Earth’s ocean temperatures are hitting record-high levels, causing South Carolina’s estuaries to heat up. That could mean trouble for native fish species that depend on those brackish waters.
Ocean surface temperatures have been breaking records every month since late March 2023, according to data from the European Union’s Copernicus Programme.
America’s climate agency, the National Oceanic and Atmospheric Administration, reports that the January-March 2024 global surface temperature was the hottest in the agency’s 175-year record. NOAA predicts with 99 percent certainty that 2024 will rank in the top five hottest years on record, with a 55 percent chance it will become the new record holder.
That warming effect is also impacting some global estuaries, according to a new study in the peer-reviewed journal “Limnology and Oceanography Letters.” The analysis compared surface-water temperatures over 737 estuaries across the planet and found nearly half of them were warming to various degrees. Of the American estuaries vulnerable to surface heating, including many along the Southeast coast, surface-water temperatures are increasing by an annual average of around 0.1 degrees Celsius (or 0.18 Fahrenheit).
Daily sea-surface temperature (in Celsius) averaged over the extra-polar global ocean (60°S–60°N) for 2016 (yellow), 2023 (red), and 2024 (black line). All other years between 1979 and 2022 are shown with gray lines.
The research found that global estuaries at higher latitudes are more susceptible to heating — with northern European estuaries increasing at an annual rate closer to 0.2 degrees Celsius (0.36 Fahrenheit).
“In a global sense, the United States’ estuaries are not the fastest-warming ones,” said John Gardner, a geology and environmental science professor at the University of Pittsburgh who co-authored the study. “Which is, I guess, not the worst news for us.”
The study and its findings rely on historic data to map warming trends, which means it doesn’t take into account future climate change predictions, Gardner said.
“This is a trend over the past four decades, so we can’t say if it’s going to get faster or slower over time,” he said.
Denise Sanger, a senior marine scientist with the South Carolina Department of Natural Resources, said Gardner’s findings largely align with the department’s temperature observations in the ACE Basin’s estuary testing sites. The basin covers about 1.6 million acres around the Ashepoo, Combahee and Edisto rivers. It was once home to rice plantations, but now is “one of the largest areas of undeveloped wetlands/uplands ecosystems remaining on the Atlantic Coast,” according to The Nature Conservancy.
Variations of a few tenths of a degree may seem small from a human perspective. But Brooke Blosser, the Land, Water and Wildlife Project Manager for the Coastal Conservation League, said such temperature change can be a problem for the native estuarine species of South Carolina.
“Having warmer estuaries in the month of February compared to March could be a huge difference for some of those species,” Blosser said. “This warming temperature of the estuaries could potentially shift the spawning season of some of these fish and shellfish. If it shifts the spawning season, it could lower the recruitment (essentially the population) of fish in the estuary next year.”
Take South Carolina’s economically important blue crabs, for example. Landings for the species have cratered in recent decades, from a high of nearly 9.4 million pounds in 1978 to 2.6 million in 2021, according to SCDNR records. Michael Kendrick, who leads the department’s crustacean research, said the best years for the species tend to coincide with colder winters.
“We were able to show that in winters that are colder, those are the years where we see higher abundance of juvenile blue crabs, suggesting they like those colder winter conditions better,” he said. “I think there’s still some work to understand the mechanism that leads to those relationships, but I think that’s an example of how these warming waters are going to impact things like our recreational and commercial invertebrate species.”
South Carolina’s estuary-dependent species aren’t just facing pressure from heat. They’re also dealing with the arrival of new climate migrants.
Looking inland from the south side of the North Edisto River, the ACE Basin stretches to a hazy horizon, rich with intertidal salt marshes and pluff mud channels.
Under pressure
There’s at least one estuary-dwelling critter basking in the state’s warming estuaries: the invasive green porcelain crab. The species first appeared on America’s East Coast in the early 1990s. Its native range is unknown, according to the Smithsonian Institution, although it’s theoretically supposed to be limited to more tropical waters.
But as the oceans have warmed, the porcelain crab has pushed farther northward from Florida, following a behavior called “range creep.” Now, it can be found as far up the coast as North Carolina, and in concentrations as high as 20,000 to 30,000 crabs per square meter, the Smithsonian reports. (The species is generally around 8 millimeters in length.)
“We would normally have colder water (in winter) that would kill off the invasive species that can’t survive, but we’re not having as many of those cold-water events,” Blosser said.
There are several ways an invasive species can hurt native inhabitants, ranging from predation to (in the porcelain crab’s case) hogging aquatic resources or altering environmental conditions, Kendrick said.
“As warming continues, then our native environment may become susceptible to colonization by invasive species, where previously our environmental conditions may have prevented their colonization,” he said. “Invasive species are an important part of understanding how our estuaries are changing.”
Building resilient estuaries
An estuary is an immensely complex system of interactions, both natural and man-made. The ebb and flow of the tides, the tendency of salt marshes to migrate with changing sea levels, a man-made structure blocking that migration or a new local development increasing runoff into feeder streams all are factors that can impact the health of an estuary.
The research of Gardner and his colleagues speaks to that complexity.
“While our headline is that there are many (estuaries) that are warming, I’m almost more interested in the fact that about half (53 percent) are not warming, which was honestly a little surprising for me,” Gardner said. “There are estuaries that are not warming, even when the local air temperature is warming.”
Gardner said the answer to that mystery could provide an important road map for protecting global estuaries and increasing their resilience to a warming world.
In the near term, Blosser said relieving man-made pressures on an estuary can bolster its resilience. That can include preserving marsh migration corridors or installing oyster reefs to help improve water quality. It’s a complex balancing game, she said.
