Interspecies differences and lake‐specificity in the δ¹³C and δ¹⁸O signatures of charophyte carbonate encrustations

This study identifies the relationships between the δ 13 C and δ 18 O signatures of extant charophyte carbonates (δ 13 C CARB and δ 18 O CARB ) and the δ 13 C of dissolved inorganic carbon and δ 18 O of water (δ 13 C DIC and δ 18 O WATER ), respectively, and the structure of charophyte beds and the physico‐chemical properties of the ambient water. Inter‐ and intra‐specific differences and lake‐related variability are assessed. Four morphologically different charophyte species ( Nitellopsis obtusa , Chara tomentosa , Chara rudis , Chara contraria ) were studied in four charophyte‐dominated lakes ( Chara ‐lakes) in Central Europe. A wide distribution of the species in calcium‐rich freshwater lakes, where they form extensive underwater beds and contribute to the deposition of calcareous sediments, makes the study representative of Chara ‐lakes. Charophytes and water above them were sampled monthly (June–September 2008) at permanent study sites. Additionally, three comparative macrophyte‐free pelagic sites were sampled in each lake. For all the charophytes studied, there was a greater variation in the δ 13 C and δ 18 O values of the encrustations than of the ambient and pelagic waters. Except for N . obtusa and C . tomentosa , the species differed significantly in the δ 13 C CARB values. The encrustations were enriched in 13 C compared to water DIC. Chara tomentosa encrustations were significantly depleted in 18 O compared to N . obtusa and C . contraria . In contrast to Chara species, N . obtusa encrustation was 18 O‐enriched compared to surrounding water. The largest isotope differences between carbonate encrustations and surrounding water were found for C . rudis . Considering the δ 13 C CARB versus δ 13 C DIC values, only slender species (i.e. N . obtusa and C . contraria ) precipitated encrustations close to isotopic equilibrium with water DIC. Based on theoretical δ 18 O CALCULATED and measured δ 18 O CARB values, only C. tomentosa precipitated encrustation in equilibrium with the δ 18 O WATER . Using redundancy analysis, the δ 13 C CARB values positively correlated with the δ 13 C DIC and δ 18 O WATER values and the percent volume of water inhabited by plants. The depth of sites, water mineralisation, alkalinity and fertility were inversely correlated with the δ 13 C CARB . The redundancy analysis revealed a clear division of samples into species and lakes; lakes were especially distinct. We postulate that the stable C and O isotope compositions of charophyte encrustations are lake‐ rather than species‐specific and that the isotopic signatures of charophyte carbonates are representative of the entire lake epilimnion, not just the vegetated littoral zone. Furthermore, the isotopic signal of bulk sediment can provide environmental information that is as useful as species‐specific isotope values.