SEASONAL VARIABILITY OF THE ORGANIC-MATTER IN A SEDIMENTARY COASTAL ENVIRONMENT - SOURCES, DEGRADATION AND ACCUMULATION (CONTINENTAL-SHELF OF THE GULF-OF-LIONS, NORTHWESTERN MEDITERRANEAN-SEA)

The temporal variations of the superficial (0-1 cm) sedimentary organi c matter were studied at a depth of 26 m on the continental shelf of t he Gulf of Lions (northwestern Mediterranean). The samples were analyz ed for total organic carbon, coarse organic carbon (>40 mu m), hydroly zable organic carbon, nitrogen, total amino acids, total and individua l sugars (HPLC), lignin-derived compounds (HPLC) and kerogens (acid-so luble, humic substances and human). Seasonal variations of the organic compounds are related to the sedimentological, hydrodynamical and phy sico-chemical environmental conditions. The mean annual values of the different organic compounds analyzed show the low quantities and their evolved character at the sediment-water interface: 0.5% total org C ( TOC) (d.w.), 0.049% N (d.w.), C/N: 11.2, coarse org C (COG): 62% of TO C, hydrolyzable org C: 45% TOC. The labile compounds represent a low p ercentage of the total organic matter (TOM), amino-acids: 12% of TOM a nd sugars: 5% of TOM. The relative proportions of soluble (humic) and insoluble kerogens (humin), respectively 6% and 94% of TOC are typical of a highly evolved organic matter. The large contribution of plant r emains confirmed by the high proportion of COG, corresponds to a low p roportion of humic substances and a high degree of condensation (H/C = 1.3). The infrared spectroscopy determination of the functional group s of the humic substances permits us to confirm both autochtonous (mar ine) and allochtonous (terrestrial) sources of organic matter in the T et prodeltaic accumulation area. Numerous functional groups identified reveal the fresh quality of the organic inputs at the sediment-water interface. Aliphaticity is well marked and nitrogenous compounds (1 an d 2 amines) correspond to autochtonous production (in spring: phyto- a nd zoo-planktonic blooms in the euphotic zone; in summer: primary prod uction under the thermocline and phytobenthic blooms). Sugars are well represented, but from two origins: phytoplanktonic, and issue from te rrestrial high plants (ligno-cellulosic complexes). The aromatic funct ional groups are well identified (aromatic ether, phenol) as are the m arkers of a large contribution of terrestrial organic matter. The acti ve hydrodynamical (resuspension) and biological (consumption, bioturba tion) events participate in the frequent reworking of the superficial deposits (muddy-silts) and, consequently, the active degradation (oxid ization) of the organic matter. The more labile organic matter is rapi dly consumed by the intense biological benthic activity. The organic C content is low (0.5% d.w.) except during the autumnal input (2.5% d.w .). This input (8 cm thick) disappears one month later, because of the resuspension induced by littoral currents. This same phenomenon induc es during all the year the reworking of the first centimeter of the de posit. In terms of budget of the carbon cycle in the coastal area, thi s active hydrodynamical environment shows the supremacy of the carbon degradation over its burial.