Stable isotopes in recent sediments of Lake Arendsee, NE Germany: Response to eutrophication and remediation measures

Recent sediments of Lake Arendsee, a small eutrophic lake in NE Germany, were examined in high temporal resolution in order to investigate the applicability of stable isotopes as proxies to trace paleoenvironmental conditions. The age model of the sediment core is based on varve counting and 137Cs/210Pb dating and reveals that the sedimentary record covers the last 70 years, during which the lake has undergone major changes in productivity and eutrophication in response to nutrient loading.

Since 1965, TOC, TIC, TN and TP accumulation rates in general reflect the change from oligo-/mesotrophic to eutrophic level. The isotopic compositions of autochthonous organic matter and authigenic calcite (whitings) demonstrate that carbon isotope fractionation takes place in a CO2-limited pool. In accordance with the historical limnological observations, enhanced eutrophication is reflected by δ13CTOC increases of over 3‰ from 1934 to 1985 as a result of less 12C/13C fractionation by photosynthesis during intense primary productivity.

Decreasing δ13CTIC values indicate that biogenically induced calcite formation occurred during enhanced primary production accompanied by CO2 limitation. The gradual trend to more light nitrogen isotopic signatures towards the sediment surface reflects the dominance of air nitrogen fixation of cyanobacteria blooms during the last decades.

The δ18OTIC distribution suggests that plankton blooms between 1934 and 1978 had shifted towards the early season with lower temperatures as a result of increasing nutrient level, which subsequently favor plankton blooms even in colder waters. Since the mid 1980s, decreasing δ13CTOC and increasing δ13CTIC values since 1985 indicate a more efficient 12C/13C fractionation during photosynthesis with no CO2 limitation.

This improvement in water quality is in accordance with several remediation measures carried out since the late 1970s. Linear isotopic trends are interrupted by high TIC, TOC, TN and TP accumulation rates in the early 1970s, which can be clearly attributed to the artificial drainage of a nearby wetland carried out during that time.

This event can be considered an important external source for nutrient-loaded water and particulate matter and is displayed by significantly heavier δ13CTIC and δ18OTIC values in the sedimentary record.