Taxon-specific delta C-13 analysis of chitinous invertebrate remains in sediments from Strandsjon, Sweden

Taxon-specific stable carbon isotope (delta C-13) analysis of chitinous remains of invertebrates can provide valuable information about the carbon sources used by invertebrates living in specific habitats of lake ecosystems (for example, sediments, water column, or aquatic vegetation). This is complementary to delta C-13 of sedimentary organic matter (SOM), which provides an integrated signal of organic matter produced in a lake and its catchment, and of diagenetic processes within sediments. In a sediment record from Strandsjon (Sweden) covering the past circa 140 years, we analyzed SOM geochemistry (delta C-13, C:N-atomic, organic carbon content) and delta C-13 of chitinous invertebrate remains in order to examine whether taxon-specific delta C-13 records could be developed for different invertebrate groups and whether these analyses provide insights into past changes of organic carbon sources for lacustrine invertebrates available in benthic and planktonic compartments of the lake. Invertebrate taxa included benthic chironomids (Chironomus, Chironomini excluding Chironomus, Tanytarsini, and Tanypodinae), filter-feeders on suspended particulate organic matter (Daphnia, Plumatella and Cristatella mucedo), and Rhabdocoela. delta C-13 of chironomid remains indicated periodic availability of C-13-depleted carbon sources in the benthic environment of the lake as delta C-13 values of the different chironomid taxa fluctuated simultaneously between -34.7 and -30.5 aEuro degrees (VPDB). Daphnia and Bryozoa showed parallel changes in their delta C-13 values which did not coincide with variations in delta C-13 of chironomids, though, and a 2-3 aEuro degrees decrease since circa AD 1960. The decrease in delta C-13 of Daphnia and Bryozoa could indicate a decrease in phytoplankton delta C-13 as a result of lower lake productivity, which is in accordance with historical information about the lake that suggests a shift to less eutrophic conditions after AD 1960. In contrast, Rhabdocoela cocoons were characterized by relatively high delta C-13 values (-30.4 to -28.2 aEuro degrees) that did not show a strong temporal trend, which could be related to the predatory feeding mode and wide prey spectrum of this organism group. The taxon-specific delta C-13 analyses of invertebrate remains indicated that different carbon sources were available for the benthic chironomid larvae than for the filter-feeding Daphnia and bryozoans. Our results therefore demonstrate that taxon-specific analysis of delta C-13 of organic invertebrate remains can provide complementary information to measurements on bulk SOM and that delta C-13 of invertebrate remains may allow the reconstruction of past changes in carbon sources and their delta C-13 in different habitats of lake ecosystems.