Space Grant Consortium - University of Wisconsin - Green Bay

each grid cell at daily resolution. The ice mask alters the exchange of heat, gas and momentum
between the lake and the atmosphere. Temporal increases/decreases in fractional ice coverage
create a heat flux to/from the lake. For this purpose, lake ice is assumed to have a constant
thickness of 0.25 meters when present.
Ecosystem Model. The ecosystem model is that of Dutkiewicz et al. [2005] and Bennington et
al. [2009] updated so phosphorous is the only limiting nutrient. It includes the cycling of carbon,
alkalinity, and oxygen. The model tracks the states of phosphorous and carbon as they pass from
dissolved inorganic forms to phytoplankton, to zooplankton, and to detritus in both dissolved and
sinking particle forms. A schematic of the ecosystem is presented in Figure 1.
Figure 1. Schematic of the model ecosystem adapted from Dutkiewicz et al., [2005].
Two size classes of phytoplankton are included, and phytoplankton growth is limited by
phosphorous availability, lake temperature, and light. Phytoplankton are assumed to assimilate
carbon into biomass at a fixed C:P ratio of 200 [Urban et al., 2006b]. Particulate organic carbon
and phosphorous remineralize at a maximum rate of 5 d -1 to include the effects of the microbial
loop and sink 0.5 m/day [Chai and Urban, 2004]. Growth and remineralization rates are
modified by temperature according to Eppley [1974]. One class of zooplankton preys upon both
phytoplankton classes, but assimilates only a portion of what it grazes into its biomass. To
include the effects of the microbial loop without an explicit tracer for bacteria, a significant
portion of the lake’s ecosystem [Biddinda et al., 2001], 95% of deceased material goes directly
into dissolved form and rapidly remineralizes. Chlorophyll is parameterized according to the
mass of phytoplankton present and light availability during the last 24 hours. Photosynthetically
active radiation (PAR) is assumed to be 45% of shortwave radiation [Frouin and Pinker, 1995].
The exchange of CO2 between the lake and atmosphere is dependent on the difference in partial
pressures across the air-lake interface and a piston velocity. We used the parameterization of
Wanninkhof [1992] to include the effects of wind speed on the air-lake CO2 flux. For coupled
model runs, the model was spun up for three years so that the lake may outgas excess carbon
from the initial conditions.
Model Forcing. The physical model was forced with hourly winds, downward short and long
wave radiation at the surface, air temperature, atmospheric pressure, and specific humidity
created by interpolating meteorological observations at three open lake buoys and from nearby
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