Annual cycle of biogenic carbon export in the Gulf of St. Lawrence

Abstract

A multi-institutional initiative was created in a context of potential oil exploration at the Old Harry site to monitor the marine ecosystem and manage resources of the Gulf of St. Lawrence (GSL). As part of this initiative, two sediment traps were deployed at depths of 34 m and 100 m at a site northwest of Cabot Strait in the Laurentian Channel to measure downward carbon fluxes from October 2014 to October 2015. Fluxes of particulate organic carbon (POC), biogenic silica (BSi), chloropigments, protists and zooplankton fecal pellets were used to evaluate vertical and temporal changes in the magnitude and composition of biogenic carbon fluxes and to provide baseline information prior to any exploration activities in the region. Peaks in POC, BSi, chloropigment and diatom fluxes at both depths at the end of April 2015 indicated the rapid export of a locally-produced bloom mainly composed of pennate diatoms. A coincident peak in fecal pellet carbon fluxes at 100 m indicated a match between the spring ascent of copepods and the diatom bloom. The distinct composition of protist fluxes at 34 and 100 m following the formation of the winter mixed layer highlighted the influence of the pycnocline as a physical barrier for the export of particles in the region. Among identifiable components, dinoflagellates and fecal pellets, followed by diatoms and foraminifera, contributed most to the annual POC flux at 100 m, indicating a balance between lateral advection of slow-sinking protists and rapid export of fast-sinking diatoms and fecal pellets in the supply of POC toward the seafloor in the GSL. These measurements of biogenic carbon export provide insight into the fate of large and small particles in the Cabot Strait region to consider in the potential development of oil and gas activities.