Impact of eutrophication on the carbon stable-isotopic baseline of benthic invertebrates in two deep soft-water lakes

Sedimentary geochemical proxies and carbon stable isotopes of subfossil chironomid head capsules (C-13) were combined to reconstruct long-term changes in organic matter sources contributing to chironomid biomass in two soft-water lakes (Vosges Mountains, France). Here, we test how basal carbon sources fueling the benthic food web responded to two millennia of environmental changes. Sedimentary organic matter characteristics were strongly variable over time, and were mainly influenced by human activities within the watershed basin. During the pre-eutrophication phase, subfossil chironomid C-13 (C-13(HC)) values were lower than those of sedimentary organic matter. These results suggested that chironomid larvae preferentially fed on the autochthonous part of the total sedimentary organic matter. During the modern period, higher accumulation rates of organic matter and a decrease in C/N ratio suggested a steep rise in aquatic productivity for both lakes. As suggested by the synchronous increase in organic matter C-13 values, this higher productivity and the subsequent increase in dissolved CO2 uptake may have induced either a marked increase in atmospheric CO2 uptake by phytoplankton or a reduction in the phytoplankton trophic fractionation. These changes led to overlaps in terrestrial and aquatic carbon stable-isotopic baselines, hampering the reliable reconstruction of the chironomid palaeo-diet. These results seem to indicate the minor role of allochthony to sustain aquatic food webs of large and soft-water lakes, and underscore the necessity to apply an independent compound-specific stable-isotope analysis to better estimate the temporal changes in isotopic baseline and, thus, strengthen the reliability of the chironomid palaeo-diet reconstructions.