Response of phytoplankton community to nutrient enrichment in the subsurface chlorophyll maximum in Yellow Sea Cold Water Mass

The subsurface chlorophyll maximum (SCM) is a prominent biological feature in stratified waters, and may contribute substantial biomass to the water column. Understanding the transient variations of SCM phytoplankton in response to episodic nutrient input is crucial to accurately estimate integrated primary production and to assess the impact of the perturbation to the pelagic ecosystem. A microcosm experiment designed to investigate the responses of SCM phytoplankton community to pulsed nutrient enrichment was conducted in summer 2011 in the Yellow Sea Cold Water Mass area. During the experiment, the incubation cultures sustained a high photosynthetic yield (Fv/Fm) indicating that the phytoplankton was photosynthetically competent and well acclimated to conditions of irradiance and nutrient supply at the SCM. Both Fv/Fm and Chlorophyll a (Chl a) responded significantly in P enriched treatments, but not in the N enriched treatments. The largest increase of Chl a and Fv/Fm occurred when P and N were added simultaneously. Synechococcus abundance decreased sharply during the incubation, while picoeukaryote abundance increased in the P and NP addition treatments. The phytoplankton community shifted from smaller dinoflagellates dominated in the natural environment to larger diatoms dominated under nutrient enrichment conditions. The results indicated that the phytoplankton at the SCM was co-limited by P and N and had a higher requirement for P relative to N. The additional nutrient supply enhanced photosynthetic activity and favored the dominance of larger diatoms which are beneficial to carbon export. Our study suggested that episodic nutrient input induced by various physical processes make a significant impact on the phytoplankton community at the SCM. This information is important for better understanding and predicting biological responses to future climate change.