Effects of Concentrated Freshwater Discharge on Hydrography and Mesozooplankton Community in a Dammed Estuary

Abstract

This study examines the different effects of high and low freshwater discharge on the water physical structure of the water column and plankton community in a tide-dominated eutrophic estuary, the upper reach of which has been dammed. Water with a low amount of dissolved oxygen (DO) rose to the surface in 2017, which was most likely due to tidal reflection. The surface DO level is also highly affected by the concentration of chlorophyll-a (Chl-a), which may increase the DO through photosynthesis. However, this effect seems to be minimal given the degree of stratification, and the freshwater appears to remain in the bay for less than 10 days according the progression in this macrotidal region. Following a freshwater discharge in 2012, there was an influx of ammonium into the bay, followed by an increase in Chl-a and mesozooplankton biomass. When the flow was lower in 2017, there were no cladocerans. Instead, a small copepod (Oithona similis) increased exponentially toward late August, potentially taking advantage of the empty niche and rapidly multiplied during the relatively low freshwater discharge. The low oxygen did not affect the mesozooplankton, which may have been due to the relatively high DO level in the bottom waters in both years.