Cyanobacteria–phosphate–calcite interactions in limestone (hardwater) streams of England

Hardwater streams often contain deposits of calcium carbonate (travertine, tufa) supporting well-defined communities of cyanobacteria. Deposition of carbonate crusts is inhibited by moderate levels (c.100 μg/l) of dissolved orthophosphate phosphorus that can result in their loss, along with their cyanobacteria. Thirteen English cyanobacterium crust communities were investigated and in all but one, orthophosphate levels were too low to influence carbonate deposition. At one site, inhibitory levels of orthophosphate were found, and here, crusts have not been recorded for over 40 years. Phosphorus was detected in both the inorganic (CaCO 3 ) and organic fractions of the crusts, and in the latter, it was highest in the living cyanobacterium mat. No interaction between the phosphorus in these two fractions was observed. Levels of total phosphorus in the crusts were positively correlated with orthophosphate in the water. The great majority of English lowland waters currently contain inhibitory levels of dissolved orthophosphate preventing the development of these communities over large tracts of the country. Estimates of the uptake of phosphorus by the cyanobacteria suggested that their growth would not significantly influence the inhibition of precipitation of the associated calcium carbonate.