Alkalinity regulation in calcium carbonate-buffered lakes

Biogenic calcite precipitation is the removal of calcite (CaCO3) from the epilimnion to the sediments of hard-water lakes during summer stratification, caused by increased pH during algal blooms and by nucleation of calcite crystals on surfaces of micro-algae. Although this phenomenon has been studied for decades, details of the underlying mechanisms are still debated. Using results of approximately 70,000 alkalinity measurements from 13 hard-water Swiss lakes (each with approximately 30 yr to 50 yr of monitoring), we demonstrate that (i) calcium carbonate-buffered lakes act as alkalinity sinks during summer stratification but act as CO2 sources during and immediately after spring overturn; (ii) as the alkalinity concentration ([Alk(mix)]) and the total phosphorus concentration ([TPmix]) at spring overturn increase, increasingly more alkalinity is lost from the epilimnion during summer stratification; (iii) [Alk(mix)] is determined by the lake's discharge-weighted average inflow concentration ([Alk(in)]), flushing rate, mean depth and [TPmix]; and (iv) [Alk(in)] depends on the mineralogy and the land use affecting in-soil nitrification of ammonia and subsequent calcite dissolution in the catchment.