After heavy tropical rainfall, rivers feeding Mobile Bay flush massive amounts of freshwater into the Gulf, creating striking plumes that can be seen from space.

While those plumes often attract fish along their edges, scientists say they can also contribute to low-oxygen conditions beneath the surface.

Brian Dzwonkowski, a professor at the Stokes School of Marine and Environmental Sciences at the University of South Alabama, said the plumes begin with heavy rainfall over the watershed.

"These storms bring lots of tropical moisture to the terrestrial environment," Dzwonkowski said. "The watersheds collect that rain, funnel into river systems, and those river systems spit that fresh water into estuaries, and those estuaries then spit that fresh water out onto the shelf."

Because freshwater is less dense than salty Gulf water, it spreads across the surface instead of immediately mixing.

"The Gulf water has a lot of salt in it. That salt makes it heavier, and so the fresher water sits on top of the heavier Gulf water," Dzwonkowski said.

Those freshwater lenses can persist for days after a major rain event. Dzwonkowski said a plume moving through Mobile Bay generally takes about five to 10 days to reach the Gulf, followed by another five to 10 days before it gradually disperses, depending largely on wind.

"You can think of the wind as like a blender," he said. "Without the wind, the water just wants to layer itself."

The boundary where freshwater meets saltwater creates what oceanographers call a front. Those fronts become gathering places for nutrients and marine life.

"Where the fresh water and the salt water meet, you get a convergence," Dzwonkowski said. "Phytoplankton, fish — there's a lot of things that aggregate at that front."

As microscopic phytoplankton concentrate along the front, they attract baitfish, which in turn draw larger predatory fish. Dzwonkowski said those conditions can make plume edges productive areas for anglers.

According to the National Oceanic and Atmospheric Administration, chlorophyll fronts — boundaries between different water masses — are associated with elevated biological productivity and fisheries because they concentrate food sources for fish, shrimp, shellfish and other marine species.

For boaters and fishermen, plume boundaries can often be spotted without scientific equipment.

"You'll see a clear color divide between the open Gulf water and the estuarine water spilling out onto the shelf," Dzwonkowski said. "You can also see foam lines. A lot of people around here call them tide lines."

But while freshwater plumes can create productive fishing areas near the surface, they may also create problems below.

Dzwonkowski said the freshwater cap limits mixing between surface water and deeper saltwater. As algae and other organic matter die and sink, bacteria consume oxygen while breaking them down. Without enough mixing, oxygen in bottom waters is not easily replenished.

Although freshwater plumes are a normal part of the northern Gulf ecosystem, Dzwonkowski said heavier rainfall associated with a warming climate could produce larger and longer-lasting plumes.

"We know with climate change precipitation events are getting heavier," he said. "The heavier the rainfall, the bigger these pulses of fresh water are going to be, and so the more likely it would be for these plumes to get larger and more persistent."

Although freshwater plumes often disappear within days, their effects can last much longer. They shape where fish feed, influence oxygen levels beneath the surface and serve as a reminder that rainfall hundreds of miles inland can affect the health of the bay. 

Gabriella Chavez is a Report for America corps member who writes about growth and development in Baldwin County and our natural spaces for Gulf Coast Media. Your donation to match our RFA grant helps keep her writing stories like this one; please consider making a tax-deductible gift of any amount today by visiting https://tinyurl.com/yaf8yf5n.