Saemangeum Reservoir in South Korea is an estuarine system enclosed by a dyke construction, where seawater inflow and retained water outflow are managed by the opening/closing of sluice gates installed in the southern part of the dyke. An exchange of the reservoir water can cause spatiotemporal fluctuations in the salinity and trophic state, which are major drivers determining variation in the composition of biological communities in estuarine systems. Here, we investigated the seasonal and spatial variability in the copepod community and environmental conditions (water temperature, salinity, transparency, chlorophyll a concentration, total nitrogen, total phosphorus and Carlson’s trophic state index) based on seasonally conducted field monitoring in the Saemangeum Reservoir from July 2013 to January 2018. In addition to the role of temperature, salinity and chlorophyll a concentration in structuring the copepod community and diversity, the biological indices of copepods with respect to salinity range and trophic state, were evaluated. The spatiotemporal variability in the salinity and trophic state variables showed contrasting patterns, and chlorophyll a concentration was negatively affected by salinity, indicating that the reservoir water was being highly exchanged with opening of the sluice gates. The mean trophic state index values, however, were constant in the eutrophic state (50―70). Dominant copepods were Acartia (A. hudsonica, A. sinjiensis, Acartia spp.) and Oithona (O. davisae and Oithona spp.), which are common species in eutrophic neritic water. Variation in the copepod community was mainly associated with the seasonal succession of the dominant species rather than a spatial gradient (from around the estuary to the sluice gates); however, site-specific differences in frequencies of several non-dominant species could be detected around the estuary (Sinocalanus tenellus) and the sluice gates (Centropages spp., Tigriopus spp. and Labidocera rotunda). The copepod diversity increased with species-richness from around the estuary to the sluice gates, which could result from variation in the site-specific location of non-dominant species. The frequency of particular species was also able to discriminate in terms of the salinity range (oligohaline: A. pacifica, S. tenellus and A. sinjiensis; mesohaline: Pseudodiaptomus inopinus; and polyhaline: C. abdominalis and Centropages spp.) and the trophic state (mesotrophic: C. abdominalis, Calanus sinicus and Centropages spp.; and hypereutrophic: S. tenellus, P. inopinus and Sinocalanus spp.). The findings from this study not only identify the factors determining spatiotemporal variation in the copepod community in the Saemangeum Reservoir, but also expand the applicability of copepods as biological indicators of conditions associated with salinity range and trophic state in other enclosed estuarine systems.

In 2016, a massive fish kill of the planktivorous pond smelt Hypomesus nipponensis occurred in Lake Suwa, Japan. Pond smelt, which is important fish species from commercial and cultural perspectives, influences the zooplankton community in the lake. Following the massive fish kill, Daphnia galeata appeared and their population density increased to previously unseen levels by summer 2017. To help recover the fish population, larval pond smelt were released into the lake. Consequently, D. galeata has rarely appeared in Lake Suwa since summer 2017, except for a low‐density population observed in May–June 2018. These phenomena indicated that fish predation is a possible primary factor for suppressing daphnid populations in shallow eutrophic lake. Zooplankton populations showed species‐specific responses to the alterations in predation pressure. When the D. galeata population peaked after the fish kill event, rotifers, which are small‐sized competitors of daphnids, and calanoids tended to decrease in density. Rotifers then increased with the decrease in the daphnid populations due to the recovery of fish. Alternatively, cyclopoids, which demonstrated a similar population density following the fish kill event, tended to decrease when the fish population recovered. The population densities of Leptodora cf. kindtii/richardi and Bosmina spp. did not significantly differ between before the massive kill event and after pond smelt recovery. It is believed that the changes in zooplankton population densities observed in the present study may provide insights for further monitoring of aquatic food webs in eutrophic water systems, including Lake Suwa, where re‐oligotrophication is occurring and biological interactions are changing.