A summertime Gulf of Mexico dead zone fueled by pollution flowing out of the Mississippi River watershed could be among the largest on record this year.
In a seasonal forecast issued this week, the National Oceanic and Atmospheric Administration said heavy spring rain over the watershed, which drains 37 states - or about 41 percent of the U.S. - was expected to flush large amounts of nitrogen and phosphorus into the northern Gulf. That could create a dead zone covering more than 7,800 square miles.
It's too early to tell what influence the zone might have on seasonal red tides that form off the Florida shelf. Following a 2017 record-setting dead zone, a toxic red tide started in October that lasted for more than a year, littering southwest Florida beaches with dead marine life and eventually sweeping up the Atlantic coast.
"This is an atypical year given the really high discharges, so it would be something to keep an eye on," said NOAA oceanographer David Schuerer.
Water at the mouth of the Mississippi generally flows west, creating dead zones in the western Gulf. But in May and June, winds and current can push it east, said Richard Stumpf, a NOAA oceanographer who forecasts harmful algae blooms. The 2017 dead zone stretched from Louisiana to Texas, covering more than 8,700 square miles. While major flood years don't always coincide with major red tide blooms, Stumpf said nutrients from the zones can reach the Florida shelf, where red tide is seeded.
Red tide "uses many sources of nitrogen, and an extra pulse of nitrogen can help [red tide] get started," Stumpf said in an email.
Heavy flooding in Louisiana also prompted water managers to open a spillway near Lake Pontchartrain that can send more dirty water east, Schuerer said.
The dead zone in the Gulf is the largest in the U.S. and the second largest in the world. A task force has been working to shrink its size to about 1,900 square-miles, Schuerer said. But the zone has continued to average about 5,900 square miles over the last five years.
The zone is caused by nitrogen and phosphorus in farm run-off and urban stormwater. Once the nutrients reach the Gulf, they trigger algae blooms, which can then die and sink to the bottom, sucking oxygen out of the water as they decompose. That causes fish and other marine life at the bottom to die, producing even more nutrients in water.
Hurricane season, which picks up in late August and early September, tend to mix up water as they cross the Gulf and clea up the zone, Schuerer said. Whether the zone, and the stew of dead wildlife it creates, impacts Florida waters will depend on currents and weather wind up in the Gulf.
"It would be a little early to make that type of prediction," he said. "We'd have to see how things unfold."