Charophyte growth in small temperate water bodies: Extreme isotopic disequilibrium and implications for the palaeoecology of shallow marl lakes

A small pond containing the charophyte Chara hispida was monitored over a one-year period for changes in growth, water chemistry, water level and stable isotopic composition. Chara growth was found to be seasonal, with maximum growth occurring from late April to July. During this period, pH rose to > 10 while the dissolved inorganic carbon (DIC) and calcium fell as a result of photosynthesis and calcification. Large gradients in pH, water temperature and irradiance were found within the Chara sward and measurements showed that most growth and photosynthesis occurred within the upper 20 cm of the water column. Chara oospore formation was also found to be seasonal but dependent upon environmental conditions.

δ¹⁸O_w rose rapidly during summer as evaporation progressed and this was correlated with the δ¹⁸O_DIC, and to some extent with δ¹⁸O_c of the Chara encrusted calcite. However, extreme isotopic disequilibrium was observed between the δ¹⁸O_c and the δ¹⁸O_w and also between the δ¹³C_c and the δ¹³C_DIC. This arose from the high pH allowing atmospheric CO₂ to enter the water and combine directly with OH⁻.

It is concluded that, within shallow eutrophic lakes containing Chara swards, inferences of climate (e.g. air temperature) cannot be made from observations of the isotopic composition of Chara carbonates. However in combination with other geochemical data, disequilibrium events may be identifiable in ancient lake basins and taken as evidence for lake shallowing and/or eutrophication.