Precipitation of calcium carbonate in a shallow polymictic coastal lake: assessing the role of primary production, organic matter degradation and sediment mixing

In this paper, the genesis of carbonates in a shallow, polymictic and eutrophic coastal lake (Lake Sarbsko, Poland) is deciphered. Emphasis is placed upon the relationship between carbonate sedimentation and microbial degradation of organic matter, as well as on vertical mixing of sediments. The cycling of carbonates in the lake was investigated through the analysis of lake water chemistry (pH, Ca²⁺, Mg²⁺, alkalinity, SI_calc) and the stable carbon isotope composition of dissolved inorganic carbon and sedimentary calcite. It is shown that the calcite is precipitated within a relatively short time owing to the coincidence between the activity of photosynthesizing phytoplankton, the microbial decomposition of organic matter, and the physical mixing of lake deposits. In turn, the precipitated calcite is prone to dissolution within the sediments. The data presented in this paper can be applied to explain the processes affecting the carbonate saturation in freshwater systems and to interpret δ¹³C signatures in the sediments of shallow lakes.