Fifty years of eutrophication and lake restoration reflected in sedimentary carbon and nitrogen isotopes of a small, hardwater lake (South Germany)

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 δ¹³C_SOM 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 O₂ L^-1), did not reverse the increasing anoxis in the sediment, as is indicated by an declining Mn/Fe ratio. Hence, we conclude that in Fischkaltersee δ¹³C_SOM 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, i.e. methanogenesis and methanotrophy, was superimposed on the primary productivity signal by crossing a certain TP threshold (approx. TP=0.04 mg L^-1). Sedimentary δ¹⁵N 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 δ¹⁵N to nitrate utilisation in the epilimnion, however, is rather weak (R²=0.33). Comparison between sediment δ¹⁵N 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^-1) in the trophogenic zone. Coincident drops in nitrate utilisation (-20%/-53%) indicate phytoplankton assimilation of ammonium. As the artificial aeration did also not affect δ¹⁵N, we conclude that water column denitrification never contributed significantly to the sedimentary N isotope signal.