Computational study of the physical features in Lake Ontario nearshore waters

Phosphorus concentrations in Lake Ontario have been successfully reduced to below the target load under the Great Lakes Water Quality Agreement (GLWQA), but appear to be much higher in the nearshore waters than in offshore waters over the past ten to fifteen years. In connection with higher phosphorus levels, many beaches become undesirable or are even closed because of Cladophora fouling in the summer. The resurgence of the benthic algae, Cladophora, is thought to be linked to the invasion of dreissenid mussels along with physical processes in the lake. This situation has motivated the Lake Ontario Nearshore Nutrient Study (LONNS). As a part of the LONNS project, the purpose of this study is to set up relatively fine-scale hydrodynamic models for each of the three nearshore sites monitored during the 2008 field year for LONNS, e.g. Oak Orchard, Rochester, and Sandy Creek. These models were developed using a 200-meter resolution, and nested within an existing 2 km grid lakewide model. Eventually, the long-range goal of LONNS is to develop an Integrated Coastal Ecosystem Model (ICEM), for which the present nested models would provide a foundation. Specific goals are to compare the nested model results with the available measurements, and to interpret the data in terms of physical processes that may contribute to the nearshore phosphorus problem. The results suggest that the whole lake model and nested model work well for thermal structure and dominant water movement in the summer and fall, whereas adding an ice component to the model might provide better simulations for the spring. The nested model results were consistent with the whole lake model results and some of the measured data, and showed more realistic details in the nearshore region, but the grid size might not be sufficiently small to reveal the steep horizontal thermal gradients associated with the thermal bar.