Distributions of oxygen, carbon, and respiratory activity in the deep layer of the Gulf of St. Lawrence and their implications for the carbon cycle

The Gulf of St. Lawrence (GSL) is a semi-enclosed sea with an estuarine circulation forced by runoff from the St. Lawrence and Great Lakes drainage systems and balanced by a deep inflow of oceanic waters through the Laurentian Channel. Based on samples collected between July 1992 and June 1994 during Phase I of the CJGOFS program conducted in the Gulf of St. Lawrence, we present new data on the carbon and nutrient chemistry as well as respiratory activity in the deep waters. Organic carbon fluxes estimated from sediment trap data, deepwater respiratory activity, and benthic respiration measurements are consistent with those obtained from the changes in the dissolved oxygen concentration of the deep waters along the Laurentian Channel. These flux estimates suggest that approximately 10% of the local primary production reaches the deep layer (>200 m) and the sediments in the GSL. The vertical carbon budget is almost balanced in the eastern part of the gulf, but approximately half of the carbon produced in the surface layer of the northwestern gulf cannot be accounted for. The difference in hydrodynamic and biological conditions prevailing in both areas may explain our observations. (author). 71 refs., 5 tabs., 6 figs