Distribution of Mesozooplankton during Spring and Autumn across the Frontal Zone of South Sea, Korea

We investigated the distributional pattern of mesozooplankton and the related environmental characteristics across the frontal zone in the South Sea of Korea during spring and autumn 2016. The frontal zone (FZ) was formed by Korean Southern Coastal Waters and Tsushima Warm Current, and the physical structure of front was more distinct during autumn than spring. The peaks of mesozooplankton abundance were not coincident (p > 0.05) with peaks of integrated chlorophyll-a (chl-a) concentrations (spring: chl-a = 44.5 mg m−2 at FZ, mesozooplankton = 8,198 inds.m−3 at coastal zone (CZ); autumn: chl-a=141.2 mg m−2 at CZ, mesozooplankton = 5,412 inds.m−3 at FZ). Cluster analysis divided the mesozooplankton community into FZ, CZ and offshore zone (OZ) mainly by the dominant copepods (adult and immature) during both seasons that was associated with chl-a based on principal component analysis. All the zones were characterized by temporal difference in occurrence of copepods, which can be explained by the cyclopoid-to-calanoid (Cy:Ca) ratio. Cy:Ca ratio was dependent mainly on the co-occurrence of higher concentrations of chl-a and cyclopoids at each zone between spring and autumn. Paracalanus copepodites dominated at all stations across the FZ during both periods. Cy:Ca ratio at FZ in spring showed a distinctively higher value than that of CZ and OZ, whereas the pattern was the opposite in autumn. These results indicated that the temporal distribution of mesozooplankton across the frontal zone in the South Sea was mainly characterized by the zonal characteristics of FZ, unlike CZ and OZ, and was the result of temporal differences in chl-a concentrations and the associated increase of cyclopoid copepods (Oithona spp.).