Geochemical signatures (C, N, delta13C, delta15N, metals) of suspended matter in the river Weisse Elster (central Germany): their seasonal and flow-related distribution 1997-2001

Geochemical studies of carbon, nitrogen, delta13C, delta15N as well as Fe, Mn, Cd, Zn, Cr and Hg in suspended matter taken from the river Weisse Elster (central Germany) between 1997 and 2001 reveal significant changes to the composition of the organic sediment load, which correlate with the hydrological period and flow rate. Using C/N ratios and the isotope values of carbon and nitrogen as source indicators, it was found that the organic suspended matter fractions in hydrological winter periods comprise both resuspended mortal plankton material from the riverbed and terrigenous C3 plant material from the clastic input. During the 6 month summer periods, increased bioproductivity results in more dissolved carbon and mineral nitrogen compounds being taken up by the freshly formed aquatic organic substance (freshwater plankton). These compounds stem from bacterial breakdown processes affecting organic components of the river sediment and/or the peripheral soil zone. Increasing fractions of freshwater plankton during the summer period are accompanied by an increase in the nitrogen content and by isotope signatures shifting (delta13C to lower but delta15N to higher values) in the suspended matter. Seasonally opposite correlations between metal contents (e.g. Cd, Zn, Cr, Hg and Fe) and the carbon and nitrogen levels of suspended matter (significantly positive in winter and significantly negative in summer) show that in suspended matter these elements mostly bind to resuspended mortal (rather than the freshly formed living aquatic) organic substance. According to long-term measuring series, between 1993 and 2002 the levels of heavy metals (especially cadmium) in the suspended matter of the river Weisse Elster decreased. Similarly, between 1997 and 2001 the oxygen level in the river Weisse Elster improved. This caused the faster breakdown of organic substance on the riverbed, resulting in the increased uptake of 15N-rich nitrogen compounds into the fresh aquatic organic substance formed every year, and an increase in the conversion of dissolved manganese in the water into insoluble manganese compounds in the river sediment.