Is Submarine Groundwater Discharge (SGD) Important for the Historical Fish Kills and Harmful Algal Bloom Events of Mobile Bay?
Large-scale fish and crustacean kills, locally known as Jubilees, and harmful algal blooms (HABs) have been occurring in Mobile Bay (Alabama) for more than a century. In fact, the first record describing a Jubilee event in Mobile Bay during 1867 was the first ever-documented case of mass mortalities of marine animals caused by hypoxia. To evaluate the importance of submarine groundwater discharge (SGD) in the occurrence of Jubilees and HABs in Mobile Bay, a 3-year study was conducted using a multi-method approach. Significant spatial and temporal variations of SGD were revealed in the bay only by applying a combination of geochemical and shallow geophysical techniques. The development of seasonal hypoxia observed in bay waters in areas impacted by Jubilees was the result of anoxic SGD inputs, which magnitude and spatial distribution were controlled by shallow lithological heterogeneities created during the modern development of the bay. Although when compared to the river discharge SGD contributed between 0.2 (wet season) and 5% (dry season) of the total freshwater inputs to Mobile Bay, 80% of the total SGD in the bay occurred in areas ecologically impacted by hypoxia and Jubilees. In these areas, SGD comprised up to 37% of the total water inputs during the dry season, coinciding with the time of the year when Jubilees and HABs occur. In conclusion, while SGD might not be a significant source of fresh water to Mobile Bay or other estuaries worldwide, enhanced SGD caused by site-specific lithological heterogeneities can have a critical role in the development of hypoxia and ecological issues in nearshore waters.
KeywordsSubmarine groundwater discharge Estuary Lithologic heterogeneities Hypoxia Jubilees Harmful algal blooms
We also want to thank the Mobile District US Army Corps of Engineers, Nathan Coburn, Adam Forkner, Richard Allen, and Steve Dykstra for their extensive help in the field. We are also very grateful for the massive help from the Weeks Bay National Estuarine Research Reserve by providing accommodation and technical support during all field campaigns.
This research was partially funded by the National Science Foundation (NSF OIA-1632825), the 2016 ExxonMobil Summer Fund, the 2015 Gulf Coast Association of Geological Societies Student Research Grant, the University of Alabama Graduate School Research and Travel Support Fund, the UA Department of Geological Sciences W. Gary Hooks Geological Sciences Advisory Board Fund, and the A.S. Johnson Travel Fund.
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