Plankton Community Response to Physico-Chemical Forcing in the Ulleung Basin, East Sea during Summer 2008

Abstract

In Summer 2008, a multidisciplinary survey was conducted onboard R/V Haeyang 2000 to understand plankton response to the three distinct physico-chemical settings that developed in the Ulleung Basin of the East Sea. Baseline settings of hydrographic conditions included the presence of the thin (<20 m) Tsushima Surface Water (TSW) on top of the Tsushima Middle Water (TMW). It extends from the Korea Strait to <TEX>$37^{\circ}N$</TEX> along the <TEX>$130^{\circ}E$</TEX> and then turns offshore and encompasses the relatively saline (T><TEX>$26^{\circ}C$</TEX>, S>33.7) Ulleung Warm Eddy surface water centered at <TEX>$36.5^{\circ}N$</TEX> and <TEX>$131^{\circ}E$</TEX>. A relatively colder and saline water mass appeared off the southeastern coast of Korea. It was accompanied by higher nutrient and chlorophyll-a concentrations, suggesting a coastal upwelling. Most of the offshore surface waters support low phytoplankton biomass (0.3 mg chl-a <TEX>$m^{-3}$</TEX>). A much denser phytoplankton biomass (1-2.3 mg <TEX>$m^{-3}$</TEX>) accumulated at the subsurface layer between 20-50 m depth. The subsurface chlorophyll-a maximum (SCM) layer was closely related to the nutricline, suggesting an active growth of phytoplankton at depth. The SCM developed at shallow depth (20-30 m) near the coast and deepened offshore (50-60 m). A fucoxanthin/zeaxanthin ratio was high in coastal waters while it was low in offshore waters, which indicated that diatoms dominate coastal waters while cyanobacteria dominate offshore waters. The community structure and biomass of phytoplanktonare closely related to nitrogen availability. Zooplankton biomass was higher in the coastal region than in the offshore region while species richness showed an opposite trend. Zooplankton community structure retained a coastal/offshore contrast. These suggest that summer hydrography is a stable structure, lasting long enough to allow a hydrography-specific plankton community to evolve.