Abstract
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.
Highlights
Coastal and estuarine ecosystems provide various ecosystem services of direct benefit to humans, so the loss of biodiversity and ecosystem function caused by excessive anthropogenic activities are major concern [1,2]
The seasonal variations and spatial gradients in water temperature, salinity, and the other parameters related to trophic state are shown in Fig 2 and Fig 3, as the mean values with standard deviation (SD)
The results indicate that the composition of the copepod community was largely different for each season but could be changed with respect to the salinity range
Summary
Coastal and estuarine ecosystems provide various ecosystem services of direct benefit to humans (such as fisheries, provision of fish nursery habitats, water filtering and detoxification), so the loss of biodiversity and ecosystem function caused by excessive anthropogenic activities are major concern [1,2]. Given the connection between inland and marine waters, the biological communities in such ecosystems can be dynamically influenced by factors such as eutrophication, chemical pollution and tidal current [3,4]. Planktonic organisms are often used as biological indicators for monitoring the impact of human activities on ecological conditions, because they are sensitive to environmental fluctuations [5]. Copepods are recognized as a major resource, supporting the abundance of economically important fish in coastal or marine environments due to their central role connecting the energy pathway from primary producers (phytoplankton) to higher trophic levels (fishes) in the food web [7,8,9,10]. In comparison to lotic water systems (e.g., lakes and reservoirs), coastal and marine environments can be dramatically regulated by tidal currents, which make it complicated to detect the factors influencing the composition of copepod communities. The influence of the Kuroshio current on the seasonal succession, distribution, species composition and diversity of the copepod community has been well studied in Taiwan [1,14,15]
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