TEMPORAL VARIATION OF DELTA-C-13 IN PARTICULATE ORGANIC-MATTER AND OYSTER CRASSOSTREA-GIGAS IN MARENNES-OLERON BAY (FRANCE) - EFFECT OF FRESH-WATER INFLOW

The temporal variability of delta(13)C in suspended particulate organi c matter (POM) and oyster Crassostrea gigas along a salinity gradient was investigated from May 1992 to September 1993 within the estuarine bay of Marennes-Oleron (France). During this period the mean daily dis charge of the Charente River exhibited large seasonal variation, with a high discharge from November 1992 to January 1993. Contrary to that at the river mouth and the marine littoral, delta(13)C in POM and in o ysters at mid-estuary was affected by the high flood period. The delta (13)C values of POM decreased in mid-estuary and remained at low level s during the high discharge period, indicating an increasing contribut ion of terrestrial inputs to the estuarine POM pool. At the same site, a remarkable decrease of delta(13)C in oysters occurred between Decem ber 1992 and March 1993 (after a time lag compared to the ambient POM) , indicating incorporation of terrestrial organic matter in oyster tis sues during the high flood discharge. The lag between the delta(13)C d ecrease in POM and oysters is attributed to the time needed for oyster tissues to incorporate enough newly terrestrial light carbon to be re cognized by the delta(13)C measure (about 1 to 2 mo). This time interv al depends on tissue turnover time. The delta(13)C POM decrease (i.e. 1.3 parts per thousand) cannot explain entirely the decrease observed in oysters (i.e. 2.3 parts per thousand). In fact, the pattern exhibit ed by mid-estuarine oysters can be explained by the increasing contrib ution of terrestrial organic matter to their feeding, and the inabilit y to preferentially utilize specific components of the estuarine POM t hat are C-13-enriched.