Discrepancies between the stable isotope compositions of water, macrophyte carbonates and organics, and mollusc shells in the littoral zone of a charophyte-dominated lake (Lake Lednica, Poland)

Abstract

The aim of this study was to better understand the relations between carbon and oxygen stable isotope values of ambient water, mollusc shells, macrophytes and their carbonate encrustations, commonly used in palaeolimnological studies. Water, molluscs and macrophytes were sampled from the littoral zone in Lake Lednica, NW Poland. The influence of carbon species assimilated during photosynthesis and the net intensity of photosynthesis resulting from the size of charophyte species and the density of their stands were postulated to be the most important factors causing the species-specific δ13C values of charophyte thalli and encrustations. It was suggested that photosynthetic activity of charophytes affected not only the δ13C values of charophyte encrustations but also mollusc shells by changing δ13C values of DIC within charophyte stands. In addition, incorporation of metabolic carbon into the shell was proposed as the main cause of both the 13C depletion of mollusc shells relative to δ13C values of DIC and the species-specific δ13C values of shells. Mollusc shells were precipitated at the isotope equilibrium or close to the equilibrium with δ18O values of lake water. Charophyte encrustations were found to be 18O depleted due to the kinetic isotope effects during intense photosynthesis and thus fast precipitation of the calcite.