CARBON AND NITROGEN STABLE ISOTOPES IN SUSPENDED MATTER AND SEDIMENTSFROM THE SCHELDE ESTUARY

The C/N and stable C and N isotope ratios (delta(13)C, delta(15)N) of sedimentary and suspended particulate matter were determined in the Sc helde Estuary. Suspended matter was divided into 2 to 5 size fractions by centrifugation. Four major pools of organic matter were recognized : riverine, estuarine, marine and terrestrial materials. Terrestrial o rganic matter (delta(13)C approximate to -26%, delta(15)N approximate to 3.5%, C/N approximate to 21) is important for the sedimentary pool, but suspended matter is dominated by the marine (delta(13)C approxima te to -18%, delta(15)N approximate to 9%, C/N approximate to 8), river ine (delta(13)C approximate to -30%, delta(15)N approximate to 9%, C/N approximate to 7.5) and estuarine (delta(13)C approximate to -29%, de lta(15)N approximate to 15%, C/N approximate to 8) end-members. In the upper estuary, the suspended matter size fractions vary systematicall y in their carbon and nitrogen biogeochemistry, with the small particl es having low C/N ratios, depleted delta(13)C and enriched delta(15)N values relative to large particles. Moreover, sedimentary and suspende d matter differ significantly in terms of C/N ratios (17 vs. 8.9), del ta(13)C (-26.3 vs. -28.9%) and delta(15)N (+ 6.9 vs. 12.0%). In the lo wer estuary, suspended matter fractions are similar and sedimentary an d suspended organic matter differ only in terms of delta(13)C (-23.5 v s. -20.1%). Our data indicate that autochthonous organic matter contri butes significantly to the total suspended matter and that the suspend ed organic matter composition cannot be explained in terms of conserva tive mixing of riverine and terrestrial sources on the one hand and ma rine sources on the other hand. (C) 1998 Elsevier Science B.V.