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

Abstract

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, <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^–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

Southern Hemispheric Westerlies control sedimentary processes of Laguna Azul (south-eastern Patagonia, Argentina)

“…Values between 2 and 6 ‰ characterize modern terrestrial plants in the catchment area of Laguna Azul (Mayr et al, 2005). (Braig et al, 2013). This interpretation agrees with the strong decline of δ 13 C and high concentration of TS ( Fig.…”

“…TOC/TN ratios suggest predominance of lacustrine production. This indicates a lake at an early stage of evolution with low availability of nitrate or less intense denitrification compared with unit C. In this unit, TOC/TN ratios increase possibly because of loss of nitrogen related to enhanced microbial denitrification, which increases δ 15 N values as evidenced by positive δ 15 N excursions (Braig et al, 2013). This interpretation agrees with the strong decline of δ 13 C and high concentration of TS (Figure 4).…”

“…However, recent studies have shown that algal organic matter in crater lakes with natural eutrophication can also exhibit TOC/TN ratios >10 because of N-limiting conditions (Heyng et al, 2012). Eutrophication and/or meromictic conditions can lead to δ 13 C values below −30‰ because of microbial degradation of organic matter (Braig et al, 2013;Heyng et al, 2012;Hollander and Smith, 2001) explaining the observed values in unit C. The lack of correlation between TOC and BSi data during this unit further confirms that sedimentary organic matter is not only controlled by photosynthetic productivity but also by microbial sources and processes of decomposition.…”

Unravelling climate change impacts from other anthropogenic influences in a subalpine lake: a multi-proxy sediment study from Oberer Soiernsee (Northern Alps, Germany)

“…The nitrogen (d 15 N) and carbon (d 13 C org ) isotope fingerprints as well as the TOC/TN (total organic carbon/total nitrogen) ratios of bulk sediment provide valuable information on the origin of sedimentary organic matter (OM) and help to distinguish between autochthonous or allochthonous sources (Meyers & Teranes, 2001). In cases where the sedimentary OM was predominantly autochthonous, testified by low TOC/TN ratios, the records of d 13 C org and d 15 N were used to reconstruct past trophic conditions and changes in nutrient availability (Braig et al, 2013). The isotopic composition of the dissolved inorganic carbon (DIC) pool is controlled by the signature of atmospheric CO 2 permanently diffusing into the lake and by photosynthesis/respiration rates (Wetzel, 2001;Braig et al, 2013).…”

Microbial electrochemical systems for nitrate removal from diluted streams

“…However, a highly increased nitrogen load is likely to cause eutrophication especially in coastal areas [9], leading to excessive growth of planktonic algae (known as algal blooms) and consequently to ecosystem destruction mainly due to the depletion of dissolved oxygen [2,10].…”