Space Grant Consortium - University of Wisconsin - Green Bay

Figure 5. Model pCO2 at the surface (thick black) and at 12m (SAMI, thin grey) compared to direct measurements
of pCO2 (thick grey) during 2001. Indirect estimates of pCO2 from EPA pH and alkalinity at the nearest EPA station
are shown in shapes. Note that the EPA station was visited overnight.
Model pCO2 agrees with the open-lake direct observations and indirect estimates of pCO2 and
estimates a lake-wide efflux of only 0.03 TgC/yr. For comparison, Urban et al. [2005] estimated
the lake efflux to be 3 TgC/yr from indirect estimates of pCO2 taken near the coastline along the
Keweenaw Peninsula. Significant spatial variability in productivity, pCO2, and carbon fluxes
exist in the model, suggesting that extrapolating spatially limited observations can be
problematic. The model is able to provide an estimate of the daily air-lake flux for all of the lake
to fill in observational gaps.
Discussion and Conclusions
The model adequately simulates lake temperature, productivity and pCO2. The open lake is
supersaturated with carbon during winter and spring and under-saturated or near equilibrium
during summer. Lake Superior mixes to the bottom twice yearly, and no water mass within the
lake is able to store carbon dioxide away from the atmosphere for long periods. The open lake is
not a significant source of carbon dioxide (0.03 TgC/yr), and the near shore waters would be
unable to maintain super-saturation without continued inputs of carbon.
Nearshore observations along the Keweenaw Current [Urban et al., 2005] suggest the lake is
heterotrophic and a source of carbon to the atmosphere in the near shore zone. The magnitude of
this source is extremely uncertain, because all pCO2 estimates were indirectly calculated from
electrode-measured pH, temperature, and alkalinity. Direct measurements of near-shore pCO2
are needed for a true understanding of the lake’s carbon cycle. When the model can agree with
both near shore and open lake pCO2 observations and estimates, the whole lake carbon budget
may be understood. The model does not include terrestrial inputs of carbon. Due to its long
residence time (176 years [Quinn, 1992]), 90% of the allochthonous carbon is respired within the
lake. The terrestrial carbon must be respired mainly in the near shore zone; otherwise, model
pCO2 in the offshore regions would be low compared to observations. The next step is to add
river and runoff inputs of terrestrial carbon to the model.
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