Origin and distribution of suspended organic matter as inferred from carbon isotope composition in a Mediterranean semi-enclosed marine system

Abstract

The origin and distribution of suspended organic matter, the trophic features and the stable carbon isotopic composition of particulate organic carbon (POC) were studied monthly in a Western Mediterranean semi-enclosed basin. Sampling stations were selected as a function of wind-exposure and the degree of vegetation cover and then compared with an adjacent unvegetated site. The predominant vegetation was seagrass (Posidonia oceanica and Cymodocea nodosa) and Caulerpa prolifera. Water samples were analyzed for total suspended matter (inorganic and organic fractions), photosynthetic pigments (chlorophyll-a and phaeopigments), dissolved organic carbon, particulate organic carbon and their isotopic composition. Temperature and salinity were also measured at the same sampling sites within range of Mediterranean limits. The suspended organic matter concentration was 1.77 ± 1.55 mg l-1; the chlorophyll-a concentration was low (0.35 ± 0.24 μg l-1); the dissolved organic carbon concentration was 2,140 ± 2,010 μg l-1; the particulate organic carbon concentration was 212 ± 106 μg l-1 and the isotopic composition was 18.77 ± 2.51‰. There were significant temporal differences except for phaeopigments, POC and its POC isotopic composition, and there were no spatial differences other than for δ13C. This picture highlighted a general seasonal trend and trophical features similar to adjacent sea. Spatial differences in δ13C showed that the source of suspended organic matter was different between stations as that between sources and wind-hydrodynamic constraints. In adjacent open sea, the phytoplankton component was identified as being the main isotropic signal of POC. In the Stagnone, resuspension from exposed sites and sedimentation into sheltered sites control biological dynamics, depending on wind exposition. Consequently, the dominance of the first or the second proces determines the typology and the development of vegetation, allowing it to recognize different sub-systems where the origin and distribution of particulate organic carbon could be different. A first sub-system was exposed to dominant winds and affected by sediment erosion which was from wind-induced turbulence, and had a main effect on limited submersed vegetation. The POC isotopic composition (δ13C = - 18.5‰) reflected a contemporary contribution of macrophyte and microphytobenthos. The combined effects of resuspension-sedimentation-lateral drifting permitted the autotrophic unicellular biomass marker to predominate, compared to Cymodocea and Caulerpa. The second sub-system was represented by the vitality of the waters and by unpredictability which was due to wind pulsing. As a result, a seasonal regime predominate by the trend in the seagrass leaves component (Posidonia oceanica and Cymodocea nodosa). The POC isotopic composition followed the seagrass, showing the heaviest values (spring and summer) in peaks in leaf biomass, and the lightest values in autumn and winter when the biomass was low.