In situ pCO2 and O2 measurements in a lake during turnover and stratification:: Observations and modeling

Sensors for the partial pressure of CO2 (pCO(2)) and dissolved O-2 (DO) were deployed near the surface and bottom of a freshwater lake (Placid Lake, Montana) during the period from ice cover to seasonal stratification. Sources of variability were examined using one-dimensional physical and biogeochemical models. Model predictions for pCO(2) and DO were compared to further constrain model parameters. A number of transient processes were documented that have not been well characterized in previous studies. The models made it possible to link these short-term events to specific forcings. We found that (1) 11 d of the 13-d turnover period occurred under ice through light-driven convective mixing, (2) phytoplankton biomass increased to its highest seasonal level under ice, (3) weak stratification set up immediately after ice-out, causing bottom water pCO(2) and DO to diverge from surface levels, (4) subsequent diel convective mixing brought bottom pCO(2) and DO back toward surface levels, and (5) before stable stratification, vertical entrainment of CO2-rich water, net production, and air-water exchange drove 100-200 muatm daily changes in pCO(2) but, because of their counterbalancing effects, surface pCO(2) remained > 1,000 muatm for nearly I month after ice-out. Upon stable stratification, net production and air-water exchange overcame pCO(2) gains from mixing and heating and reduced pCO(2) to near atmospheric levels within 20 d. Net production and gas exchange accounted for similar to75% and 25%, respectively, of the decrease in surface pCO(2) observed after ice-out. Diel convection was the dominant mixing process both under ice and after ice-out and may be an important underrepresented mechanism for CO2 and DO exchange between surface and bottom water.