Spatial distributions of mesozooplankton biomass, community composition and grazing impact in association with hypoxia in the Pearl River Estuary

Abstract

Hypoxia in estuarine and coastal water is a critical environmental stressor on marine ecosystem worldwide. However, the inter-relationships between hypoxia and mesozooplankton have not been fully understood. This study investigated the biomass, community structure and grazing impact of mesozooplankton in the Pearl River Estuary during summer 2017 when bottom water hypoxia occurred. At the hypoxic stations, the biomass of mesozooplankton was relatively higher, the taxonomic diversity of mesozooplankton was lower, and the communities were more dominated by Acartia spp. and Paracalanus spp. than those of the non-hypoxic stations. The slopes of the NBSS (normalized biovolume size spectrum) were steeper at the hypoxic stations than non-hypoxic stations, indicating a higher contribution of small zooplankton species to the total community therein. Mesozooplankton consumed 0.06%–14.1% of phytoplankton standing stock daily. The ingestion rates of mesozooplankton in the hypoxic and non-hypoxic stations showed no significant difference, but the grazing impact was lower at the hypoxic stations than that at non-hypoxic stations. This may be due to the disproportionally high phytoplankton standing stock, and more importantly the changes of the community structure and physiology (e.g., feeding behavior and trophic mode) of mesozooplankton in the hypoxic condition. Overall, our study indicates the ecological relevance of mesozooplankton to bottom water hypoxia in estuarine and coastal waters and provides information for a better understanding of the estuarine and coastal ecosystems impacted by episodic hypoxia.