Abstract. Even though human-induced eutrophication has severely
impacted temperate lake ecosystems over the last centuries, the effects on
total organic carbon (TOC) burial and mineralization are not well
understood. We study these effects based on sedimentary records from the
last 180 years in five Swiss lakes that differ in trophic state. We compare
changes in TOC content and modeled TOC accumulation rates through time to
historical 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 of
microbial TOC mineralization and vertical distributions of microbial
respiration reactions in sediments. Our results indicate that the history of
eutrophication is well recorded in the sedimentary record. Overall,
eutrophic lakes have higher TOC burial and accumulation rates, and
subsurface peaks in TOC coincide with past periods of elevated P
concentrations in lake water. Sediments of eutrophic lakes, moreover, have
higher rates of total respiration and higher contributions of methanogenesis
to total respiration. However, we found strong overlaps in the distributions of
respiration reactions involving different electron acceptors in all lakes
regardless of lake trophic state. Moreover, even though water column P
concentrations have been reduced by ∼ 50 %–90 % since the
period of peak eutrophication in the 1970s, TOC burial and accumulation
rates have only decreased significantly, by ∼ 20 % and 25 %,
in two of the five lakes. Hereby there is no clear relationship between the
magnitude of the P concentration decrease and the change in TOC burial and
accumulation rate. Instead, data from one eutrophic lake suggest that
artificial ventilation, which has been used to prevent water column anoxia
in this lake for 35 years, may help sustain high rates of TOC burial and
accumulation in sediments despite water column P concentrations being
strongly reduced. Our study provides novel insights into the influence of
human activities in lakes and lake watersheds on lake sediments as carbon
sinks and habitats for diverse microbial respiration processes.
Peer Review #2 of "Rethinking the lake trophic state index (v0.2)"
