Abstract: An NPZD(nutrient, phytoplankton, zooplankton, detritus) model with incorporation of a diazotroph function group was used to simulate the temporal change of ecosystem in the upstream Kuroshio Current. The modeled nutrients, chlorophyll-a, and productivity showed good agreements with the observations. The impacts of N₂-fixation on nutrient concentrations, plankton biomass, and detritus were assessed by comparisons of the modeled simulations with and without diazotrophs. Our results suggested that diazotrophs were most abundant in summer and fall when other phytoplankton were nitrogen limited. The mean growth of phytoplankton could increase by 64% with the input of new nitrogen from N₂-fixation, leading to 30% increase of primary production, regenerated production, and new production, respectively. In summer, N₂-fixation supported 50%~80% of the new production in the upper 50 m, but only 10%~50% in the depths of 50~200 m. This finding suggested that N₂-fixation was an important source of new nitrogen for phytoplankton production above 50 m in the upstream Kuroshio, but new production below 50 m was largely contributed by the vertical nutrient fluxes from below.

Key words: NPZD model, Kuroshio, N2-fixation, ecosystem, new production