Impact of an anticyclonic eddy on the summer nutrient and chlorophyll a distributions in the Ulleung Basin, East Sea (Japan Sea)

Abstract

The impact of the anticyclonic Ulleung Warm Eddy (UWE) on the vertical distributions of nutrient and chlorophyll a (Chl a) concentrations in the Ulleung Basin (UB) was investigated during the contrasting summers of 2005 and 2007. The physical structure of the water column was characterized by an intrathermocline eddy (ITE) in 2005, whereas the UWE remained distant from the sampling transect in 2007. Water column structures appeared to be highly stratified, and nutrients in the surface waters were totally depleted at all stations. In 2005, an exceptionally high concentration of Chl a (5.5 mg m(-3)) was measured below the surface mixed layer in the eddy core (station D3), and values of similar to 2.5 mg m(-3) were observed at the eddy edge (stations D2 and D4). Formation of an ITE efficiently mixed surface and deep-ocean waters, the latter supplying sufficient nutrients to generate an extremely high concentration of Chl a at the base of the subsurface layer. Overall, the results indicated that the anticyclonic UWE plays a key ecological role in supporting substantial phytoplankton biomass in the nutrient-depleted surface waters in summer and maintaining high benthic mineralization in the deep-sea sediments of the UB.