Spatio-temporal distribution of nekton community structure and diversity change in Hangzhou Bay, CHINA

The stock of nekton resources and spatiotemporal variation of community in the gulf of estuary area are closely related to the quality of habitat and ecological environment (e.g., habitat degradation, freshwater input, eutrophication, and water exchange). However, their relative importance in shaping the spatial heterogeneity of nekton in bay of China estuary remains rarely reported. In this study, the spatiotemporal dynamics of the nekton assemblage structure were investigated in Hangzhou Bay (HZB), a semi-enclosed bay of China estuary. It was based on a comprehensive survey constituting 40 collections from 10 stations over 4 seasons within a year. A wide taxonomic diversity was encountered during the nekton survey, including 57 taxa distributed among 23 families. Average nekton biomass was significantly higher in winter which dominated by the fish biomass than in summer with the shrimp biomass had the higher proportion. The average nekton abundance was significantly higher in warm seasons with the especially high abundance of shrimp than in cold seasons which the abundance of shrimp relatively low. Nekton of HZB were mainly composed of small near-shore species. Little differences between subtropical (27 species) and warm-temperate (30 species) species of nekton were surveyed on the basis of thermal adaptability, but the seasonal variation was obvious. The number of demersal species in warm seasons (54 species) was higher than that in cold seasons (43 species). The dominant species (IRI>1000) were fishes such as Miichthys miiuy, Collichthys lucidus and shrimps such as Exopalaemon annandalei, Palaemon gravieri and crabs such as Portunus trituberculatus. The biomass diversity index (H'), evenness index (J') and richness index (d) of nekton in HZB were higher in summer and autumn (warm seasons) than in winter and spring (cold seasons). Nevertheless, the abundance diversity index (H') and evenness index (J') of nekton were showed opposite seasonal variation characteristics. The seasonal alternate index (AI) and migrate index (MI) of nekton community varied from 94 to 218 and -73 to 35 respectively, which meant that the nekton community in the survey waters were in large variation and unstable comfortably. The survey stations could be divided into 3~4 groups in the 55%~65% similarity levels by the clustering and NMDS sequencing analysis of nekton during four seasons. Important environmental correlates of assemblage structure were identified using redundancy analysis (RDA). Strong physical gradients in salinity, water temperature, dissolved oxygen and depth correlated predominantly with nekton assemblage structure, and reflected substantial spatiotemporal variation. And chemical variables like DIP and Chla were also highly correlated with nekton community structure. Estuarine embayments in the central bay of China, like Hangzhou Bay, might be viewed appropriately as landwardmost sections of the wider, highly productive spawning and nursery grounds of this region with the good fisheries monitoring and management strategies adapted to the needs of ecosystems and national conditions.