Origin and fate of organic carbon in the freshwater part of the Scheldt Estuary as traced by stable carbon isotope composition
Hellings, L.; Dehairs, F.A.; Tackx, M.; Keppens, E.; Baeyens, W.F.J. (1999). Origin and fate of organic carbon in the freshwater part of the Scheldt Estuary as traced by stable carbon isotope composition. Biogeochemistry 47(2): 167-186. http://dx.doi.org/10.1023/A:1006143827118
We investigated the seasonal and geographical variation in the stable carbon isotope ratios of total dissolved inorganic carbon (d13CDIC) and suspended matter (d13CPOC) 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 d13CDIC values are observed due to enhanced input of CO2 from decomposition of C-13-depleted terrestrial plant detritus (average d13CDIC = -14.3 parts per thousand). 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 d13CDIC values (average d13CDIC = -9.9 parts per thousand). In winter the phytoplankton biomass is low, as indicated by low chlorophyll a concentrations (Chl a < 4.5 µgl-1), compared to summer when chlorophyll a concentrations can rise to a maximum of 54 µgl-1. Furthermore, in winter the very narrow range of d13CPOC (from -26.5 to -27.6 parts per thousand) 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 d13CPOC 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.
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