Blue–green algae and freshwater carbonate deposits

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Abstract
Twenty-seven contemporary freshwater carbonate deposits were investigated (26 in the British Isles and 1 in S. Australia). The blue–green algaeSchizothrix calcicolaandMicrocoleus vaginatusoccurred at 23 of the sites. The remaining sites represented areas where deposition had ceased. About 1% of the dry mass of the deposits consisted of Cyanophyta. The assimilation rates of these algae, measured by14CO2uptake, were low. It was estimated that the equilibrium change caused by CO2uptake due to photosynthesis could account for only 1–2% of the precipitated carbonate. Cultures ofS. calcicolafailed to deposit carbonates in solid and liquid media. Deposition rates varied between 0.006 and 8 mm a-1(mean 1.2 mm a-1). The laminations encountered at some sites could be correlated to seasonal differences in cyanophyte growth which enabled some deposits to be dated. Thin sections revealed small crystals adhering to the muci­laginous sheaths of the algae. The transmission of light in some deposits was investigated. The algal layer corresponded to the depth to which light was attenuated to about 1% of the surface intensity.