Phytoplankton productivity enhancement and assemblage change in the upstream Kuroshio after typhoons

Abstract

The primary production (PP), nitrate-uptake-based new production (NO 3-NP), chlorophyll aα concentration, and phytoplankton assemblage in the upstream Kuroshio Current neighboring the northern South China Sea (SCS) before and after the consecutive passage of 3 typhoons during the summer of 2007 were compared. The aim was to elucidate the effects of riverine mixing on phyto-plankton dynamics in the oligotrophic Kuroshio, to which northern SCS and Taiwan coastal water spread after the typhoons. Spatial changes in surface salinity were used to differentiate the effects of riverine mixing from wind-induced upwelling. After the typhoons, PP and NO3-NP in the Kuroshio both were higher due to enriched nutrients from entrainment of riverine-mixed waters. Abundances of diatoms had increased but the abundances of Trichodesmium spp., Richelia intracellularis, and unicellular potential diazotrophs that typically thrive in the summer were reduced. Specifically, olig-otrophic coccolithophores, such as Umbellosphaera tenuis and Discosphaera tubifera, were replaced by Gephyrocapsa oceanica and Emiliania huxleyi, and Prochlorococcus sp. was replaced by Synechococcus spp. The shift of the phytoplankton community from Trichodesmium spp. to diatoms suggests that the biogenic carbon enhanced by the typhoons tended to sink rather than be recycled in the upper-water food web. © Inter-Research 2009.