THESIS
2019
xii, 79 pages : illustrations (some color), color maps ; 30 cm
Abstract
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
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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.
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