Effects of Coastal Upwelling and Downwelling on Hydrographic Variability and Dissolved Oxygen in Mobile Bay

Upwellling and downwelling events are important coastal processes that strongly influence shelf ecosystem dynamics. Though changes on the shelf have been well studied, the impact of these events on estuarine systems has received less focus. In summer 2016 a downwelling and upwelling event were observed near the mouth of Mobile Bay. The impact of these events were examined throughout the bay with high spatial resolution observations. Five boat surveys were conducted to capture the spatial response of offshore forcing and its changes in the estuary. In addition to the surveys, 16 conductivity, temperature, and depth (CTD) sensors were deployed and measured temporal changes. A 7 degrees C temperature change was observed in the channel as a result of the offshore changes. In the shipping channel and offshore, advection was a major source of changes in heat content and often exceeded the surface heat flux. Advection of low dissolved oxygen water into the estuary was also observed in the shipping channel and at the Dauphin Island station nearest to the estuary mouth. The extent of these changes highlights the importance of estuary-shelf connectivity in river-dominated systems where offshore forcing can drive large changes in estuaries during low flow summer conditions. Plain Language Summary Upwelling is a coastal process where winds cause surface waters to diverge offshore, this then leads to cold water from deeper offshore moving in to replace it. These types of changes on the shelf have been well studied, and the impact of these offshore events on estuarine systems has received less focus. In summer 2016 a downwelling and upwelling event were observed Mobile Bay, Alabama. Five boat surveys were conducted to measure how the estuary changed in response to these offshore events. The largest observed result was a 7 degrees C temperature change in the estuary as a result of the offshore changes. In the shipping channel that runs the length of Mobile Bay and offshore, larger temperature changes occurred when upwelled water replaced the existing water and the changes exceeded the warming that occurs from the surface. In addition to temperature changes, low dissolved oxygen water entered the estuary and was associated with the upwelling event. The extent of these changes highlights the importance of estuary coastal connectivity where offshore changes can drive large changes in estuaries.