Space Grant Consortium - University of Wisconsin - Green Bay

Figure 4. Chlorophyll estimated from SeaWiFS imagery (personal communication with Colleen Mouw) and model
chlorophyll (mg Chl / m 3 ).
Lake pCO2 and CO2 Flux. The difference in the partial pressures of carbon dioxide between
the atmosphere and the lake surface drive the air-lake CO2 gas exchange. The partial pressure of
CO2 (pCO2) has been directly measured only twice in Lake Superior. Direct measurements of
pCO2 indicate the lake is under-saturated and a sink of carbon dioxide during the summer bloom.
pCO2 is indirectly estimated by measuring lake pH and alkalinity. Measuring pH with an
electrode has a significant bias (-0.13) in freshwater [French et al., 2001] that introduces an
uncertainty in pCO2 up to hundreds of µatm (if bias is present, actual pCO2 would be
significantly lower than shown) during summer. This can completely change the scientific
understanding of whether the lake is a sink or source of atmospheric CO2. To further complicate
scientific understanding of the lake’s pCO2, the EPA measurements of pH are taken after the lake
water is heated to 25 C. When estimating pCO2 from EPA measurements of pH and alkalinity,
the pH must be corrected to the in situ temperature. The model pCO2 compares extremely well to
directly measured pCO2 provided by M. Baehr (Figure 5). The model would not be expected to
capture the extreme magnitude of the CO2 drawdown seen in the data, as the model is a 10km by
10km region. The model pCO2 is also reasonable compared to indirect estimates of pCO2 from
the nineteen stations visited by the EPA each year in April and August (Figure 5). Uncertainties
of indirect pCO2 estimates are unknown, as the pH bias is unknown.
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