A three-dimensional model of Lake Superior with ice and biogeochemistry

a r t i c l e i n f o The formation of winter ice on Lake Superior has been shown to be important in determining the annual thermal cycle of the lake and long-term trends of surface water temperature increase. However, modeling studies of Lake Superior to date have not included dynamic and thermodynamic ice cover. These physical characteristics of the lake in turn can have significant impacts on biogeochemical cycling within the lake. We present a new three-dimensional model of Lake Superior that includes a dynamic and thermodynamic ice model and a biogeochemical model. Results from the model forced by observed meteorological conditions for the period 1985 to 2008 are discussed and compared with available observations. Modeled long-term interannual trends in increasing water temperature and decreasing ice cover are compared with observed rates. In the model, total annual gross primary productivity is found to correlate positively with mean annual temperature and negatively with mean winter ice-cover magnitude. Introduction Lake Superior is a minimally disturbed, large aquatic system. It is the world's largest lake by surface area and contains approximately 10% of the Earth's surface freshwater. The lake plays a significant role in influencing regional meteorology, as well as being very important to the surrounding region as a source of clean and abundant water and a medium for inexpensive, waterway transportation. It is unusual, when compared to the other Laurentian Great Lakes, in that human impact has been limited. First, Lake Superior is upstream of the other Laurentian Great Lakes and receives runoff from its own catchment basin only. Lake Superior's catchment basin area is relatively small compared to the lake surface area with a ratio of 1.55 (Cotner et al., 2004). Second, development within its basin is not significant: only 3% of the Superior basin is used for agriculture as compared to 27–67% for the other Great Lake basins; over 90% of its basin is covered with forest, which is far less abundant elsewhere (Sea Grant College Program, 1985). Also, the amount of water withdrawn for consumption from Lake Superior is at most a small fraction of the amounts withdrawn from the other lakes (The Great Lakes Commission, 2006). While the disturbance of Lake Superior has been minor compared to the other Laurentian Great Lakes, recent work by Austin and Colman (2007) demonstrates that the thermal characteristics of Lake Superior have been changing over the past quarter …