The nutrient dynamics and the lower trophic ecosystems in the warm waters in the vicinity of Japan

Abstract

Understanding oceanic environments is important for sustainable use of ocean biological resources. This paper reviews studies on low productivity in the Kuroshio and Tsushima Warm Current regions using routinely observed nutrient concentration and the zooplankton community structure. Nutrient concentrations show the region-specific characteristics of the Kuroshio and Tsushima Warm Current. Seasonal observations in the Kuroshio reveal that spatiotemporal variation of the nutrient supply from the deep layer leads to spatiotemporal variation of the nutrient concentration in the surface mixed layer. However, upstream of the Kuroshio and Tsushima Warm Current, horizontal advection creates unique vertical structures of nutrient concentrations during the summer season. Besides the routine observations, we develop a highly sensitive ammonium analysis method and apply this method to the study of ammonium excretion of the zooplankton. The zooplankton community structures in the Japan Sea are also analyzed. Long-term observations in the coastal area of the Japan Sea in the spring season indicate that the coastal branch of the Tsushima Warm Current makes the spatial distributions of the zooplankton community heterogeneous. In the summer season, the abundance of appendicularians increases with warming, and the zooplankton community structures change with this increase of appendicularian abundance. The lower trophic ecosystem had been considered both oligotrophic and homogenous in the Kuroshio and Tsushima Warm Current, but our studies suggest that it varies spatiotemporally with the physical and chemical environment.