Dissolved organic carbon on Georges Bank

Dissolved organic carbon (DOC) in sea water from Georges Bank was measured by a high temperature combustion/direct injection (HTC/DI) technique during the spring bloom period in April 1993. Concentrations in surface waters (72-85 μM) and deep waters (54-56 μM) were similar to DOC concentrations measured in the oligotrophic north-west Atlantic Ocean by a number of other investigators by various techniques. Although surface values for Chl-a concentrations ranged from 2 to 5 μg l^-1, NO₃^- ranged from 3 to 9 μM and particulate organic carbon (POC) concentrations ranged from 1.0 to 10 μM, DOC concentrations only varied by < 18‰. DOC was slightly higher (∼5 μM) in the highly productive central bank region than in surrounding stratified surface waters. Relatively constant stable carbon isotopes (DO¹³C = -22.0 ± 0.5‰) could not be used to identify a source for these small variations. Slight decreases of DOC (5-12 μM) and dissolved organic nitrogen (DON) (0.5-1.0 μM) in filtered sea water kept in the dark was observed over six months, suggesting the presence of a small, labile pool of dissolved organic matter in addition to a large, more refractory reservoir of DOC (∼70 μM) during the spring bloom period on Georges Bank.