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

Abstract

Sensors for the partial pressure of CO 2 (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 lightdriven 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 CO 2 -rich water, net production, and air-water exchange drove 100-200 atm daily changes in pCO 2 , but, because of their counterbalancing effects, surface pCO 2 remained Ͼ1,000 atm for nearly 1 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 ϳ75% 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 CO 2 and DO exchange between surface and bottom water.Spring thaw heralds not only a new growing season but also an important transitional period for ice-covered lakes and reservoirs. Physical forcings change dramatically between ice cover and open water, creating large and rapid changes in lake biology and geochemistry. As ice and snow cover diminishes, light penetration can stimulate phytoplankton growth (e.g., Wright 1964) and convective mixing (Bengtsson 1996). Surface water temperatures Ͻ4ЊC continue to warm and sink or, after ice-out, to be mixed by winds to greater depths, which results in an isothermal water column. Nutrients accumulated over winter combined with increased insolation produce the characteristic spring phytoplankton bloom. With further solar heating, the water column becomes thermally stratified, isolating the bottom water from the atmosphere until turnover occurs again, which is usually in the fall for dimictic lakes. The duration and magnitude of ice cover, turnover, and stratification can dramatically influence the concentrations of biogeochemical species both seasonally and interannually (e.g., Cornett and

Short‐term dynamics of oxygen and carbon in productive nearshore shallow seawater systems off Taiwan: Observations and modeling

“…This comprehensive model can be simplified by focusing on the key processes and thus adapted to various environments (Kraines et al 1996;Baehr and DeGrandpre 2004;DeGrandpre et al 2004).…”

“…There have also been modeling efforts to distinguish the biological forcing from physical influences (Kraines et al 1996;DeGrandpre et al 1997;Baehr and DeGrandpre 2004). The challenge now is to differentiate the fractional contribution of the different sources modulating the biogeochemical variations in hydrologically dynamic areas.…”

Uptake and sequestration of atmospheric CO2 in the Labrador Sea deep convection region

“…Similar models were used by Baehr and DeGrandpre [2004] and DeGrandpre et al [2004]. Others have interpreted variability using 1-D models with reasonable success in the Labrador Sea [e.g., Tian et al, 2004].…”

The Potential for CO2-Induced Acidification in Freshwater: A Great Lakes Case Study

“…In western Lake Superior, highfrequency measurements were taken 7.5 km offshore of Split Rock Lighthouse (47.19°N,91.34°W) at 12 m depth using a subsurface mooring equipped with a Submersible Autonomous Moored Instrument for pCO 2 (SAMI-CO 2 ; Baehr and DeGrandpre, 2004;Atilla et al, 2011) and a temperature sensor. Data were collected every half hour from June 6, 2001, through September 11, 2001.…”