The seasonal cycle of phytoplankton biomass and primary productivity in the Ross Sea, Antarctica

Phytoplankton standing stocks and carbon assimilation were measured during four cruises to the southern Ross Sea, Antarctica during 1996 and 1997 in o rder to assess the details of the seasonal cycle of biomass and productivit y. The seasonal composite showed that phytoplankton biomass increased rapid ly during the austral spring, and integrated chlorophyll reached a maximum during the summer (January 15) and decreased thereafter. Particulate matter ratios (carbon:nitrogen, carbon:chlorophyll) also showed distinct seasonal trends with summer minima. Carbon assimilation increased rapidly in the sp ring, and reached a maximum of 231 mmol C m(-2) d(-1), ca, four weeks earli er than the maximum observed biomass (during early December). It decreased rapidly thereafter, and in austral autumn when ice formed, it approached ze ro. The time of maximum growth rate coincided with the maximum in C-assimil ation, and at 0.66 d(-1) equaled predictions based on laboratory cultures. Growth rates over the entire growing season, however, were generally much l ess. Deck-board incubations suggested that photoinhibition occurred at the greatest photon flux densities, but in situ incubations revealed no such su rface inhibition. We suggest that due to the nature of the irradiance field in the Antarctic, assemblages maintained in on-deck incubators received mo re light than those in situ, which resulted in photoinhibition. This in tur n resulted in a 17% underestimate in on-deck productivity relative to in si tu determinations. The phytoplankton bloom appeared to be initiated when ve rtical stability was imparted in austral spring, coincident with greater da ily photon flux densities. Conversely, decreased productivity likely result ed from trace metal limitation, whereas biomass declines likely resulted fr om enhanced loss rates, such as aggregate formation and enhanced vertical f lux of larger particles. The seasonal progression of productivity and bioma ss in the southern Ross Sea was similar to other areas in the ocean that ex perience blooms, and the cycling of carbon in this region is extensive, des pite the fact that the growing season extends no more than five months. (C) 2000 Elsevier Science Ltd. All rights reserved.