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

Abstract

Abstract. Even though human-induced eutrophication has severely\nimpacted temperate lake ecosystems over the last centuries, the effects on\ntotal organic carbon (TOC) burial and mineralization are not well\nunderstood. We study these effects based on sedimentary records from the\nlast 180\xa0years in five Swiss lakes that differ in trophic state. We compare\nchanges in TOC content and modeled TOC accumulation rates through time to\nhistorical data on algae blooms, water column anoxia, wastewater treatment,\nartificial lake ventilation, and water column phosphorus (P) concentrations.\nWe furthermore investigate the effects of eutrophication on rates of\nmicrobial TOC mineralization and vertical distributions of microbial\nrespiration reactions in sediments. Our results indicate that the history of\neutrophication is well recorded in the sedimentary record. Overall,\neutrophic lakes have higher TOC burial and accumulation rates, and\nsubsurface peaks in TOC coincide with past periods of elevated P\nconcentrations in lake water. Sediments of eutrophic lakes, moreover, have\nhigher rates of total respiration and higher contributions of methanogenesis\nto total respiration. However, we found strong overlaps in the distributions of\nrespiration reactions involving different electron acceptors in all lakes\nregardless of lake trophic state. Moreover, even though water column P\nconcentrations have been reduced by ∼ \u200950\u2009%–90\u2009% since the\nperiod of peak eutrophication in the 1970s, TOC burial and accumulation\nrates have only decreased significantly, by ∼ \u200920\u2009% and 25\u2009%,\nin two of the five lakes. Hereby there is no clear relationship between the\nmagnitude of the P concentration decrease and the change in TOC burial and\naccumulation rate. Instead, data from one eutrophic lake suggest that\nartificial ventilation, which has been used to prevent water column anoxia\nin this lake for 35 years, may help sustain high rates of TOC burial and\naccumulation in sediments despite water column P concentrations being\nstrongly reduced. Our study provides novel insights into the influence of\nhuman activities in lakes and lake watersheds on lake sediments as carbon\nsinks and habitats for diverse microbial respiration processes.