In the Pearl River Estuary (PRE), hypoxia has been frequently observed during summer since the 1990s. Yet, limited data exist with regard to the effects of hypoxia on mesozooplankton in this region. In this study, both field investigations and laboratory experiments were conducted to show the distribution patterns and physiological changes of mesozooplankton in response to hypoxia. Field results indicated that mesozooplankton biomass was higher in the water column where hypoxic bottom water was detected. The stations with bottom hypoxic waters showed a lower taxonomic diversity of mesozooplankton and were dominated by Acartia spp. and Paracalanus spp. Mesozooplankton consumed 0.06–14.1% of phytoplankton standing stock daily, and the grazing impact was lower at the stations with...[ Read more ]

In the Pearl River Estuary (PRE), hypoxia has been frequently observed during summer since the 1990s. Yet, limited data exist with regard to the effects of hypoxia on mesozooplankton in this region. In this study, both field investigations and laboratory experiments were conducted to show the distribution patterns and physiological changes of mesozooplankton in response to hypoxia. Field results indicated that mesozooplankton biomass was higher in the water column where hypoxic bottom water was detected. The stations with bottom hypoxic waters showed a lower taxonomic diversity of mesozooplankton and were dominated by Acartia spp. and Paracalanus spp. Mesozooplankton consumed 0.06–14.1% of phytoplankton standing stock daily, and the grazing impact was lower at the stations with bottom hypoxia. Strong mesozooplankton diel vertical migration (DVM) exist and most of mesozooplankton could reside in the hypoxic waters during daytime due to the DVM. Physiology changes, in terms of survival, feeding, and respiration rates of three representative mesozooplankton species (copepods) in response to hypoxia were further investigated in the laboratory. The three representative copepods Paracalanus parvus, Acartia erythraea, and Temora turbinate were isolated from the PRE. We found that P. parvus and A. erythraea showed no decline in survival at hypoxic conditions, while T. turbinate was more sensitive to hypoxia, with a survival rate of 79% at hypoxic conditions. Feeding of P. parvus was also not influenced by hypoxia, while the feeding rate of A. erythraea and T. turbinate dropped significantly to 68.1% and 33.5%, respectively, under the hypoxic condition as compared with those in the controls. A decrease of respiration rates was observed in all three species at hypoxic conditions, but the small copepod P. parvus seemed less been affected by hypoxia than the other species. Overall, these results improved our understanding of the interplay between mesozooplankton assemblage and hypoxia episodes and provided insight into how the hypoxia can alter the ecological and physiological response of mesozooplankton species.