Stable isotope composition of Chara rudis incrustation in Lake Jasne, Poland

Abstract

Stable isotope composition (δ13C and δ18O) was analysed in mineral incrustation of Chara rudis and surrounding waters. This macroalga forms dense and extensive charophyte meadows and may significantly contribute to the calcium carbonate precipitation and deposition of marl lake sediments. The study aimed to find out if charophyte calcium carbonate was precipitated in an isotopic equilibrium with lake water and if the precipitation was related to the environmental conditions. Two apical internodes of 10 individuals of C. rudis were collected monthly between June and late October 2008 at three permanent study sites (1.0 m, 1.5 m and 2.0 m deep) in a small (15.1 ha) and shallow (mean depth: 4.3 m) mid-forest lake with extensively developed charophyte meadows (Lake Jasne, mid-Western Poland). Basic physical–chemical analyses were performed at each study site, and water samples for further laboratory determinations, including stable isotope analyses, were collected from the above searched C. rudis stands and, simultaneously, at three comparative sites in the macrophyte-free pelagial. The difference in δ13C between incrustation and water from above C. rudis exceeded 2‰ V-PDB at each site. In the case of δ18O, it exceeded 2‰ between July and September. Accordingly, it is postulated that calcium carbonate was not precipitated in an isotopic equilibrium with lake water. Incrustation was enriched in heavier carbon isotope, 13C, and water was enriched in 18O. δ13C of incrustation and DIC were positively correlated, whereas negative relation was found between δ18O of incrustation and water. Several dependencies were found with water chemistry above the plants. The content of mineral incrustation in Chara dry weight had negative influence on the δ18O but not on the δ13C. Community depth, structure and PVI had no effect. No significant differences appeared between isotope composition in the pelagic zone and Chara stands.