Enhanced phytoplankton and bacterioplankton production coupled to coastal upwelling and an anticyclonic eddy in the Ulleung basin, East Sea

Abstract

We investigated the biomass and production rates of microorganisms associated with coastal upwelling and the anticyclonic Ulleung warm eddy (UWE) of the Ulleung basin in the East Sea. Shipboard observations revealed that depth-integrated phytoplankton biomass and production were higher in the moderately stratified ring of the UWE than in the vertically well-mixed eddy core or in the stratified region outside of the eddy in the Ulleung basin. Similarly, heterotrophic bacterial production was higher in the eddy ring than in the eddy core or outside of the eddy. Bacterial biomass in the highly productive eddy ring showed little difference compared to the bacterial biomass of the core site due to the impact of grazing by heterotrophic protozoa. Satellite imagery and diatom species composition data in conjunction with physico-chemical parameters demonstrated that wind-driven coastal upwelling in the southeast of Korea was largely responsible for the phytoplankton bloom and enhanced bacterial production along the UWE. Overall, the results indicated that the UWE entrained highly productive upwelling coastal waters and delivered enhanced microbial biomass and production into the central Ulleung basin. The results further implied that the UWE and the subsequent effect on vertical particulate carbon flux may play a significant role in stimulating benthic respiration and in sequestering organic carbon produced by coastal upwelling down into the deep Ulleung basin.