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)

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 delta C-13 values of charophyte thalli and encrustations. It was suggested that photosynthetic activity of charophytes affected not only the delta C-13 values of charophyte encrustations but also mollusc shells by changing delta C-13 values of DIC within charophyte stands. In addition, incorporation of metabolic carbon into the shell was proposed as the main cause of both the C-13 depletion of mollusc shells relative to delta C-13 values of DIC and the species-specific delta C-13 values of shells. Mollusc shells were precipitated at the isotope equilibrium or close to the equilibrium with delta O-18 values of lake water. Charophyte encrustations were found to be O-18 depleted due to the kinetic isotope effects during intense photosynthesis and thus fast precipitation of the calcite.