Export of biogenic carbon and structure and dynamics of the pelagic food web in the Gulf of St. Lawrence Part 2. Inverse analysis

The effects of the structure and dynamics of the pelagic food web on the export of organic carbon from the euphotic zone (BC export) are poorly understood. In this paper, we use inverse methods to calculate trophic flows and BC export during two contrasting seasonal regimes in the Gulf of St. Lawrence, Canada. The inverse estimates take into account forms of export other than sinking particles, such as downward mixing of dissolved and suspended particulate organic carbon. The inverse analysis shows that the ratio of BC export to primary production (E/P) is ca. 0.70 for the winter–spring period (November–April), which is characterized by weak stratification, high primary production, and high abundance of large phytoplankton cells (>5 μm). The E/P declines to between 0 and 0.4, depending on the model used, during the summer–fall period (May–October), which is characterized by strong stratification, low production, and dominance of small phytoplankton cells. The higher export relative to production during winter–spring primarily results from the export of detritus, particularly dissolved organic carbon (DOC), by physical processes. Direct sinking of the larger phytoplankton cells plays a minor role. Conversely, the small export from the summer–fall food web appears to result primarily from reduced grazing activity and DOC production. Herbivory by microbial grazers (flagellates, dinoflagellates and ciliates) dominates consumption of phytoplankton throughout the year and is a significant trophic link to the mesozooplankton. In contrast, transfer of recycled DOC through bacteria and the microbial grazers is a less significant source of carbon for the larger consumers.