Temporal and spatial variations in feeding behaviors of mesozooplankton assemblages in a low-turbidity temperate estuarine bay system

Abstract

Zooplankton are important component in transferring energy and materials to higher-level organisms in marine ecosystem. They consume the primary production and link microbial food web to classic food web in a way that is eaten by predators. Gwangyang Bay receives fresh water directly from the Seomjin River and thereby gradients of salinity and nutrients are well developed from the river mouth through estuary to open sea. Environmental characteristics are unique along a gradient of the river, estuary, inner bay and outer bay. Three study stations representing the estuary, inner bay, and outer bay were selected, and a bottle incubation experiment were conducted at each station. Clearance rates of mesozooplankton ranged from −0.79 to 2.94 mg−1 d−1, −0.68 to 3.10 mg−1 d−1 and −0.86 to 1.82 mg−1 d−1 at station 1, station 2, and station 3, respectively. Four different patterns in clearance rate were observed depending on the size of phytoplankton. These patterns were determined by (1) the composition of mesozooplankton assemblages, (2) size-dependent prey selectivity, (3) omnivory, and (4) water temperature. Furthermore, average grazing impacts measured in the present investigation ranged from −0.1 to 1.6% (mean 0.3%) at station 1, −3.1 to 4.4% (mean 1.2%) at station 2, and −1.8 to 2.2% (mean 0.4%) at station 3. The observed values in Gwangyang Bay are slightly low compared to the previous measurements from other coastal and offshore sea areas all over the world. This result suggests the importance of omnivorou feeding by mesoaooplankton and trophic cascading through planktonic food web processes in Gwangyang Bay.