Weak Response of Animal Allochthony and Production to Enhanced Supply of Terrestrial Leaf Litter in Nutrient-Rich Lakes
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Ecosystems are generally linked via fluxes of nutrients and energy across their boundaries. For example, freshwater ecosystems in temperate regions may receive significant inputs of terrestrially derived carbon via autumnal leaf litter. This terrestrial particulate organic carbon (POC) is hypothesized to subsidize animal production in lakes, but direct evidence is still lacking. We divided two small eutrophic lakes each into two sections and added isotopically distinct maize litter to the treatment sections to simulate increased terrestrial POC inputs via leaf litter in autumn. We quantified the reliance of aquatic consumers on terrestrial resources (allochthony) in the year subsequent to POC additions by applying mixing models of stable isotopes. We also estimated lake-wide carbon (C) balances to calculate the C flow to the production of the major aquatic consumer groups: benthic macroinvertebrates, crustacean zooplankton, and fish. The sum of secondary production of crustaceans and benthic macroinvertebrates supported by terrestrial POC was higher in the treatment sections of both lakes. In contrast, total secondary and tertiary production (supported by both autochthonous and allochthonous C) was higher in the reference than in the treatment sections of both lakes. Average aquatic consumer allochthony per lake section was 27–40%, although terrestrial POC contributed less than about 10% to total organic C supply to the lakes. The production of aquatic consumers incorporated less than 5% of the total organic C supply in both lakes, indicating a low ecological efficiency. We suggest that the consumption of terrestrial POC by aquatic consumers facilitates a strong coupling with the terrestrial environment. However, the high autochthonous production and the large pool of autochthonous detritus in these nutrient-rich lakes make terrestrial POC quantitatively unimportant for the C flows within food webs.
Keywordsstable isotopes terrestrial subsidy carbon budget ecological efficiency benthic food web pelagic food web
We thank A. Türck, C. Helms, J. Schreiber, S. Schuchort, S. Oksanen, and T. Wanke for their help in the field. We further acknowledge discussion and contributions by M. Gessner, R. Jones, S. Devlin, A. Vogt, K. Kuntze, M. Graupe, A. Busse, D. Thompson, S. Schmidt-Halewicz, N. Walz, P. Casper, K. Premke, G. Nützmann, J. Rääpysjärvi, and M. Kaupenjohann. Two anonymous reviewers provided comments, which helped improving the text. We thank K. Metzdorf (Technoplan Zelte und Planen GmbH) for lake divisions. R. Mauersberger (Förderverein Feldberg-Uckermärkische Seen e.V.) and R. Tischbier (Stiftung Pro Artenvielfalt) kindly provided background information and access to the lakes. This study was financed by the TERRALAC-project (http://terralac.igb-berlin.de) of the Wissenschaftsgemeinschaft Leibniz (WGL). J. Syväranta and M.J. Vanni were supported by the IGB Fellowship program in Freshwater Science and K. Scharnweber was further supported by the German Academic Exchange Service (DAAD).
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