Zooplankton community variations in response to summer hypoxia in Sishili Bay, North Yellow Sea, China

Hypoxia may cause severe disturbances to coastal ecosystems. The potential effects of summer hypoxia on zooplankton communities in Sishili Bay, China, was investigated in three cruises conducted during the summer from 2015 to 2017. Comparative analysis was performed on zooplankton communities at hypoxic and normoxic sites. Results show that the number and abundance of zooplankton species at hypoxic sites were lower than those at normoxic sites. Copepods exhibited greater dominance at the hypoxic sites compared to the normoxic sites, suggesting a higher tolerance to low oxygen concentrations than other taxonomic groups. Specifically, Oithona spp. was particularly prevalent at hypoxic sites and displayed a negative correlation with dissolved oxygen concentration at sea bottom. Additionally, the zooplankton community at hypoxic sites also exhibited lower Shannon-Wiener diversity and Margalef richness indices. The normalized biovolume size spectrum of the hypoxic sites showed steep slopes, suggesting a shift towards smaller species and lower transfer efficiency in pelagic food webs for the zooplankton community.