Assessing transport timescales in Lake Huron's Hammond Bay: The crucial role of the Straits of Mackinac's exchange flows.

The oscillating bidirectional exchange flows between Lakes Michigan and Huron in the Straits of Mackinac generate complex hydrodynamics and the exchange flows are known to alter hydrodynamics in regions as far down as 50-60 km from the Straits modulating physical, chemical, and biological processes in the region. Although previous research examined the effects of exchange flows on hydrodynamics, their impacts on transport time scales, including residence and flushing times, have not been quantified. We used observations and a three-dimensional hydrodynamic model to simulate bidirectional exchange flows in the Straits and their effects on hydrodynamics, temperature, and transport timescales in the Hammond Bay area, Lake Huron for the summers of 2018 and 2019. Comparisons with field observations showed that hydrodynamics can only be accurately described when the bidirectional flows are included in the modeling of the bays close to the Straits. Spectral analysis showed that the exchange flows play an important role in controlling conservative solute transport in bays close to the Straits. The residence time in the Hammond Bay area was calculated using a dye release approach with (without) the effects of bidirectional exchange flows producing estimates of 9.87 (16.00) and 13.75 (23.62) days for years 2018 and 2019 respectively based on a combined model of the two lakes and a model of Lake Huron only. Similarly, flushing times in the Hammond Bay area were estimated as 12.14 (14.38) and 8.96 (10.80) days for 2018 and 2019, respectively with (without) the exchange flows. Ignoring the exchange flows in the Straits was found to overestimate the residence time and flushing time in the Hammond Bay area by roughly 74 and 20 %, respectively. These results highlight the importance of including the bidirectional exchange flows in biophysical models of bays in Lake Huron closer to the Straits and in similar systems elsewhere.