Initial research by a team of University of Alabama marine scientists hints groundwater contributes contaminants into Mobile Bay, a situation that could worsen as rising sea levels awaken contaminants dormant in the sediment. Scientists are searching for clues from the past to see if this happened before.
By Adam Jones
Photos by Zach Riggins
Mobile Bay is the fourth largest estuary in the United States fed by the second largest delta in the country, Mobile–Tensaw River Delta. Hugged by rivers and the Gulf of Mexico, it is difficult to imagine groundwater as a major player in the Bay’s ecology.
But, simply put, it is. Accounting for a trickle of water in the bay compared to water from the Mobile River, groundwater delivers half of the nitrogen derived from ammonium into the bay, according to studies by Dr. Natasha Dimova, assistant professor and a marine and environmental geochemist in UA’s geological sciences department. Nitrogen is critical for the health of the estuary, but too much of it brings problems that can affect water quality and economic activity such as the lively seafood industry.
A motivation to study submarine groundwater discharge in Mobile Bay is a unique phenomenon called jubilees, moments when huge numbers of marine life such as fish and crabs swarm shallow waters. They can have severe economic impact on the seafood industry and tourism that thrive in and around the bay.
One of the important triggers of jubilees are an increase in algae, or small marine plants, in what are named harmful algal blooms. These blooms occur when an abundance of nutrients that algae feed off release into the bay.
While rivers bring spikes of nutrients, Dimova and team's work clearly shows hot spots of submarine groundwater discharge coincide with frequently observed areas of jubilees in Mobile Bay during the dry season.
When sorting out the influence of groundwater on the health of the bay and influence on harmful algal blooms, Dimova wants to know what is in the sediment. Below the surface, the sediment can tell the history of the bay, shedding light on when there have been harmful algal blooms.
Initial work shows the sediments are storehouses for nutrients in an organic layer that Dimova’s team discovered around Mobile Bay just below the surface, seen when a rod, or core, of sediment is carved out and pulled from the soil. The sediment also holds onto toxic metals such as mercury, which, when consumed by fish, hurts the seafood industry.
“Imagine that sediment that has never been touched by sea water and now that sea water is going up, it’s going to shake everything,” she said. “It’s a ticking bomb to me.”