A three-dimensional model of Lake Superior with ice and biogeochemistry: investigating interannual Lake Trends and the deep chlorophyll maximum

Abstract

The development of a realistically configured three-dimensional model for Lake Superior including prognostic ice and biogeochemistry models is documented. The addition of a prognostic ice model is a significant advance over previous modeling efforts. The hydrodynamic, ice and biogeochemical models are described and behavior of the model during the period 1985 to 2008 and focusing on the annual cycle of 2005 is discussed. The model is found to sufficiently reproduce many observed physical and biological characteristics of Lake Superior. It is also successfully applied in two scientific investigations: interannual trends in lake temperature, ice cover and primary productivity and elucidation of the causal mechanisms of Lake Superior's deep chlorophyll maximum. 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. 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.The deep chlorophyll maximum (DCM) is a near ubiquitous feature in Lake Superior during the summer stratified season. Previous studies have elucidated observable characteristics of the DCM in Lake Superior but the physical and biological mechanisms controlling the creation and maintenance of the DCM remained unclear. Sensitivity runs are performed to explore the influence of photoadaptation, photoinhibition, zooplankton grazing, and phytoplankton sinking on the vertical distribution of chlorophyll in the water column. The role of a nutricline in determining the presence and nature of the DCM is also explored. The presence of the DCM is dependent upon the presence of thermal stratification in the model. The sensitivity runs reveal that photoadaptation plays a primary role in determining the depth of the DCM in the model while zooplankton grazing and phytoplankton sinking affected the magnitude but not the presence or depth of the DCM. Photoinhibition showed negligible effects on chlorophyll concentration distribution. The presence of a nutricline in the model is also a necessary condition for the formation of the DCM and it influences both the depth and magnitude of the DCM.