that the added DOC either acted as a subsidy by increasing the production of the top trophic level (mesozooplankton), or as a sink causing decreased top consumer production (planktivorous fish). [ABSTRACT FROM AUTHOR]

A tracer experiment using isotope N was performed to study the assimilation and retention of nitrogen from feces by two planktivorous fish, silver carp Hypophthalmichthys molitrix and tilapia Oreochromis niloticus, in Lake Taihu (China). Microcystis was enriched with N-NHCl, lyophilized to produce feed for fish, and traced to establish the fate of feces nitrogen. Samples of organisms and abiotic substances were analyzed for excess N and nutrient concentrations in the water column were determined on days 0, 1, 5, 10, 15, and 20. Nutrient concentration analyses indicated that TN and TP were about 4 times higher in the tilapia enclosure than in the silver carp enclosure due to the digestive capacity of Microcystis, which suggests that the ichthyoeutrophication potential of tilapia is greater than that of silver carp. 11.05 % of the N was assimilated by the tilapia whereas 3.58 % of the N was assimilated by the silver carp, suggesting that tilapia has a higher capacity to assimilate and retain Microcystis nitrogen than silver carp, although the absorptivities of both species were relatively low. At the end of the experiment, 8.48 % of the N was detected in sedimentary detritus in the tilapia enclosure, as compared to 6.07 % of the N in the silver carp enclosure, which suggests that only a small fraction of the Microcystis-derived nitrogen sank to the bottom. In conclusion, much of the Microcystis-derived nitrogen was neither assimilated by fish nor accumulated on the sediment floor. It presumably floated in the water column, contributing to phytoplankton propagation and thus degrading the water quality. [ABSTRACT FROM AUTHOR]

The foreign and Russian literature devoted to studying the effect of enhancing the growth of colonies of the cyanobacterium Microcystis aeruginosa Kutz em. Elenk. after their passage in a viable state through the digestive tract of filter-feeding aquatic animals (planktivorous fish, daphnia, and bivalves) has been analyzed. The main mechanisms of this effect are considered. Its role in the functioning of eutrophic reservoirs is discussed. The prospects and the need for further studies of the effect of enhancing Microcyctis growth after its viable passage through the digestive tracts of filter-feeding animals are shown for the development of a complete theory of the functioning of aquatic ecosystems. [ABSTRACT FROM AUTHOR]

The biomass and population dynamics of crustacean zooplankton were determined in oligotrophic Lake Toya in Japan over 5 years from May 1992 to May 1997. In 1992 and 1993, zooplankton biomass was up to 4.3 g dry weight m, whereas it decreased to <1 g dry weight m after 1994. This extreme change in biomass was associated with the succession of dominant species from larger ones, such as Daphnia longispina and Cyclops strenuus ( s. lat.), to smaller ones, such as Eubosmina tanakai and Bosmina longirostris. Consequently, this biomass change seemed to cause an increase in the chlorophyll a concentration in the euphotic zone and a decline in lake transparency. Because the birth rates of the dominant species were somewhat higher after 1994, the decline in the populations of larger crustaceans seemed to depend more on their rate of death rather than rate of birth, and this higher death rate is not considered to be attributed to food shortage. Although these results strongly suggest a top-down cascading effect of fish predation upon crustaceans, annual catches of two commercially important planktivorous fish species have also decreased in the lake, coincidentally with decreases in zooplankton biomass. This may be attributable to fishing regulations that prohibit catching smaller fish, implying that such smaller fish affect zooplankton and phytoplankton, as well as lake transparency. [ABSTRACT FROM AUTHOR]

While the structuring role of fish in lakes is well studied for the summer season in North temperate lakes, little is known about their role in winter when fish activity and light irradiance potentially are lower. This is unfortunate as the progressing climate change may have strong effects on lake winter temperature and possibly on trophic dynamics too. We conducted an enclosure experiment with and without the presence of fish throughout winter in two shallow lakes with contrasting phosphorus concentrations. In hypertrophic Lake Søbygård, absence of fish led to higher biomass of zooplankton, higher grazing potential (zooplankton:phytoplankton ratio) and, accordingly, lower biomass of phytoplankton and chlorophyll a (Chl a), while the concentrations of total nitrogen (TN), total phosphorus (TP), oxygen and pH decreased. The average size of egg-bearing Daphnia and Bosmina and the minimum size of egg-bearing specimens of the two genera rose. In the less eutrophic Lake Stigsholm, zooplankton and their grazing potential were also markedly affected by fish. However, the decrease in Chl a was slight, and phytoplankton biovolume, pH and the oxygen concentration were not affected. TN was higher when fish were absent. Our results indicate that: (i) there is a notable effect of fish on zooplankton community structure and size during winter in both eutrophic and hypertrophic North temperate lakes, (ii) Chl a can be high in winter in such lakes, despite low light irradiance, if fish are abundant, and (iii) the cascading effects on phytoplankton and nutrients in winter may be more pronounced in hypertrophic lakes. Climate warming supposedly leading to reduced winter mortality and dominance of small fish may enhance the risk of turbid state conditions in nutrient-enriched shallow lakes, not only during the summer season, but also during winter. [ABSTRACT FROM AUTHOR]

Despite the presence of high nutrient concentrations, most ponds located around Brussels (Belgium) show a considerable variation in turbidity. The importance of submerged macrophytes in maintaining the clear-water state requires identification of the main factors determining macrophyte abundance and diversity in ponds and small lakes. In this study, the inter-relationships between submerged macrophyte cover, fish abundance and turbidity were investigated in 13 eutrophic peri-urban ponds. Along a turbidity gradient, vegetation switched from dominance by Stoneworts ( Chara and Nitella spp.) in the clearest ponds, to dominance by Potamogeton pectinatus in ponds with a slightly lower water transparency. Despite the presence of both P. pectinatus and Stoneworts in each of the vegetated ponds, only one became dominant. Only a very low abundance (around 20%) of submerged vegetation was found in ponds of intermediate turbidity, while macrophytes were absent in turbid ponds. Multi- and univariate analysis showed a marked difference in chemical, physical and biological properties between ponds deliberately used for fish stocking and ponds that were not. Macrophyte cover was significantly negatively correlated with turbidity and plankti-benthivorous fish abundance. No such correlation was observed with piscivorous fish abundance, except for pike that were associated with a charophyte vegetation in the study ponds. The strong relationship found between fish abundance and turbidity, its negative effect on submerged vegetation cover, and the importance of submerged vegetation in controlling phytoplankton abundance, should be taken into account when selecting ponds for fish stocking. It also suggests that the study ponds have a good potential for ecological quality restoration by biomanipulation. [ABSTRACT FROM AUTHOR]

The re-emergence of Gymnodinum catenatum blooms after a 10 year hiatus of absence initiated the present investigation. This study aims to evaluate the exposure of small pelagic fishes to paralytic shellfish toxins (PST) during blooms of G. catenatum. Sardines ( Sardina pilchardus) were selected as a representative fish species. In order to assess toxin availability to fish, both intracellular PSTs (toxin retained within the algal cells) and extracellular PSTs (toxin found in seawater outside algal cells) were quantified, as well as toxin levels within three fish tissue matrices (viscera, muscle and brain). During the study period, the highest cell densities of G. catenatum reached 2.5 × 10 cells l and intracellular PST levels ranged from 3.4 to 398 ng STXeq l as detected via an enzyme linked immunosorbent assay (ELISA). Measurable extracellular PSTs were also detected in seawater (0.2-1.1 μg STXeq l) for the first time in Atlantic waters. The PST profile in G. catenatum was determined via high performance liquid chromatography with fluorescence detection (HPLC-FLD) and consisted mostly of sulfocarbamoyl (C1+2, B1) and decarbamoyl (dcSTX, dcGTX2+3, dcNEO) toxins. The observed profile was similar to that reported previously in G. catenatum blooms in this region before the 10-year hiatus. Sardines, planktivorous fish that ingest a large number of phytoplankton cells, were found to contain PSTs in the viscera, reaching a maximum of 531 μg STXeq kg. PSTs were not detected in corresponding muscle or brain tissues. The PST profile characterized in sardine samples consisted of the same sulfocarbamoyl and decarbamoyl toxins found in the algal prey with minor differences in relative abundance of each toxin. Overall, the data suggest that significant biotransformation of PSTs does not occur in sardines. Therefore, planktivorous fish may be a good tracer for the occurrence of offshore G. catenatum blooms and the associated PSTs produced by these algae. [ABSTRACT FROM AUTHOR]

Zooplankton were 3-8 times more abundant during the day near the surface than elsewhere in the water column over a 1-2.4 m deep back reef in Moorea, French Polynesia. Zooplankton were also significantly more abundant near the surface at night although gradients were most pronounced under moonlight. Zooplankton in a unidirectional current became concentrated near the surface within 2 m of departing a well-mixed trough immediately behind the reef crest, indicating that upward swimming behavior, rather than near-bottom depletion by reef planktivores, was the proximal cause of these gradients. Zooplankton were highly enriched near the surface before and after a full lunar eclipse but distributed evenly throughout the water column during the eclipse itself supporting light as a proximal cue for the upward swimming behavior of many taxa. This is the first investigation of the vertical distribution of zooplankton over a shallow back reef typical of island barrier reef systems common around the world. Previous studies on deeper fringing reefs found zooplankton depletion near the bottom but no enrichment aloft. In Moorea, where seawater is continuously recirculated out the lagoon and back across the reef crest onto the back reef, selection for upward swimming behavior may be especially strong, because the surface serves both as a refuge from predation and an optimum location for retention within the reef system. Planktivorous fish and corals that can forage or grow even marginally higher in the water column might have a substantial competitive advantage over those nearer the bottom on shallow reefs. Zooplankton abundance varied more over a few tens of centimeters vertical distance than it did between seasons or even between day and night indicating that great care must be taken to accurately assess the availability of zooplankton as food on shallow reefs. [ABSTRACT FROM AUTHOR]

In littoral zones of aquatic systems, submerged macrophytes have marked structural variation that can modify the foraging activity of planktivores. Swimming and feeding behavior of Pseudorasbora parva and Rasbora daniconius (Cyprinidae) on their prey Daphnia pulex and Artemia salina, respectively, was studied in a series of laboratory experiments with varying stem densities. A range of stem densities was tested for each of the two species to compare the effect of simulated macrophytes on prey attack rates and swimming speed, average stem distance ( D) was measured in fish body lengths for each of the two fish species. We found that, with reducing average stem distance, the attack rate decreased in the similar trend and this trend was similar for both fish species. However, the species differed in the degree to which swimming activity was hindered at increased stem densities, and this was due to species-specific differences in the distance moved with one tail beat. Therefore, we conclude that the reductions in swimming speed with reduced average stem distance are due to the differences in fish movement per tail beat. [ABSTRACT FROM AUTHOR]

Planktivorous fish, both visual predators and filter feeders, enhance eutrophication processes in lakes. In pampean shallow lakes several planktivorous species may coexist but often two species dominate: silverside ( Odontesthes bonariensis), a visual planktivorous fish when young adult, and sabalito ( Cyphocharax voga), an omnivorous filter feeder. To assess the relative strength of the cascading trophic effects of the two species, a mesocosm experiment was conducted using different proportions of both species as treatments. Differences were found in water transparency, phytoplankton biomass, crustacean abundance, individual size and biomass. Our results suggest that visual predators intensify eutrophication effects more strongly than filter feeders do. [ABSTRACT FROM AUTHOR]

Subfossil zooplankton assemblages (Cladocera 22 taxa, Rotifera 1 taxon) were identified from the surface sediments of 36 shallow (median depth = 0.7 m) Danish coastal brackish lakes differing in epilimnic salinity (SAL, range 0.2–17.4‰), summer-mean total phosphorus (TP, 27–327 μg l−1) and total nitrogen (TN, 0.850–2.629 mg l−1), as well as in submerged macrophyte coverage and planktivorous fish density (PL-CPUE). Cladoceran species richness declined significantly with increasing SAL, TP and TN, while no significant correlation was found to either PL-CPUE, macrophyte coverage or lake surface area. Bonferroni-adjusted forward selection within canonical correspondence analysis (CCA) showed that 22.1% of the variation in zooplankton data was explained by PL-CPUE, SAL and TP uniquely; each variable explaining an almost equally significant amount of variation in the zooplankton data. Predictive models to infer PL-CPUE, SAL and TP were developed using variance weighted-averaging (WA) procedures. Almost similar values of boot-strapped coefficient of determination ( r2boot-strapped 0.22–0.38) were produced by the WA inference models of PL-CPUE, SAL and TP, while the inference models of TP produced the lowest boot-strapped root-mean-squared-error of prediction (RMSEPboot-strapped 0.29–0.36 log(TP + 1), μg l−1). Yet, zooplankton TP and SAL optima (WA) were strongly correlated ( r2 = 0.46), while PL-CPUE optima (WA) were independent of both TP and SAL optima, indicating that only the PL-CPUE inference models are suitable for making reconstructions. [ABSTRACT FROM AUTHOR]

Limnologists have long recognized the importance of predation in freshwater communities. The majority of study of predator effects has involved vertebrate predators, with emphasis on planktivorous fish. Documented effects of planktivorous fish have been so dramatic that manipulations of their populations are seen by many as potential tools in lake management. However, the success of such manipulations is often less than desired due to the ubiquitous complexity of food webs and the pervasiveness of compensatory responses to food web manipulation. Recently, enormous effort has been applied to the Lake Kinneret pelagic food web in effort to reduced the abundance of the planktivorous Kinneret bleak Acanthobrama terraesanctae and thereby increase the biomass of herbivorous zooplankton in the hopes of increasing water clarity. We compared potential predation pressure on Lake Kinneret herbivorous zooplankton by bleak and the other major zooplankton predators in the lake, the cyclopoid copepods Mesocyclops ogunnus and Thermocyclops dybowskii. We found that, despite having much lower biomass, cyclopoid copepods accounted for a greater portion of the predation mortality on herbivorous zooplankton than bleak. Our results suggest that reductions in predation pressure by bleak will not yield subsequent increases in herbivorous zooplankton biomass. Rather, reductions in bleak predation pressure may allow for increases in cyclopoid copepod abundance and thereby a net increase in predation pressure on herbivorous zooplankton. [ABSTRACT FROM AUTHOR]

The effectiveness of fish removal as a tool for biomanipulation and restoration was studied from 1993 to 1997 in shallow Lake Pohjalampi (North Karelia, eastern Finland). The external nutrient loading into this lake was low and nearly stable throughout the study period. During a period of 5 years, a total of > 200 kg ha-1 of fish, mainly roach and bream, were caught and thus the total fish biomass was reduced by nearly 80%. However, in 1996 a very dense year-class ofperch hatched and the number of planktivorous fish increased again. These changes in the fish community resulted in a decreased fish predation on benthic invertebrates, whereas the predation of fish on zooplankton increased. The responses in the prey communities were consistent with these changes: the biomass of benthic invertebrates increased and that of zooplankton decreased. At the same time, nutrient and chlorophyll a concentrations declined. The decreasing chlorophyll a:P ratio indicated also that the nutrient cycling in the lake was reduced and despite the increased predation on herbivorous zooplankton the lake was shifted to a more oligotrophic state. [ABSTRACT FROM AUTHOR]

To examine the relationships between the biomass of pelagic communities and to study how these relationships differ between lakes with different fish assemblages, we used data on nutrients, chlorophyll, zooplankton, and fish from 96 Argentinean lakes and reservoirs. Significant differences in regressions between chlorophyll and total phosphorus, macrozooplankton and chlorophyll, and between total fish and macrozooplankton, in lakes with different fish assemblages, were found. Lakes with zooplanktivorous fishes but without piscivores, and lakes without planktivores, had the highest and the lowest phytoplankton biomass, respectively. The regression line for lakes with both types of fish were intermediate. Furthermore, lakes with planktivores, but without piscivores, had the lowest macrozooplankton. The analysis of thetrend in simple regression residuals suggests that the changes in chlorophyll, macrozooplankton, and total fish with lake trophy, are related to changes in fish assemblages. [ABSTRACT FROM AUTHOR]