1ST-ORDER ORGANIC-CARBON BUDGET IN THE ST-LAWRENCE LOWER ESTUARY FROM C-13 DATA

Carbon isotope ratios and concentrations of suspended particulate matter and surface sediments of the Lower St Lawrence Estuary were used to determine the seasonal fluxes of particulate organic matter to the estuarine floor. Light carbon particles [-26·0‰≤δ13C(PDB) ≤ -24·5‰] brought by the St Lawrence River during spring freshet are carried to the intermediate and deep watermasses of the Lower Estuary until July. In contrast, heavy loads of 13C-enriched particles (δ13C up to -19·8‰) in the surface waters at the head of the Laurentian Channel are directly attributable to the early summer phytoplankton bloom or indirectly to the related zooplankton populations. Only small amounts of terrigenous particles (-25‰ ≤ δ13C ≤ -24‰) from the turbid zone are still found in the early fall water column. All particles have a short residence time of a few days within the St Lawrence Estuary, limiting most seaward export. The surface sediments of the Lower Estuary have an average C-isotope composition of -22·4‰. Using a simple two-components 13C-mixing equation, this value represents 43±8% and 57±8% of terrigenous and marine particulate organic carbon, respectively. A first-order carbon budget was then derived from published sedimentation rates and primary production estimates. About 75% of the terrigenous particulate organic matter introduced in the Upper Estuary is deposited within the limits of the Lower Estuary. Simultaneously, the equivalent of less than 30% of the annual phytoplanktonic carbon production in the Lower Estuary reaches the estuarine floor, mostly under the form of copepod faecal pellets. Over 70% of the photosynthesized carbon is biodegraded and recycled in the water column. © 1991.