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 ex port of organic carbon from the euphotic zone (BC export) are poorly unders tood. In this paper, we use inverse methods to calculate trophic flows and BC export during two contrasting seasonal regimes in the Gulf of St. Lawren ce, Canada. The inverse estimates take into account forms of export other t han sinking particles, such as downward mixing of dissolved and suspended p articulate 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 prim ary production, and high abundance of large phytoplankton cells (> 5 mu m). 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 str atification,low production, and dominance of small phytoplankton cells. The higher export relative to production during winter-spring primarily result s from the export of detritus, particularly dissolved organic carbon (DOC), by physical processes. Direct sinking of the larger phytoplankton cells pl ays a minor role. Conversely, the small export from the summer-fall food we b appears to result primarily from reduced grazing activity and DOC product ion. Herbivory by microbial grazers (flagellates, dinoflagellates and cilia tes) dominates consumption of phytoplankton throughout the year and is a si gnificant trophic link to the mesozooplankton. In contrast, transfer of rec ycled DOC through bacteria and the microbial grazers is a less significant source of carbon for the larger consumers. (C) 1999 Elsevier Science Ltd. A ll rights reserved.