Export of biogenic carbon and structure and dynamics of the pelagic food web in the Gulf of St. Lawrence Part 1. Seasonal variations

The seasonal changes in photosynthetic production, respiration, sinking flu x of organic carbon, and food web structure are described in the Gulf of St . Lawrence over a two-year period during the Canadian Joint Global Ocean Fl ux Study (JGOFS) program. The results show contrasts in net metabolism betw een periods of low (winter and spring) and high (summer and fall) vertical stability. The winter-spring period was associated with an autotrophic pela gic food web: predominance of large phytoplankton cells, large zooplankton, and high herbivorous potential transfers towards the zooplankton. The stra tified summer-fall period was associated with a heterotrophic food web: dom inance of small phytoplankton cells, replacement of the size class occupied by large phytoplankton with large heterotrophic dinoflagellates and ciliat es, smaller zooplankton, and dominance of omnivorous transfers towards the zooplankton, Despite differences in algal size and composition as well as i n size structure of the trophic compartments between winter-spring and summ er-fall, the particulate organic carbon fluxes observed at 50 m depth was q uantitatively similar during these two periods. Even though winter photosyn thetic production was relatively low, the high chlorophyll a concentration, the size structure of the trophic compartments, and the high contribution of large phytoplankton cells (mainly diatoms) to biological activity were s imilar to those observed during the spring and could explain the high heter otrophic biomass observed during winter. Crown copyright (C) 1999 Published by Elsevier Science Ltd. All rights reserved.