A model of the three-dimensional hydrodynamics, transport and flushing in the Bay of Quinte

The Bay of Quinte, Ontario, receives excessive nutrient loads and suffers from poor water quality. The 70 km long z-shaped bay traps the nutrients due to limited flushing with Lake Ontario, leading to increased nutrient residence times. Therefore, it is important to understand the three-dimensional hydrodynamic conditions within the bay, as these drive horizontal transport, dilution of nutrient rich inflows and water exchange to the lake. In this study, the effects of meteorological forcing, river inflows and Lake Ontario exchange on the hydrodynamics and mixing were investigated using a numerical model. The model was validated against temperature time series and profile data, with a maximum root-mean-square deviation < 2.3 °C in comparison to observed temperature profiles. Six methods were applied to estimate flushing from the bay, with three methods (tracer release, drifter paths, bulk residence time) converging to predict the main channel of the bay flushes 5 times a year. Isolated embayments have higher water ages (4-5 months) and may trap nutrients with sufficient time and conditions for algae blooms to occur. Strong advection is modeled in the main channel, with low horizontal transport in the embayments and efficient flushing near the connection with Lake Ontario. This provides insight for watershed management, for example, to design ideal locations for nutrient discharges (e.g. wastewater plumes), to target specific rivers for nutrient load reductions, and to support future coupled hydrodynamic and biogeochemical modeling of the bay.