Phytoplankton response to deep seawater nutrient addition in the North Pacific Subtropical Gyre

摘要

ABSTRACT: We examined the phytoplankton response to the addition of nutrient-enriched deep seawater (DSW) and of nitrate only during shipboard experiments performed between July 2004 and May 2007 in the North Pacific Subtropical Gyre (NPSG). Chlorophyll a (chl a), community size structure and composition, carbon fixation (CF) rates, and nutrient concentrations were measured daily for 5 to 7 d under simulated in situ conditions. Despite the fact that the NPSG is a permanently stratified, oligotrophic biome, there was a seasonal response to the addition of DSW and nitrate. In summertime experiments, chl a and CF rates increased up to 18- and 22-fold, respectively, relative to unamended controls after an incubation period of 5 to 6 d. Nutrients were assimilated to below control concentrations. A shift from a Prochlorococcus-dominated to a Synechococcus- and diatom-dominated community, and an 8- to 12-fold increase in the chl a content of the 2 µm size fraction were observed. Addition of nitrate only increased chl a and CF rates 10- and 7-fold, respectively. In wintertime experiments, chl a and CF rates increased up to 4- and 9-fold, respectively, after an incubation period of up to 7 d. Only 50% of nutrients were assimilated and nitrate alone stimulated a 2-fold increase in chl a and CF rates. We explore the role of nutrient limitation, community composition, grazing, and light in explaining our observations. Findings from this seasonal and multi-year field experiment demonstrate how little we know about bloom development in the NPSG and highlight the potential risk in extrapolating the response of phytoplankton to natural or artificial fertilization from short-term studies.