Geochemical signatures (C, N, δ¹³C, δ¹⁵N, metals) of suspended matter in the river Weiße Elster (central Germany): their seasonal and flow-related distribution 1997–2001

Abstract

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.

High frequency measurements of reach scale nitrogen uptake in a fourth order river with contrasting hydromorphology and variable water chemistry (Weiße Elster, Germany)

“…We then estimated the assimilatory demand for nitrogen ( D A ) by converting NPP to molar units, assuming 1 mol C fixed per mol O 2 produced, and multiplying by the ecosystem autotrophic C/N ratio. The molar C/N ratio used was 7 for July and 9 for September, corresponding to data collected during a previous study in the Weisse Elster middle reach [ Junge et al ., ]. These estimates of D A were then compared to U A calculated from the observed signals.…”

Heavy metal inflow into the floodplains at the mouth of the river Weiße Elster (Central Germany)

“…3). Irrespective of the hydrological situation, one sample was taken monthly at WSP2 as of 1993 [6].…”

Characteristics of suspended matter in the River Sava watershed, Slovenia

“…Diagrams 4 A, B, C and D show similar patterns; higher C/N ratio typical of a higher plant input with lower δ 13 C values are observed in the late summer sampling season, but correlation between δ 13 C POC and C/N ratios is poor. Higher δ 13 C POC values could be attributed to biological degradation and anthropogenic influences, while samples of industrial and coalmine wastewaters representing anthropogenic influences deviate from δ 13 C values and C/N ratios in suspended organic matter and have little importance on C/N ratios, δ 13 C and δ 15 The results of δ 13 C and δ 15 N in the suspended matter in River Weiβe Elster [32] showed the opposite trend in comparison with River Sava in Slovenia. Lower δ 13 C and higher δ 15 N in the suspended matter in River Weiβe Elster are due to increasing fractions of freshwater plankton during the summer period.…”