Using timescales to interpret dissolved oxygen distributions in the bottom waters of Chesapeake Bay

Abstract

A simplified conceptual model based on timescales of gravitational circulation, vertical exchange, and total oxygen consumption rate of the biochemical processes is presented to provide insight into the relationships between estuarine dynamics and bottom water dissolved oxygen (DO). Two dimensionless parameters are introduced to diagnose the relationship between the vertical exchange process and the biochemical DO consumption and the influence of gravitational circulation on replenishment of bottom DO. The relative magnitudes of these timescales provide a linkage between the physical and biochemical processes. The hypoxic and anoxic conditions in deep waters of Chesapeake Bay are successfully interpreted with these three proposed timescales. Because the Bay is a long estuary, the replenishment of the bottom DO due to gravitational circulation diminishes as the bottom water travels farther upstream. The bottom DO is mainly modulated by the vertical exchange process in the middle and upper portions of the Bay. In addition to other physical processes that affect vertical exchange, wind and freshwater are the major predictors of the vertical exchange time. The model is applicable to Chesapeake Bay and other estuaries with persistent gravitational circulation if the dimensionless parameters can be appropriately estimated