Thousands of dead fish washed up on the gulf coast of Texas over the weekend, and massive globs of seaweed carrying plastic pollution infested with flesh-eating bacteria are raising public health concerns at Florida’s famous beaches.
Like a cat leaving a mangled rat on the doorstep, the oceans seem to be trying to get our attention.
A large school of Gulf menhaden died over the weekend and left miles of beaches at Texas’s Quintana Beach County Park covered in dead fish, leading to officials warning people to avoid the beach due to the stench and thousands of sharp bones and fins. The menhaden were likely feeding in shallow water when an increase in temperature robbed the fish of dissolved oxygen that must pass through their gills.
Fish kills are alarming because they send a distressing signal about the health of an ecosystem — in this case, the embattled Gulf of Mexico — even if, in this event, the menhaden got unlucky and trapped in warm shallow water. Wildlife officials in Texas pointed to natural causes and local factors to explain the fish kill, but one reality of human-caused pollution and climate disruption is more mass fish die-offs, more often.
Warming temperatures and agricultural pollution are also likely nurturing the troublesome mat of seaweed stretching from the Gulf of Mexico to the shores of Africa, but more on that in a minute.
Cooler water holds oxygen much more efficiently than warm water, and fish kills occur in shallow waters across the country every summer as algae blooms fed by sunlight and water pollution further deplete oxygen levels.
If a relatively still body of water heats up and turns green on a hot, sunny day, fish can often be seen gulping for air at the surface before large numbers panic and die from hypoxia. Plenty of sunlight and warm, calm water are set conditions for fish kills; waves and disturbances otherwise help dissolve oxygen into the water.
Water pollution contributes to fish kills, particularly agricultural runoff, which contains nutrients and fertilizer that feed massive algae blooms. Hypoxia hot spots include the shallow, western end of Lake Erie and at the mouth of the Mississippi River in the Gulf of Mexico, where a “dead zone” stretching for an estimated 3,275 square miles is devoid of fish and other species.
However, the fish kill in Texas is particularly massive, stinky and affecting beachgoers, earning headlines across the country. While it’s difficult to link a single fish kill to global warming, climate disruption is increasing the frequency of mass fish die-offs, threatening both ecosystems and fisheries around the world, according to a 2022 study by researchers at the University of Arkansas. Research has also linked mass die-offs of starfish to environmental stressors caused by climate change.
“Specifically, climate change is more than gradually increasing temperatures because it also increases temperature variation, such as we experienced much of this summer,” co-author Simon Tye said at the time. “In turn, our findings suggest these rapid changes in temperature affect a wide range of fish regardless of their thermal tolerance.”
Katie St. Clair, the manager of the sea life facility at Texas A&M University at Galveston, said fish kills can provide nutrients to other animals and are a regular summer occurrence, but rising temperatures in oceans generally must be monitored by scientists.
“Water can only hold so much oxygen at certain temperatures, and certainly we know that seawater temperatures are rising,” St. Clair told NPR on Monday.
Which brings us to the Great Atlantic Sargassum Belt, the largest seaweed bloom in the world. Stretching from the beaches of Miami to the Congo in Africa, the belt is a 5,000-mile stretch of seaweed mats called sargassum, which form island-like clumps in the ocean. Massive clumps are taking over beaches in Florida, where the seaweed turns brown and rots in the sun, effectively canceling any plans to surf or sunbathe.
Large sargassum mats are not new to the Atlantic and provide valuable habitats for all sorts of species in open water. However, scientists discovered that the Sargassum Belt had exploded in size only a few years ago.
Like the phytoplankton algae that contribute to fish kills, scientists hypothesize that warming waters and agricultural pollution released by large rivers into the ocean could be fueling the mass of sargassum — a mix of natural and human-caused factors. Scientists caution that more research is needed, but the sargassum bloom clearly follows a seasonal pattern linked to temperature and nutrient inputs.
“Earth’s ocean biogeochemistry is changing in response to natural and human forcings,” said Paula Bontempi, who manages NASA’s Ocean Biology and Biogeochemistry Program, in a statement.
Beached sargassum is now causing a health scare in Florida after researchers found that large amounts of plastic pollution (certainly from humans) got caught in the web of seaweed and created the perfect microbiological environment for various species of vibrio bacteria, including about a dozen varieties that attack the internal organs of humans and are commonly called “flesh-eating bacteria.”
Officials are warning cleanup crews to approach the seaweed mats on the beach with care. Everyone else should simply stay away from beaches in southern Florida where the seaweed washes up.
While it may look like the oceans are crying for help under an onslaught of human-caused environmental crises, both fish kills and the sargassum bloom are evidence that certain species are doing what they do best and exploiting changing conditions to propagate and survive.
Phytoplankton and sargassum are crucial parts of the ocean ecosystem, but humans have given them a leg up by altering the climate and filling rivers that flow into the ocean with pollution and fertilizer. It’s up to humans to decide whether we want an ocean full of dead fish, seaweed and algae, or a multitude of species, including those we harvest to feed millions of people.
“The Great Atlantic Sargassum Belt suggests that we may be witnessing ecosystem shifts that could have important implications for marine organisms and ecosystem services on which humans depend,” Bontempi said.
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