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 carbon isotope ratios of total dissolved inorganic carbon (¹³C_POC) and suspended matter (¹³C_POC) in the freshwater part of the River Scheldt. Two major sources of particulate organic matter (POM) occur in this riverine system: riverine phytoplankton and terrestrial detritus. In winter the lowest ¹³C_DIC values are observed due to enhanced input of CO₂ from decomposition of ¹³C-depleted terrestrial plant detritus (average ¹³C_DIC = –/14.3). During summer, when litter input from terrestrial flora is the lowest, water column respiration on POM of terrestrial origin is also the lowest as evidenced by less negative ¹³C_DIC values (average ¹³C_DIC = –9.9). In winter the phytoplankton biomass is low, as indicated by low chlorophyll a concentrations (Chl a < 4.5 gl^–1), compared to summer when chlorophyll a concentrations can rise to a maximum of 54 gl^–1. Furthermore, in winter the very narrow range of ¹³C_POC (from –26.5 to –27.6) is associated with relatively high C/N ratios (C/N > 9) suggesting that in winter a major fraction of POC is derived from allochthonous matter. In summer ¹³C_POC exhibits a very wide range of values, with the most negative values coinciding with high Chl a concentrations and low C/N ratios (C/N < 8). This suggests predominance of phytoplankton carbon in the total particulate carbon pool, utilising a dissolved inorganic carbon reservoir, which is already significantly depleted in ¹³C. Using a simple two source mixing approach a reconstruction of the relative importance of phytoplankton to the total POC pool and of ¹³C/¹²C fractionation by phytoplankton is attempted.