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 flux 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 Flux Study (JGOFS) program. The results show contrasts in net metabolism between periods of low (winter and spring) and high (summer and fall) vertical stability. The winter–spring period was associated with an autotrophic pelagic food web: predominance of large phytoplankton cells, large zooplankton, and high herbivorous potential transfers towards the zooplankton. The stratified summer-fall period was associated with a heterotrophic food web: dominance of small phytoplankton cells, replacement of the size class occupied by large phytoplankton with large heterotrophic dinoflagellates and ciliates, smaller zooplankton, and dominance of omnivorous transfers towards the zooplankton. Despite differences in algal size and composition as well as in size structure of the trophic compartments between winter–spring and summer–fall, the particulate organic carbon fluxes observed at 50 m depth was quantitatively similar during these two periods. Even though winter photosynthetic 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 similar to those observed during the spring and could explain the high heterotrophic biomass observed during winter.