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. Even though human-induced eutrophication has severelyimpacted temperate lake ecosystems over the last centuries, the effects ontotal organic carbon (TOC) burial and mineralization are not wellunderstood. We study these effects based on sedimentary records from thelast 180 years in five Swiss lakes that differ in trophic state. We comparechanges in TOC content and modeled TOC accumulation rates through time tohistorical data on algae blooms, water column anoxia, wastewater treatment,artificial lake ventilation, and water column phosphorus (P) concentrations.We furthermore investigate the effects of eutrophication on rates ofmicrobial TOC mineralization and vertical distributions of microbialrespiration reactions in sediments. Our results indicate that the history ofeutrophication is well recorded in the sedimentary record. Overall,eutrophic lakes have higher TOC burial and accumulation rates, andsubsurface peaks in TOC coincide with past periods of elevated Pconcentrations in lake water. Sediments of eutrophic lakes, moreover, havehigher rates of total respiration and higher contributions of methanogenesisto total respiration. However, we found strong overlaps in the distributions ofrespiration reactions involving different electron acceptors in all lakesregardless of lake trophic state. Moreover, even though water column Pconcentrations have been reduced by ∼ 50 %–90 % since theperiod of peak eutrophication in the 1970s, TOC burial and accumulationrates have only decreased significantly, by ∼ 20 % and 25 %,in two of the five lakes. Hereby there is no clear relationship between themagnitude of the P concentration decrease and the change in TOC burial andaccumulation rate. Instead, data from one eutrophic lake suggest thatartificial ventilation, which has been used to prevent water column anoxiain this lake for 35 years, may help sustain high rates of TOC burial andaccumulation in sediments despite water column P concentrations beingstrongly reduced. Our study provides novel insights into the influence ofhuman activities in lakes and lake watersheds on lake sediments as carbonsinks and habitats for diverse microbial respiration processes.

Effects of eutrophication on sedimentary organic carbon cycling in five temperate lakes