Fine-scale dynamics of calcite precipitation in a large hardwater lake

In hardwater lakes, calcite precipitation is an important yet poorly understood process in the lacustrine carbon cycle, in which catchment-derived alkalinity (Alk) is both transformed and translocated. While the physico-chemical conditions supporting the supersaturation of water with respect to calcite are theoretically well described, the magnitude and con-ditions underlying calcite precipitation at flne temporal and spatial scales are poorly constrained. In this study, we used high frequency, depth-resolved (0-30 m) data collected over 18 months (June 2019 - November 2020) in the deeper basin of Lake Geneva to describe the dynamics of calcite precipitation fluxes at a flne temporal resolution (day to sea-son) and to scale them to carbon flxation by primary production. Calcite precipitation occurred during the warm strat-ifled periods when surface water CO2 concentrations were below atmospheric equilibrium. Seasonally, the extent of Alk loss due to calcite precipitation (i.e., [30-42] g C m-2) depended upon the level of Alk in surface waters. Moreover, interannual variability in seasonal calcite precipitation depended on the duration of stratiflcation, which determined the volume of the water layer susceptible to calcite precipitation. At flner timescales, calcite precipitation was charac-terized by marked daily variability with dynamics strongly related to that of planktonic autotrophic metabolism. In-creasing daily calcite precipitation rates (i.e., maximum values 9 mmol C m-3 d-1) coincided with increasing net ecosystem production (NEP) during periods of enhanced water column stability. In these conditions, calcite precipita-tion could remove as much inorganic carbon from the productive layers as NEP. This study provides mechanistic in-sights into the conditions driving pelagic calcite precipitation, and quantifles its essential contribution to the coupling of organic and inorganic carbon cycling in lakes.