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Copepods are common crustaceans in aquatic systems and one of the most important producers of carotenoid astaxanthin pigments, which can enhance the animals’ resistance against potentially damaging ultraviolet radiation (UVR), but at the same time, increases the risk of fish predation. Previous studies have demonstrated that copepods have different pigmentation levels matching the current threat level in terms of UVR and fish occurrence. However, these other studies have quantified population-levels changes in pigmentation, making it difficult to disentangle the role of individual phenotypic colour changes from that of selection. We quantified carotenoid-based pigmentation with colorimetric methods, which enabled us to track changes within individual copepods. Two species of copepods, Diaptomus castor and Eudiaptomus gracilis, were exposed to high and low UVR and fish cues in a factorial design. L*a*b* colour values (CIE; Commission International de l’Eclairage) were extracted from digital photographs of each copepod and used as proxies for carotenoid concentration. Our results showed that individual copepods significantly changed their pigmentation in response to both UVR and fish cues within a period of 2 weeks. However, the responses differed between sexes and between adults and juveniles. UVR effects were present in all life-stages whereas fish effects were only detected in juveniles, with largest responses in D. castor. This confirms that carotenoid pigmentation is a phenotypically plastic trait, and highlights that strategies for trading off risks of UVR and predation differ between males and females as well as between life-stages.

Although nickel (Ni) frequently enters into water, documents on speciation of the metal and its toxicity to freshwater organisms are scanty. Laboratory experiments, made in this study, with 1.0–5.0 mg/L of Ni revealed that Ni was not quickly removed from water. Application of cubic regression followed by classical optimization technique showed that maximum reduction time (T) of Ni in water ranged between 60 to 65 h. Ninety-six hours of LC 50 value of Ni to crustacean Diaptomus forbesi , fish Cyprinus carpio , and worm Branchiura sowerbyi was respectively 5.43, 14.70, and 19.73 mg/L. Normalizing the lethal values and plotting them against time, it was observed that C . carpio was more sensitive than D . forbesi , which was not reflected in the 96-h LC 50 values. However, sensitivity of these organisms to Ni was better explained by power regression equation (M = aNi b ), which exhibited that mortality (M) of D . forbesi and B . sowerbyi increased slowly between 24 to 72 h, increasing sharply at 96 h, while mortality of C . carpio increased steadily every 24 till 96 h. Experiment with 1.0 mg/L of Ni in outdoor vats showed that crustacean zooplankton and worms accumulated high concentration of Ni, while only gut and liver tissues of fish C . carpio accumulated trace and moderately high concentration of Ni, respectively. It is assumed that toxicity of Ni to fish is mediated primarily through gill.

Historical plankton samples from the St. Lawrence Great Lakes were subjected to taxon-specific 15N analysis to test the hypothesis that the changes recorded in zooplankton communities during the 21st Century are related to changes in the trophic positions of large-bodied carnivorous copepods. Daphnia mendotae was used as the reference herbivore for trophic-level comparisons. The results were that Limnocalanus macrurus, Diaptomus (Leptodiaptomus) sicilis as well as the cladoceran Bythotrephes cederstroemi show evidence of elevated carnivory compared to data from the 20th Century. The large diaptomid Diaptomus (Leptodiaptomus) sicilis has a stable isotope signature that is significantly more carnivorous in Lake Superior than in Lakes Michigan and Huron by approximately one-half trophic level. Differences were found in 10 cases out of 15 for Limnocalanus (Huron, Michigan Superior), 6 cases out of 15 for Diaptomus (Huron, Michigan) and in 1 out of 1 for Senecella (Superior). We did not find evidence to support the theory that large-bodied calanoid copepods may have improved their representation in the food webs of the upper Great Lakes by shifting their trophic position downward. Instead, large-bodied Calanoida have increased their trophic positions in parallel with their increased relative abundance. More research is thus needed to explain the driving forces for changing food web dynamics in the Great Lakes.

As an intermediary connection between primary producers and higher trophic levels, zooplankton are an important component of the aquatic food chain, contributing significantly to aquatic biological productivity. This study describes the zooplankton diversity and community structure, as well as their relationships with ecological factors, in homestead ponds of a coastal district along the northern Bay of Bengal region. Significant differences (p < 0.05) were detected in the mean values of temperature, pH, DO, TDS, transparency, and phosphates from the ponds during December 2020, but no significant differences were found in the nitrate levels. However, no significant variances in the ecological parameters between months were found due to the study taking place in the same season. Sixteen zooplankton species, representing four groups, were found in the study area, with the highest mean abundance of 301.19 ± 40.55 ind./L recorded in February 2021 and the lowest of 293.02 ± 21.13 ind./L recorded in December 2020. The diversity (H’), evenness (e), richness (J), and dominance (D) ranged from 2.49 to 2.12, from 0.93 to 0.84, from 2.05 to 1.56, and from 0.13 to 0.09, respectively. Based on the SIMPER analysis, Diaptomus gracilis and Brachionus calyciflorus were found to be significant contributors (>10%) to the zooplankton community structure in different months. The ANOSIM results revealed that 10 species of zooplankton were significant contributors based on their average dissimilarity. The canonical correspondence analysis (CCA) identified that pH, transparency, nitrates, and phosphates have a significant impact on the abundance of zooplankton species in the homestead ponds in the study area.

Experiments with environmentally relevant concentrations of Cu in glass aquaria revealed that Cu was quickly removed from water. Cubic regression of Cu concentration against time showed that maximum rate of removal was around 69.34–72.11 h irrespective of treatment. The 96 h LC 50 value of Cu was respectively 0.18, 0.19 and 0.35 mg/L for fish Cyprinus carpio , crustacean Diaptomus forbesi and worm Branchiura sowerbyi . Normalizing the lethal values and plotting them against time it was observed that there was sharp differences in mortality over time between the organisms and 96 h lethal values could misrepresent susceptibility of the organisms to Cu. Treatment of 0.1 mg/L of Cu in water resulted in accumulation of 10.57, 4.38, 1.46 and 2.44 µg/g of Cu, respectively in sediment, worm, crustacean zooplankton and whole body of fish. But, Cu deposited in high concentrations in gut and liver of fish indicating that Cu was principally accumulated through food.

A Siberian diaptomus, identified as Leptodiaptomus cf. angustilobus (Sars G.O., 1898), has been found for the first time in Europe (Russia, Nizhny Novgorod oblast, mouth area of the Kerzhenets River (Middle Volga), 56°6′5″ N, 44°57′28″ E). A brief morphological description with illustrations of the species is presented. Differences in the morphometry of abdominal somites, length of antennules, and size of the fifth pair of thoracic legs are shown between females from Nizhny Novgorod oblast and Siberia. We have also obtained data on the abundance of L. cf. angustilobus in the plankton community of the mouth area of the Kerzhenets River and described habitat conditions for the Diaptomidae species new to this region.

This work aims to analyze the zoogeographic distribution of Calanoida in Kazakhstan. Kazakhstan belongs to the Palaearctic region, and its territory is ascribed to the European–Siberian and Nagorno–Asian biogeographical subregions. The European–Siberian subregion includes the Volga–Ural, Irtysh, and Turkestan–Aral provinces. The Balkhash province belongs to the Nagorno–Asian subregion. Studies of the Calanoida fauna were carried out between 1997 and 2019. For this purpose, 7250 zooplankton samples were taken in 130 different water bodies. Findings of 26 species of Calanoida have been documented. The richest in species composition (20) is the Calanoida fauna of the Irtysh province. Ten Calanoida species have been recorded in the Volga–Ural province, 8 in the Turkestan–Aral province, 7 in the Balkhash province, and 5 in the Ponto–Caspian region. The distribution of the species richness of the order is determined by a complex of climatic factors, including the density of the hydrographic network, a variety of hydrochemical conditions, and accidental acclimatization of species. Far Eastern species (Sinodiaptomus sarsi, Neutrodiaptomus incongruens, Neodiaptomus schmackeri) entered the inland water bodies of Kazakhstan, most likely through the introduction of non-native fish species. The Black Sea species Acartia tonsa and Calanipeda aquaedulcis were introduced into the Caspian Sea with ballast waters. Three autochthonous species (Limnocalanus macrurus, Eurytemora grimmi, Eurytemora minor), formerly inhabiting the Caspian Sea, can now be considered extinct. Acanthodiaptomus denticornis, Arctodiaptomus (R.) salinus, Phyllodiaptomus blanci, and Eudiaptomus graciloides are widespread in the region. Endemic species (Gigantodiaptomus irtyshensis, Arctodiaptomus naurzumensis) and species are new for Kazakhstan (Diaptomus (Chaetodiaptomus) mirus, Eudiaptomus transylvanicus, Arctodiaptomus dentifer, A. (Rh.) ulomskyi were found in small waterbodies; they are known only from single occurrence sites as well as Eurytemora caspica. The last one was described from the northern part of the Caspian Sea, in the coastal zone. Further research into small water bodies that are poorly studied may expand our knowledge of the diversity of Calanoida in Kazakhstan. Calanoida fauna of Kazakhstan was closest to the fauna of countries with a continental climate and most strongly differed from countries with subtropical and Mediterranean types of climates.

We collected suspended particulate matter (seston) and zooplankton samples from Lake Washington in Seattle, Washington, USA, over a 10‐month period to investigate the effects of food availability on zooplankton fatty acid (FA) composition. The percentage of nutritionally critical ω3 polyunsaturated fatty acids (PUFA) in the seston varied from 8% of the FA pool in midsummer to 30% during the spring diatom bloom. Zooplankton accumulated much higher percentages ω3 PUFA than was available in the seston. In particular, cladocerans preferentially accumulated eicosapentaenoic acid (EPA, 20:5ω3), copepods accumulated docosahexaenoic acid (DHA, 22:6ω3), and both copepods and cladocerans accumulated 18 carbon chain ω3 PUFAs (C₁₈ ω3). By comparison, the FA of zooplanktivorous juvenile sockeye salmon (Oncorhynchus nerka) were strongly dominated by EPA (12.5% ± 2.1%) and DHA (28.2% ± 8.7%). The saturated fatty acid and the arachidonic acid (ARA, 20:4ω6) composition of Diaptomus ashlandi was strongly (r² = 0.76) and moderately (r² = 0.54) correlated with the prevalence of these FAs in the seston. Furthermore, the DHA content of Diaptomus was moderately correlated with the seston's DHA content (r² = 0.45) and very strongly correlated with seston EPA (r² = 0.89). Since EPA was the most prevalent PUFA in the seston and DHA was the most prevalent PUFA in Diaptomus, these results suggest that Diaptomus may synthesize DHA from the EPA in their food. In general, zooplankton species in Lake Washington were strongly enriched with those FA molecules that are most physiologically important for fish nutrition (i.e., ARA, EPA, and DHA), indicating a clear mechanism by which changes in seston composition influence fisheries ecology.

The large bloom-forming flagellate Gonyostomum semen has been hypothesized to be inedible to naturally occurring zooplankton due to its large cell size and ejection of long slimy threads (trichocysts) induced by physical stimulation. In a grazing experiment using radiolabelled algae and zooplankton collected from lakes with recurring blooms of G. semen and lakes that rarely experience blooms, we found that Eudiaptomus gracilis and Holopedium gibberum fed on G. semen at high rates, whereas Daphnia cristata and Ceriodaphnia spp. did not. Grazing rates of E. gracilis were similar between bloom-lakes and lakes with low biomass of G. semen, indicating that the ability to feed on G. semen was not a result of local adaptation. The high grazing rates of two of the taxa in our experiment imply that some of the nutrients and energy taken up by G. semen can be transferred directly to higher trophic levels, although the predominance of small cladocerans during blooms may limit the importance of G. semen as a food resource. Based on grazing rates and previous observations on abundances of E. gracilis and H. gibberum, we conclude that there is a potential for grazer control of G. semen and discuss why blooms of G. semen still occur.

This research was focused on distribution of the Calanoida species in the Kharkiv region. Three species were found in the Kharkiv Region for the first time: Hemidiaptomus rylowi Charin, 1929, Hemidiaptomus hungaricus Kiefer, 1933, and Eudiaptomus transylvanicus (Daday, 1890). Additional information on the distribution of Hemidiaptomus rylowi Charin, 1929 were provided, which is listed in the Red Data Book of Ukraine as a threatened species. We did not register some five species - Diaptomus amblyodon Marenzeller, 1873, Diaptomus arnoldi Siewerth, 1928, Diaptomus insignis Smith, 1909, Diaptomus castor (Jurine 1820) in Kharkiv region since first record by G. Fadeev in 1929.