Chloride and total phosphorus budgets for Green Bay, Lake Michigan

Abstract

Green Bay is an elongated freshwater embayment located in northwestern Lake Michigan. Due to its short residence time, the lower bay is heavily influenced by the Fox River's large nutrient load. The inner bay is classified as hypereutrophic and a well-defined trophic gradient is observed moving away from the Fox River towards Lake Michigan, where the bay is nearly oligotrophic. Recent chloride and total phosphorus loading estimates were used to update a chloride and total phosphorus mass-balance model for the bay for 1994–2008. The chloride model provided a means to estimate turbulent eddy diffusion within the bay and exhibited excellent agreement with observed data. The total phosphorus model agreement with observed data was generally good, with the exception of a large deviation in lower Green Bay during 1999–2004. The model was used to estimate the internal loadings necessary to account for the deviation in phosphorus concentrations. The source of the unexpected increase remains unclear, but we speculate significant internal loading due to wind-driven sediment resuspension and hypoxia-induced phosphorus diffusion was significant. These models allow needed reductions to be identified and sourced and also indicate the role internal loading may play in the Green Bay phosphorus budget.