Dissolved organic carbon in the Gulf of St. Lawrence

Dissolved organic carbon in the Gulf of St. Lawrence was measured throughout the water column by high-temperature catalytic oxidation during and after the 1994 spring phytoplankton bloom. DOC ranged from 28 to 111 μM. Except for the waters near the Magdalen Islands, the DOC in the euphotic zone was higher after the bloom (84.2–99.1 μM) than during the bloom (81–83 μM). Lowest euphotic zone DOC occurred near the Magdalen Islands (49 μM) where it was nearly as low as the deep-water DOC (35–50 μM) in the Anticosti Channel and Cabot Strait. In the deep water below 150 m the DOC levels in the inner regions of the Gulf were higher during the bloom than after it. These results along with the results of phytoplankton, bacteria, and ocean chemistry analyses were subjected to multiple regression, Spearman Rank, and principal component analyses to elucidate the role of DOC in transferring carbon between planktonic components of the Gulf of St. Lawrence pelagic ecosystem. The analyses showed a relationship between DOC and phytoplankton primary production in the euphotic zone during the bloom and a relationship between DOC and bacterial production in the euphotic zone after the bloom. A diel study showing DOC production in the morning and DOC degradation in the evening implicated phytoplankton in generating DOC during the day.