Observations of dissolved oxygen variability and physical drivers in a shallow highly stratified estuary

Abstract

Evaluating the role of long-term anthropogenic changes on dissolved oxygen (DO) in coastal systems is often a challenge due to variability in a system and measurement sensitivity to spatial and temporal trends. Physical processes can significantly modify the DO variability and act as a major driver of uncertainty when it comes to quantifying the bio-chemical changes associated with the DO budget. Stratification has been observed to be a dominant factor in controlling the hypoxic conditions that can develop in Mobile Bay, AL, a shallow highly stratified estuary prone to episodic hypoxia. Using CTD transect data and long-term water quality monitoring stations the variability of DO was examined throughout Mobile Bay. This study examined the physical drivers of these trends and highlighted the role stratification, temperature, and advection play in driving the bay wide variability. Under stable conditions, the spatial trend in Mobile bay will reflect the along-estuary gradient. When stable conditions don't occur due to random episodic mixing events and cross estuary exchange, the Bay can be driven by a number of factors: the along-estuary gradients, time since the previous mixing event, level of stratification, and biochemical oxygen demand. The combination of these elements provides an increased understanding of the complex dynamics driving low DO in this system. Long-term trends show the DO is decreasing in Mobile Bay based on changes in DO in the shipping channel, northern region, and Bon Secour region.