Inter-annual variations of summertime plankton communities in Sishili Bay, Northern Yellow Sea, China

Abstract

With an intensive increase in anthropogenic activities, plankton communities in coastal areas show deterioration, such as shifts from diatom-dominated to more dinoflagellates and cyanobacteria, and increasing gelatinous zooplankton. To further understand the dynamic changes in plankton communities under anthropogenic activities in coastal areas, a multi-annual survey comprising nine cruises in summer during 2012–2020 was conducted in Sishili Bay, China. The results showed that the phytoplankton community in Sishili Bay was mainly dominated by diatoms, with an abundance of 94.86%; copepods were the dominant taxa in the zooplankton community, accounting for 64.57% of the total abundance, followed by zooplankton larvae (24.74%) including bivalvia, copepoda nauplius, ophiopluteus, etc, whereas other taxonomic groups, such as cladocerans, tunicates, chaetoganatha, cnidarians, protozoa, mysidacea, and ctenophores only accounted for 10.69% of the total abundance. Noticeable changes were observed in the plankton community, accompanied by some signals of community deterioration: dinoflagellates presented an increasing trend compared with that in diatoms; zooplankton larvae and gelatinous organisms also displayed apparent increasing trends. Restriction of dissolved inorganic phosphorus (DIP) combined with an increase in total phosphorus was conducive to a proportional increase in dinoflagellates. Overall, these results suggest that sea reclamation and marine cage aquaculture have promoted an increase in gelatinous organisms. The observed juvenilization of zooplankton might be driven by a trophic cascade effect caused by an increase in carnivorous zooplankton. Overall, the present study highlights the importance of anthropogenic activities affecting plankton inter-annual changes and suggests that the nutrient structure and control of anthropogenic activities should be investigated in detail for the protection and restoration of marine ecological environments.