Organic carbon isotope ratios and implications for the maximum turbidity zone of the St Lawrence Upper Estuary

Abstract

Carbon isotope ratios of suspended particulate matter of the St Lawrence Upper Estuary exhibit a different linear correlation with the ambient salinities for each sampling period, if one excludes the stations from the early mixing zone. These relationships reflect the tidal advection of particles in the turbid zone and their conservative mixing with marine inputs. The year-round constant δ 13C value of the marine pole (−23·6‰, S.D. = 0·3) is representative of an average long term mixture of terrigenous particles and planktonic cells. In May, the upstream pole (δ 13C = −26·6‰) is dominated by terrigenous particulate organic carbon brought by the river with the spring freshet. After spring run-off, the sedimentary exchanges between the large tidal platforms of the Cap Tourmente region and the nearby estuarine channels control the particulate organic carbon composition of the upstream pole (δ 13C = −25·4‰ in June to −24·4‰ in October). The influence of this pole extends to the entire maximum turbidity zone, blurring the isotopic characteristics of the freshwater particulate organic carbon inputs. The summer fluvial planktonic production (δ 13C = −24·0‰) for instance has a negligible influence on the gradual seasonal 13C enrichment of the upstream pole. Similarly, the debris of C 3 vascular plants growing on the marshes ( Scirpus and Sagittaria) does not appear to have a dominating influence on the fall particulate organic carbon composition of the turbid zone. The ‘averaged out’ upstream pole reflects the long residence time (6–12 months) of the particles kept in suspension in the turbid zone.