Cyanobacterial tufa calcification in two freshwater streams: ambient environment, chemical thresholds and biological processes

Calcareous freshwater streams near Bad Urach, southwest Germany, were studi ed to determine the environmental limits to cyanobacterial calcification. D aily variations in temperature, pH, calcium concentration, and alkalinity w ere measured at seasonal intervals from September 1993 to January 1995 in t wo small woodland streams with lengths of 1.0 and 1.5 km. The principal cau se of supersaturation in these fast-growing streams is inorganic carbon dio xide outgassing from resurging groundwater, locally enhanced by turbulence at waterfalls and cascades. Photosynthetic uptake of carbon dioxide and tem perature effects are negligible. Nonetheless, organic substrates, particula rly cyanobacteria-dominated microbial mats and biofilm, significantly local ize precipitation by providing suitable nucleation sites. Precipitation rat es on artificial substrates, up to 2.2 mm/yr on limestone, correlate with h igh Saturation Index values. Copper substrates inhibited microbial coloniza tion and received negligible encrustation. Tufa formation and external calc ium carbonate encrustation of cyanobacteria are conspicuous where the annua l WATEQ4F Saturation Index exceeds 0.8, and are slightly below 0.7. Calcium carbonate impregnation of cyanobacterial sheaths has not been observed. We infer that in these fast-flowing streams cyanobacteria utilize CO2 in phot osynthesis whereas elsewhere, in sluggish freshwater, cyanobacteria utilize HCO; leading to sheath impregnation by calcium carbonate even where Satura tion Index is only I 0.2-0.3. Thus, photosynthetic influence on cyanobacter ial calcification appears to be negligible in fast-flowing CO2-rich streams and cyanobacterial calcification takes the form of external encrustation. In slow-flowing CO2-poor streams and lakes cyanobacteria preferentially uti lize bicarbonate and sheath impregnation can result. Modern tropical marine carbonate shelves have saturation indices commonly in the range 0.62-0.82 and cyanobacterial calcification is negligible. Extrapolation suggests that cyanobacterial calcification could occur in present-day seas if Saturation Index exceeded 0.8. This level, corresponding with heavy calcification in the freshwater streams studied, may have been widespread in marine environm ents in the geological past at times when calcified cyanobacteria and envir onmentally influenced non-skeletal calcium carbonate precipitates were abun dant. (C) 1999 Elsevier Science B.V. All rights reserved.