Residence time of a freshwater embayment connected to a large lake

Abstract

The average length of time water remains within the boundaries of an aquatic system is a key parameter controlling the system's biogeochemical behavior. This timescale, generally referred to as the hydraulic residence time, provides a first-order description of multiple and complex processes that drive transport. The procedures to estimate these transport timescales are reviewed and, through the analysis of numerical simulations, the links between residence timescales and the underlying hydrodynamic processes in a canonical freshwater embayment, Little Sodus Bay (LSB), are explored. LSB has negligible through-flow and is connected permanently to Lake Ontario (LO) through a narrow and shallow channel. Exchange in the channel is the result of a multiple balance where spatial thermal variations (baroclinic forcing), oscillations in water level (barotropic forcing), frictional mixing, wind, and the effects of unsteadiness are all important. The simulations indicate that baroclinic processes are the dominant exchange and mixing mechanisms in embayments like LSB. The largest density gradients across the channel are caused by episodic upwelling events in LO during the stratified season, when exchange rates increase by at least an order of magnitude. The mean residence timescales undergo dramatic variations in time and space and, in general, are comparable to the timescales of the systems' variability itself. The simulations reveal that temporal variations of mean residence timescales occur at interannual, seasonal, and down to synoptic timescales, and are closely related to the occurrence and frequency of upwelling events.