TEMPORAL VARIATIONS IN THE CHEMICAL AND CARBON-ISOTOPE COMPOSITIONS OF MARINE AND TERRESTRIAL ORGANIC INPUTS IN THE BAY OF MARENNES-OLERON,FRANCE

Composition of particulate organic matter inputs within Marennes-Olero n Bay (France) was investigated over various times scales, through mul tiple measurements (SPM, C:N, Chi. a, delta(13)C). Oceanic and terrest rial end-members show little delta(13)C variations over the seasonal c ycle or between years. Seasonal variations in chemical and isotopic co mposition of riverine POM reflect the relative contribution of riverbo rne phytoplankton, but, except during spring bloom, POM appears to be mainly detrital. Between years, differences in rainfall regime greatly affect all the measured parameters. At the estuary mouth, seasonal va riations observed within the river are blurred by sedimentation-resusp ension processes linked to hydrodynamic constraints, leading to only l ittle variations in the delta(13)C values. The larger variations were observed here in C:N values and were ascribed to the effect of high fl oods bringing more nitrogenous detritus in the lower part of the estua ry. At the marine pole of the bay, POM is mainly dominated during wint er by aged terrestrial detritus during low river flow periods, and ref lects new ''fresh'' continental inputs during runoff events. In spring and fall, its composition appears to be primarily controlled by prima ry production. C-13/C-12 ratios of bay phytoplankton are markedly more negative than true oceanic values, probably due to differences in spe cies composition. In Marennes-Oleron Bay, POM composition cannot be de scribed by a simple mixing model between oceanic and terrestrial end-m embers, since many biological and physical processes alter their chemi cal and isotopic signatures. Variations between years emphasize also t he need of considering meteorological events such as high floods or lo ng drought periods.