Space Grant Consortium - University of Wisconsin - Green Bay

EPA protocol also requires the water sample be heated to 25 C before measuring the pH and
alkalinity. Heating does not change the alkalinity of water but can drastically change the pH, so
pH must first be corrected to in situ lake temperature to estimate lake pCO2. Previous studies
[Alin et al., 2007; Urban et al., 2005] have been unaware of this protocol, leading to large
overestimates of springtime lake pCO2 (hundreds of µatm) and moderate overestimates in
August (~50 µatm). Such extreme changes in estimates of lake pCO2 can completely change the
understanding of whether the lake is a significant source of carbon dioxide or a sink during
certain times of the year.
There are two time series of direct measurements of Lake Superior pCO2. These observations are
limited in space, because they measure pCO2 at one point in the lake, but you can see daily and
hourly changes in pCO2 at the point location. Direct observations of lake pCO2 suggest that the
open lake is a source of carbon dioxide during spring and a sink during the bloom. During the
cold months, water and respired carbon dioxide are able to mix to the surface and efflux. When
the lake warms, biological productivity removes carbon dioxide from the surface waters, leading
to reduced pCO2 [Atilla et al., 2009].
Primary productivity and carbon fluxes are orders of magnitude different in the ocean coastal
region than in the open ocean, and significant spatial variability in Lake Superior in satellite
chlorophyll are observed as well. However, the majority of physical and chemical observations
in Lake Superior have been taken near shore along the Keweenaw Peninsula or near Duluth in
the western arm of the lake. Scientists must extrapolate over the largest lake in the world in both
space and time and cannot be expected to balance the carbon budget or capture the expected
spatial variability. In an attempt to balance the lake carbon budget and understand the
mechanisms of the air-lake CO2 flux, we used a coupled model to close the open-lake carbon
budget, estimate lake efflux, and understand mechanisms controlling the efflux. We simulated
lake conditions for 2001, because both indirect estimates and direct observations of lake pCO2
exist for 2001.
Methods
An ecosystem model of Lake Superior was coupled to a three dimensional hydrodynamic model
of the lake to estimate the air-lake flux of carbon dioxide in the absence of any terrestrial carbon
inputs. The couple model simulated lake circulation, productivity, and air-lake gas exchange for
2001.
Physical Model. We used the MIT general ocean circulation model [Marshall et al., 1997a,
1997b] configured to the bathymetry of Lake Superior [Schwab and Seller, 1996] with a uniform
horizontal resolution of 10 x 10 kilometers. The model uses a z-coordinate system of 29 vertical
layers. The top 50 meters have finest vertical resolution, with layer thicknesses of 5 meters.
Vertical resolution gradually becomes coarser with depth to a thickness of 33.8 meters at 322
meters depth. The Smagorinsky [1963] horizontal diffusivity scheme and the KPP vertical
mixing scheme [Large et al., 1994] simulate the effects of sub-grid scale processes. The time
step of numerical integration is 200 seconds.
A bulk formula is used to calculate momentum exchange and net heat fluxes between the
atmosphere and lake. Ice cover data from NOAA [Assel, 2003] is applied as a fractional mask to
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