Water age simulation in Lake George, NY

Whole lake residence time has been associated with various water quality parameters, including harmful algal blooms. Despite observations of spatial variability in commonly measured lake water quality parameters, little attention is given to the spatial variability of residence time in lakes. We use water age as a surrogate for residence time and we examine its spatial and temporal distribution in ten bays of varying size in Lake George, New York (USA). We simulated the physical processes in the lake with the simulator SUNTANS, and we validated the outputs against water temperature profiles as well as measured water currents. Using simulated water age, we show that the average residence time in 9 of the 10 bays is less than three days. A bay-averaged timeseries analysis shows that water age can sharply decrease within one day due to strong wind events. The average spatial distribution within each bay is shown to be non-uniform, with only a small section of the bottom layer having a substantially greater age (> 1 week in certain bays). Snapshots of water age transects indicate that strong wind events substantially change the vertical distribution of water age in some bays, even to the extent of inverting the distribution. A substantial decrease in water age within the bays was associated with the shallowing and deepening of the thermocline. Our results highlight how variations in water residence times within lakes can introduce substantial variation in water quality attributes. Whole lake residence times may serve as a poor proxy to understand the dynamics of water masses, especially in large and morphologically complex waterbodies.