Variability of primary production among basins in the East/Japan Sea: Role of water column stability in modulating nutrient and light availability

Abstract

The primary productivity of the East/Japan Sea (EJS) displays strong spatio-temporal variability, largely driven by the oceanographic forcings that vary across the different regions of the sea. Consequently, the ecosystem responses to climate variability could be region-dependent. A model-based analysis from this study synthesized the complex biological-physical interactions across the EJS, and suggested an intricate role of water column stability in driving the observed seasonal and basin-scale variability. Compared to the northern EJS (e.g., the Japan Basin), the southern EJS (e.g., the Ulleung and Yamato Basins) has a more favorable mixing regime for higher productivity in both winter and summer, balancing the need for phytoplankton growth in terms of nutrient and light availability. The wintertime water column in the southern EJS is relatively stable compared with the northern EJS due to the influx of warm Tsushima Warm Current (TWC) water and also weaker surface atmospheric cooling, leading to decreased light limitation for primary production in the south than in the north. During the stratified summer season, the surface stirring and mixing, due to the energy dissipation of strong TWC inflow, promote a relatively less stable surface mixed layer in the south than that in the north, and thus alleviate the nutrient limitation at the surface. In addition, the current- and wind-induced upwelling along the east coast of Korea further enhances the productivity in the southwestern EJS. The general south-north ‘see-saw’ pattern of mixing regime, combined with upwelling-downwelling switching, could be the key driver for the observed production patterns in the EJS.