Specific oceanographic characteristics and phytoplankton responses influencing the primary production around the Ulleung Basin area in spring

Abstract

The East Sea (Sea of Japan) is a marginal, semi-closed sea in the northwestern Pacific. The Ulleung Basin area, which is located near the subpolar front of the East Sea, is known to have high primary production and good fisheries in spring season. After episodic wind-driven events during the spring of 2017, horizontal and vertical profiles of physical chemical biological factors were investigated at 29 stations located in the Ulleung Basin area. In addition, growth responses of phytoplankton communities to nutrient additions were evaluated by bioassay experiments to understand the fluctuation of phytoplankton biomass. Because of strong northwestern wind, phytoplankton biomass was scattered and upwelling phenomenon might be suppressed in this season. The phytoplankton abundances in the coastal stations were significantly higher than offshore and island stations. In contrast, the nutrient and chlorophyll a (Chl a) concentrations and the phytoplankton biomass were quite low in all locations. Bacillariophyceae was dominated group (75.1% for coastal, 40.0% for offshore and 43.6% for island stations). In the algal bioassays, the phytoplankton production was stimulated by N availability. The in vivo Chl a values in the +N and +NP treatments were significantly higher than the values in the control and the +P treatments. Based on the field survey, the higher nutrients in coastal waters affected the growth of diatom assemblages, however, little prosperity of phytoplankton was observed in the offshore waters despite the injection of sufficient nutrients in bioassay experiments. The growth of phytoplankton depended on the initial cell density. All of results indicated that a dominant northwestern wind led to a limited nutrients condition at euphotic layers, and the low level of biomass supply from the coasts resulted in low primary production. Both supplying nutrients and introducing phytoplankton through the currents are critical to maintain the high productivity in the Ulleung Basin area of the East Sea.