Physical and Biogeochemical Processes across Seasons in Missisquoi Bay, Lake Champlain: Insights from a Three-dimensional Model

The transboundary Missisquoi Bay in Lake Champlain, situated between Vermont (USA) and Quebec (Canada), is a shallow eutrophic embayment (depth < 5 m) with a limited water flux and connectivity to the rest of the lake. The bay has experienced persistent toxic cyanobacterial blooms during summer months in recent decades impairing recreational and other uses of the bay. These blooms are driven by high levels of non-point source nutrient inputs from the bay's large watershed dominated by agricultural land uses and internal nutrient loading from the bay bottom sediments. Analyses of field data suggested that winter weather exerts a strong control on biogeochemical cycling of phosphorus in the bay and its sediments that manifests differently depending on particular winter weather conditions with potential cascading impacts on the water quality in the warmer months. Further, climate change is likely to worsen toxic cyanobacterial blooms in the bay. Here, we present the implementation of a three-dimensional coupled hydrodynamic-ecological model to Missisquoi Bay. The model performance was tested against a high-frequency physical and biogeochemical dataset from Missisquoi Bay for the year 2017. This comparison allowed us to use the model to understand the underlying seasonal dynamics (ice off and ice on periods) of the circulation and its influence on the nutrient cycling processes and occurrence of cyanobacteria blooms in the bay in warmer months. Finally, the model offered a valuable framework for exploring management scenarios to alleviate the potential effect of climate change on cyanobacteria concentrations in the bay.