Seasonal and annual variations in the organic matter contributed by the St Lawrence River to the Gulf of St. Lawrence

The St. Lawrence River discharges a substantial volume of water (405 km³/a) containing suspended (SPM; 3.42 × 10⁶t) and dissolved (68.0 × 10⁶t) materials to the Gulf of St. Lawrence. The total load contains organic carbon in paniculate (POC; 3–14% of SPM), and dissolved (DOC; 3.76 ± 0.63 mg/l) form. The concentration of POC (and particulate organic nitrogen) is positively correlated with discharge (increased during the spring flood and the fall enhancement of flow), but concentration of DOC is not so simply related to discharge. In consequence, the total organic carbon (POC + DOC) load is relatively invariant, and increased annually by only 2–3% despite a progressive increase of 8% in discharge over the years of this study. Seasonal differences in the composition of the particulate organic matter (POM) are interpreted as reflecting dominant contributions from within-river production in summer and from terrestrial sources in spring and fall. In years when the annual discharge was greater than average, a higher proportion of the POM was terrigenous. The organic matter in surface sediments of the estuary to which the river discharges is predominantly of terrestrial provenance.