@article{Braig_Mayr_Kirchner_Hofmann_Raeder_Melzer_2013, title={Fifty years of eutrophication and lake restoration reflected in sedimentary carbon and nitrogen isotopes of a small, hardwater lake (south Germany)}, volume={72}, url={https://www.jlimnol.it/jlimnol/article/view/jlimnol.2013.e21}, DOI={10.4081/jlimnol.2013.e21}, abstractNote={This study analyses the response of the carbon and nitrogen isotopic composition of sedimentary organic matter to rapid human-induced eutrophication and meromixis as well as subsequent restoration efforts [in-lake phosphorus (P)-Precipitation, P-remediation of the well inflow and multiannual destratification] in a 46-yr sediment core sequence (1963-2009) from Fischkaltersee, a small hard-water lake (S-Germany). In addition, the sediment record was compared with detailed data on water column chemistry during almost (1977-2009) the recorded history of eutrophication and trophic recovery of the named lake. While the onset of eutrophication resulted in an abrupt positive excursion (+2.4‰), the overall reaction of δ<sup>13</sup>C<sub>SOM</sub> to ongoing eutrophication and meromixis as well as to permanent hypolimnion aeration and trophic recovery is a continous negative trend (-3.7‰) with the most depleted signatures (-38.8‰) present in the youngest part of the core. This negative trend was not influenced by multiannual hypolimnion aeration, which although oxygenating bottom waters (>2 mg O<sub>2</sub> L<sup>–1</sup>), did not reverse the increasing anoxis in the sediment, as is indicated by an declining Mn/Fe ratio. Hence, we conclude that in Fischkaltersee δ<sup>13</sup>C<sub>SOM</sub> was controlled by photoautotrophic input only during an early phase in the eutrophication process. The signal of intensifying microbially mediated carbon cycling processes in the sediment, <em>i.e.</em> methanogenesis and methanotrophy, was superimposed on the primary productivity signal by crossing a certain TP threshold (approx. TP=0.04 mg L<sup>–1</sup>). Sedimentary δ<sup>15</sup>N values exhibit an overall increase (+3.4‰) in reaction to the eutrophication process, while trophic recovery produces a continous decrease in the signal (-2.7‰). Linear correlation of δ<sup>15</sup>N to nitrate utilisation in the epilimnion, however, is rather weak (R<sup>2</sup>=0.33). Comparison between sediment δ<sup>15</sup>N values and water column data reveals that two negative shifts in the isotopic signal (-1‰/-0.6‰) around 1980 and 1984 respectively, can be attributed to the existence of significant amounts of ammonium (up to 2 mg L<sup>–1</sup>) in the trophogenic zone. Coincident drops in nitrate utilisation (-20%/-53%) indicate phytoplankton assimilation of ammonium. As the artificial aeration did also not affect δ<sup>15</sup>N, we conclude that water column denitrification never contributed significantly to the sedimentary N isotope signal.}, number={2}, journal={Journal of Limnology}, author={Braig, Emanuel and Mayr, Christoph and Kirchner, Gerald and Hofmann, Andrea and Raeder, Uta and Melzer, Arnulf}, year={2013}, month={Apr.}, pages={e21} }