Phytoplankton photosynthetic parameters and primary production in Japan Sea and the East China Sea: Toward improving primary production models

Abstract

Vertical variations in the photosynthesis–irradiance (P–E) parameters and their effects on the primary production (PP) were examined in four different characteristic coastal and offshore areas in Japan Sea and the East China Sea, during a stratified season. In Japan Sea, no significant regional variations were observed in the P–E parameters at a depth of 10m between the coastal and the offshore areas. The daily water column PP (∑PP(Total)) was lower in the offshore area than in the coastal area due to the lower surface chlorophyll a (Chl a) in the offshore area and reductions in Pmax* and α* in the subsurface chlorophyll maximum (SCM) being influenced by the cold Japan Sea Proper Water existing below the euphotic zone. In the East China Sea, all the P–E parameters showed depth variations in both the shelf and Kuroshio waters. Regional variations were observed in the vertical distributions of Chl a and PP; within the surface layer, they were higher in the shelf water than in the Kuroshio water. Within the subsurface layer, this trend was reversed. In the entire water column, these regional variations cancelled each other, and no significant variation was observed in ∑PP(Total) between the two areas. ∑PP(Total) was underestimated when the wavelength dependency of the P–E parameters was ignored in all the four areas (by 21–35% on average). The error in ∑PP(Total) increased in comparison to those obtained by ignoring the depth variations in the P–E parameters (−25% to −39%) and the depth variations in both the P–E parameters and Chl a (−25% to −55%). The present study reveals the effects of vertical variations in the P–E parameters and spectral dependency of photosynthesis on the PP in Japan Sea and the East China Sea, and their relationships with hydrographic conditions such as the water column stratification, influences of Japan Sea Proper Water, and Kuroshio Current.