Biogenic calcite-phosphorus precipitation as a negative feedback to lake eutrophication

Lakes in carbonate-rich watersheds commonly precipitate calcium carbonate as calcite, and this is accelerated by photosynthetic uptake of carbon dioxide, Co-precipitation of phosphate with calcite is one reason that algal growth in such lakes tends to be strongly phosphorus (P) limited. The extent to which calcite precipitation can be a sink for added P. and thus act as a potential negative feedback to eutrophication, was demonstrated using large enclosures within a Michigan lake. Nitrogen and P were added over 45 days in the summer to produce mesotrophic and eutrophic conditions. Algal biomass and production increased markedly with the nutrient additions, elevating the pH (9-10) and greatly increasing calcite precipitations, reducing concentrations of calcium and alkalinity by up to 60%. Sediment traps indicated that calcite sedimentation was a major sink for added P. By the end of the experiment, only about of the added P remained in the water column. Major ion concentrations in a larger set of Michign lakes showed calcite precipitation to be widespread, reflecting the abundant carbonate minerals in this glacial region.