Even though lake sediments are globally important organic carbon (OC) sinks, the controls on long-term OC storage in these sediments are unclear. Using a multiproxy approach, we investigate changes ...in diatom, green algae, and vascular plant biomolecules in sedimentary records from the past centuries across five temperate lakes with different trophic histories. Despite past increases in the input and burial of OC in sediments of eutrophic lakes, biomolecule quantities in sediments of all lakes are primarily controlled by postburial microbial degradation over the time scales studied. We, moreover, observe major differences in biomolecule degradation patterns across diatoms, green algae, and vascular plants. Degradation rates of labile diatom DNA exceed those of chemically more resistant diatom lipids, suggesting that chemical reactivity mainly controls diatom biomolecule degradation rates in the lakes studied. By contrast, degradation rates of green algal and vascular plant DNA are significantly lower than those of diatom DNA, and in a similar range as corresponding, much less reactive lipid biomarkers and structural macromolecules, including lignin. We propose that physical shielding by degradation-resistant cell wall components, such as algaenan in green algae and lignin in vascular plants, contributes to the long-term preservation of labile biomolecules in both groups and significantly influences the long-term burial of OC in lake sediments.
Summary
Lake sediments are globally important carbon sinks. Although the fate of organic carbon in lake sediments depends significantly on microorganisms, only few studies have investigated controls ...on lake sedimentary microbial communities. Here we investigate the impact of anthropogenic eutrophication, which affects redox chemistry and organic matter (OM) sources in sediments, on microbial communities across five lakes in central Switzerland. Lipid biomarkers and distributions of microbial respiration reactions indicate strong increases in aquatic OM contributions and microbial activity with increasing trophic state. Across all lakes, 16S rRNA genes analyses indicate similar depth‐dependent zonations at the phylum‐ and class‐level that follow vertical distributions of OM sources and respiration reactions. Yet, there are notable differences, such as higher abundances of nitrifying Bacteria and Archaea in an oligotrophic lake. Furthermore, analyses at the order‐level and below suggest that changes in OM sources due to eutrophication cause permanent changes in bacterial community structure. By contrast, archaeal communities are differentiated according to trophic state in recently deposited layers, but converge in older sediments deposited under different trophic regimes. Our study indicates an important role for trophic state in driving lacustrine sediment microbial communities and reveals fundamental differences in the temporal responses of sediment Bacteria and Archaea to eutrophication.
Lakes are recipients of allochthonous organic matter and nutrients. However, the importance of these subsidies for food webs and how they vary with lake trophic status remains unclear, especially for ...large lakes.
We assessed the source and fate of organic matter and nutrients in seven perialpine lakes across a gradient of trophic status. We measured carbon and nitrogen stable isotopes of amino acids of lake‐residing Atlantic trout, Salmo trutta, to determine the source of primary production (i.e., how carbon is fixed in the ecosystem) and how it is transferred through food webs, respectively. Based on essential amino acid carbon fingerprinting, we estimated the probability of organic carbon originating from autochthonous (algal), allochthonous (terrestrial plant), and recycled (bacterial) sources. In addition, we used amino acid δ15N to track how this primary production is transferred to consumers in general, and by using different trophic amino acids (glutamic acid and alanine), identify the trophic pathways involving either metazoan or protozoans.
We found a high likelihood of autochthonous origin of organic carbon (86 ± 9%) in trout that contrasted with allochthonous origins of particulate organic matter and some sediments. We showed that those estimates are good proxies of source reliance. Our results also highlighted the importance of bacterial origin of organic carbon in fish (12%). The likely autochthonous origin of this carbon was supported by trophic markers (Ala δ15N) that suggest the role of protists in transferring recycled organic carbon up the food web. While the sources of nitrogen sustaining food webs varied among lakes, we found a conserved carbon fingerprinting of fish. Overall, this suggests an uncoupling between the source of nutrients and organic carbon in large perialpine lakes.
Across a wide range of trophic status (c. 2 orders of magnitude range of phosphorus concentration), several lines of evidence suggested that perialpine lake food webs shared a common reliance on autochthonous and bacterial production.
Our study is the first to quantify the dependence on allochthonous organic carbon in lake food webs based on new amino acid stable isotope markers (carbon fingerprinting and Ala δ15N) and shows promise for estimating the source of carbon fixation in ecosystems. Our results support previous suggestions that terrestrial organic carbon is a relatively minor source for aquatic consumers despite contributing to the pool of organic matter, and more importantly, its contribution does not vary substantially with trophic status in perialpine lakes.
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