Whole‐lake experiments reveal the fate of terrestrial particulate organic carbon in benthic food webs of shallow lakes

Abstract

Abstract. Lake ecosystems are strongly linked to their terrestrial surroundings by material and energy fluxes across ecosystem boundaries. However, the contribution of terrestrial particulate organic carbon (tPOC) from annual leaf fall to lake food webs has not yet been adequately traced and quantified. In this study, we conducted whole-lake experiments to trace artificially added tPOC through the food webs of two shallow lakes of similar eutrophic status, but featuring alternative stable regimes (macrophyte rich vs. phytoplankton dominated). Lakes were divided with a curtain, and maize (Zea mays) leaves were added, as an isotopically distinct tPOC source, into one half of each lake. To estimate the balance between autochthonous carbon fixation and allochthonous carbon input, primary production and tPOC and tDOC (terrestrial dissolved organic carbon) influx were calculated for the treatment sides. We measured the stable isotope ratios of carbon (d 13 C) of about 800 samples from all trophic consumer levels and compared them between lake sides, lakes, and three seasons. Leaf litter bag experiments showed that added maize leaves were processed at rates similar to those observed for leaves from shoreline plants, supporting the suitability of maize leaves as a tracer. The lake-wide carbon influx estimates confirmed that autochthonous carbon fixation by primary production was the dominant carbon source for consumers in the lakes. Nevertheless, carbon isotope values of benthic macroinvertebrates were significantly higher with maize additions compared to the reference side of each lake. Carbon isotope values of omnivorous and piscivorous fish were significantly affected by maize additions only in the macrophytedominated lake and d 13 C of zooplankton and planktivorous fish remained unaffected in both lakes. In summary, our results experimentally demonstrate that tPOC in form of autumnal litterfall is rapidly processed during the subsequent months in the food web of shallow lakes and is channeled to secondary and tertiary consumers predominantly via the benthic pathways. A more intense processing of tPOC seems to be connected to a higher structural complexity in littoral zones, and hence may differ between shallow lakes of alternative stable states.

Reconciling the role of organic matter pathways in aquatic food webs by measuring multiple tracers in individuals

“…Terrestrial inputs to freshwater food webs, as suggested by stable isotopes, are viewed as subsidies that contribute a significant fraction of the trophic base for animals , Bartels et al 2012, Scharnweber et al 2014). We found considerable use of a terrestrial subsidy by insects, crustaceans, reptiles, and fishes, largely originating with pasture grasses that use the C 4 photosynthetic pathway.…”

Taxonomic identity and phytoplankton diet affect fatty acid composition of zooplankton in large lakes with differing dissolved organic carbon concentration

“…). tPOM derived from the catchment area can sediment before reaching the pelagic areas of lakes, and thus, can be more important in littoral than in pelagic food webs (Scharnweber et al ). It is also possible that LC–SAFA are not efficiently assimilated or they are preferentially excreted by zooplankton, which would make them unsuitable as biomarkers.…”

Empirical correspondence between trophic transfer efficiency in freshwater food webs and the slope of their size spectra

“…We conducted whole‐lake experiments as part of an unrelated project starting in October 2010 (Scharnweber et al. , Mehner et al. ), for which both lakes were divided in an east‐west direction by a plastic curtain, producing fully isolated, equally sized halves.…”