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

Sensors for the partial pressure of CO₂ (pCO₂) and dissolved O₂ (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 pCO₂ 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 pCO₂ and DO to diverge from surface levels, (4) subsequent diel convective mixing brought bottom pCO₂ and DO back toward surface levels, and (5) before stable stratification, vertical entrainment of CO ₂-rich water, net production, and air-water exchange drove 100-200 μatm daily changes in pCO₂, but, because of their counterbalancing effects, surface pCO₂ remained > 1,000 μatm for nearly 1 month after ice-out. Upon stable stratification, net production and air-water exchange overcame pCO₂ gains from mixing and heating and reduced pCO₂ to near atmospheric levels within 20 d. Net production and gas exchange accounted for ∼75% and 25%, respectively, of the decrease in surface pCO₂ 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.