Currents and heat fluxes induce stratification leading to hypoxia in Green Bay, Lake Michigan.

Hector Bravo, Sajad Hamidi, J Val Klump, James Waples

Wednesday 1 july 2015

11:15 - 11:30h at Asia (level 0)

Themes: (T) Hydro-environment, (ST) Ecohydraulics and ecohydrology

Parallel session: 9G. Environment - Impact

Summer bottom water hypoxia has been a recurring water quality issue for decades in Green Bay, Lake Michigan’s largest embayment. The nutrient loading of the bay has been fairly constant, yet the magnitude and duration of hypoxia is highly variable from year to year. The bay’s morphology (~ 20km x 200 km) with high riverine inflow and restricted mixing at the southern end of the bay, combined with extensive water mass exchange with Lake Michigan at the northern end of the bay, causes that changes in the hydrodynamic structure play a key role in the set up and persistence of the water column stratification that leads to hypoxic conditions. The onset of hypoxia is related to thermal stratification, which results from both direct meteorological forcing, i.e. low winds, high air temperatures, and increased solar radiation, and from indirect meteorological forcing that drives circulation patterns resulting in the southerly incursion of cooler bottom waters onto highly reducing organic rich sediments. This circulation pattern can stratify a well mixed water column within hours, and can set up stable stratification that persist for days to weeks during which time sediment oxygen demand rates are sufficient to deplete hypolimnetic oxygen. Therefore the morphometry, circulation patterns, and the thermal input-output balance of the bay interact to produce conditions that lead to variations in the degree, extent and duration of hypoxia from day to day and from year to year. Previous field measurements showed that it is common for two layers to flow through the mouth of the bay in opposite directions during the stratified season. A 3D hydrodynamic model developed in this study, supported by existing and new field measurements, demonstrates the onset of stratification by the combined effect of surface heat flux and near bottom cold water transported southerly from Lake Michigan. Model and field data reveal layered flow, with near-bottom cold water transported into the bay and near surface warmer water transported out of the bay. Model visualizations and calculation of net transport for a cross section of the bay help to understand the time scales involved in the stratification process.