Origin and fate of organic carbon in the freshwater part of the Scheldt Estuary as traced by stable carbon isotope composition

We investigated the seasonal and geographical variation in the stable carbo n isotope ratios of total dissolved inorganic carbon (delta(13)C(DIC)) and suspended matter (delta(13)C(POC)) in the freshwater part of the River Sche ldt. Two major sources of particulate organic matter (POM) occur in this ri verine system: riverine phytoplankton and terrestrial detritus. In winter t he lowest delta(13)C(DIC) values are observed due to enhanced input of CO2 from decomposition of C-13-depleted terrestrial plant detritus (average del ta(13)C(DIC) = -14.3 parts per thousand). During summer, when litter input from terrestrial flora is the lowest, water column respiration on POM of te rrestrial origin is also the lowest as evidenced by less negative delta(13) C(DIC) values (average delta(13)C(DIC) = -9.9 parts per thousand). In winte r the phytoplankton biomass is low, as indicated by low chlorophyll a conce ntrations (Chl a < 4.5 mu gl(-1)), compared to summer when chlorophyll a co ncentrations can rise to a maximum of 54 mu gl(-1). Furthermore, in winter the very narrow range of delta(13)C(POC) (from -26.5 to -27.6 parts per tho usand) is associated with relatively high C/N ratios (C/N > 9) suggesting t hat in winter a major fraction of POC is derived from allochthonous matter. In summer delta(13)C(POC) exhibits a very wide range of values, with the m ost negative values coinciding with high Chl a concentrations and low C/N r atios (C/N < 8). This suggests predominance of phytoplankton carbon in the total particulate carbon pool, utilising a dissolved inorganic carbon reser voir, which is already significantly depleted in C-13. Using a simple two s ource mixing approach a reconstruction of the relative importance of phytop lankton to the total POC pool and of C-13/C-12 fractionation by phytoplankt on is attempted.