Contrasting currents drive geographic variability in the biomass of Pacific saury (Cololabis saira), zooplankton, and phytoplankton in the northwestern Pacific

Abstract

Exploring the biomass relationships among fish, zooplankton, and phytoplankton and the potential mechanisms that drive changes in their biomass are important for us to understand the structure and dynamics of aquatic ecosystems. In this study, we thus measured the biomass of planktivorous Pacific saury (Cololabis saira), zooplankton, and phytoplankton in the less-explored northwestern Pacific to illustrate their biomass relationships, and to explore underlying mechanisms driving the spatial patterns of biomass for these trophic organisms. Our results showed that the sampling area could be divided into the Kuroshio-driven low-nutrient and high-chlorophyll region (i.e, LNHC) and the Oyashio-driven high-nutrient and low-chlorophyll region (i.e, HNLC). In the LNHC, the phytoplankton biomass was on average threefold higher than that in the HNLC. By contrast, the biomass of zooplankton in the LNHC was averagely twofold lower than that in the HNLC. Differently, the average stock of Pacific saury in the LNHC was generally consistent with that in the HNLC. Apart from the positive correlation between zooplankton biomass and the stocks of Pacific saury, there were no linear relationships of biomass across the three organisms. The implication is that the top-down effects by zooplankton grazing or Pacific saury feeding are unlikely to cascade further down to the phytoplankton biomass in the northwestern Pacific. Based on the canonical correspondence analysis, we found that the biomass of Pacific saury, zooplankton, and phytoplankton between HNLC and LNHC responded differently to specific environmental factors (e.g., temperature, salinity, and nutrients), indicating that the contrasting Oyashio and Kuroshio currents had an impact on the geographic variation of the biomass for these organisms. Altogether, our findings provide insights into the biomass relationships among Pacific saury, zooplankton, and phytoplankton, and the researches for the dynamics of these trophic organisms in marine ecosystems.