When 'below average' is a good thing: Gulf's dead zone not as big this year, scientists say

Amy Bennett Williams
Fort Myers News-Press

It’s not the happiest date on the environmental calendar, but this year’s Day of the Dead Zone was a bit cheerier than usual.

Every summer, National Oceanic and Atmospheric Administration scientists measure the Gulf of Mexico’s hypoxic zone — an area of low to no oxygen caused by nutrients washing in from the surrounding landscape. At some 3,275 square miles, 2022’s is below average, they announced Wednesday. Even so, it covers more than 2 million acres — larger than Rhode Island and Delaware combined. 

“The size this summer …was one of the smaller mapped areas since we began these NOAA-sponsored cruises in 1985,” said Nancy Rabalais chief scientist on the survey cruise that mapped the area, “but it’s also two times larger than the environmental goal of the Hypoxia Task Force.” The multi-agency body would like to see the zone be 1,900 square miles or smaller, which has happened just once in 2000,

What causes a dead zone?

Since 1985, the federal researchers have been tracking the vast underwater dead zone that forms when mobile sea creatures flee, while those that can’t sicken and die. Humans are mostly to blame, agency researchers say; agriculture and urban runoff are key contributors. “While some hypoxia is natural, the size and scale of what we’ve seen here in the last several decades is unusually large and detrimental," said Nicole LeBoeuf, director of NOAA’s National Ocean Service in a release. 

2022's Gulf of Mexico dead zone

Spring rains flush nutrients from the Mississippi watershed – 40 percent of the continental U.S. –  into the warming Gulf. That runoff fertilizes its waters, causing microscopic algae called phytoplankton to grow, then die. They settle on the sea floor, where they become food for microscopic creatures and small fish, whose waste products also pile up on the bottom. If the water stays calm, bacteria start breaking down all that organic matter, depleting oxygen as they do.

Tidal flooding:Climate experts predict Southwest Florida will see daily tidal floods by the year 2100

From 2020:Gulf of Mexico dead zone is smaller this year, thanks to Hurricane Hanna

Related:FGCU student researchers head to sea to check the red tide-devastated zone they found last year

The long-term average for the area, which ranges between 10 and 60 miles off the Louisiana coast, hovers around 4,280 square miles; with the record high set in 2017: 8,776 square miles. The dead zone and its effects often stretch west toward Texas and sometimes east toward Mississippi and Alabama. Though confined to the northern Gulf, it becomes a no-go zone for Florida-based shrimp and fishing fleets.

Scientists sailing to measure the Gulf of Mexico's 2022 dead zone

How does the Gulf of Mexico's dead zone affect Florida?

Further south, the Florida Gulf has its own set of dead zones, though none so big as the one NOAA maps. The Sanibel-Captiva Conservation Foundation and Florida Gulf Coast University have mapped and studied the areas in years past. SCCF research scientist Rick Bartleson says the worst events usually happen when large dense red tide blooms coincide with stratification, when the water column forms distinct temperature layers instead of mixing, as happened in 2018, 2005 and 1970, he says.

But hypoxia happens near shore and in area rivers and creeks as well.

“Matlacha Pass is the most impaired,” Bartleson said, and “The lower layer of the mid and upper Caloosahatchee goes hypoxic almost yearly.” As of now, though, things look good. “A transect I made into the Gulf from Sanibel on the 20th showed good oxygen levels for the depth profiles down to 45 feet."

And, points out Calusa Waterkeeper John Cassani, some areas are regularly plagued by low oxygen levels.

“The Caloosahatchee near Beautiful Island as one example (has) chronically low dissolved oxygen, likely due to the very warm water entering the river via FPL discharge at the mouth of the Orange River.”

Professors with FGCU’s Coastal Water Institute took students 30 miles off shore in the Gulf of Mexico to study water quality/ecosystem health. The main objective was to give students enrolled in the marine chemistry and oceanography courses hands-on experience in collecting samples and using oceanographic gear.

In Fort Myers, Billy's Creek has chronically low dissolved oxygen and to the south, the Imperial River in Bonita Springs is impaired for dissolved oxygen from excess nutrients.

To monitor conditions and make its forecasts, the USGS operates more than 3,000 real-time stream gauges, 50 real-time nitrate sensors, and 35 long-term monitoring sites to measure nutrients in rivers throughout the Mississippi-Atchafalaya watershed. Scientists use these data to track long-term changes in nutrient inputs to the Gulf and help build models of nutrient sources and hotspots within the watershed.

They’re also trying to figure out how humans can work around the zone, says Steve Thur, director of NOAA’s National Centers for Coastal Ocean Science.

”We’re making ongoing investments to better understand the long-term impacts” and create models connecting watershed activities with downstream impacts, leading to new tools to help commercial fishermen. For example, “This year for the first time, scientists from NOAA Fisheries and North Carolina State University began using experimental models to better understand where shrimp could be found relative to the hypoxia.”

Low oxygen zones are a global issue mostly related to nutrient loads in coastal waters, Rabalais says, and until we humans get a handle on that, the problem will continue.