Warming water can create a tropical ecosystem, but a fragile one

Warm water discharged into the Sea of Japan let tropical fish flourish in an artificial hot spot

A cutribbon wrasse (Stethojulis interrupta) is one of the tropical species that disappeared from the Otomi Peninsula in Japan after a nearby power plant shut down operations and stopped releasing warm water.

Mark Rosenstein/iNaturalist.org (CC BY-NC-SA 4.0)

A decade ago, the waters off the Otomi Peninsula in the Sea of Japan, were a tepid haven. Schools of sapphire damselfish flitted above herds of long-spined urchins. The site was a hot spot of tropical biodiversity far from the equator, thanks to warm water exhaust from a nearby nuclear power plant. But when the plant ceased operations in 2012, those tropical species vanished. 

After the plant shut down, Otomi’s average bottom temperature fell by 3 degrees Celsius, and the site lost most of its tropical fishes, fisheries scientist Reiji Masuda of Kyoto University reports May 6 in PLOS ONE. The die-off of tropical fishes and invertebrates was “striking,” he says. Otomi quickly reverted to a cool-water ecosystem.

The life and death of the reef is providing a sneak peek into the future of temperate habitats under climate change. This research suggests that even modest warming can result in dramatic changes to cool-water reefs, with some temperate habitats converting to more tropical ones. But these emerging reefs may not match the diversity or health of other more established tropical reefs at first, leaving them as ecologically fragile as the Otomi reef proved to be.

While some temperate reefs are changing rapidly with global warming, they aren’t exact transplants of more established tropical ecosystems, says David Booth, a marine ecologist at the University of Technology Sydney not involved in the new study. Booth studies increasingly tropical Australian reefs. 

“People always ask us, ‘Oh, that means even though the Barrier Reef’s in trouble with bleaching, in a couple of years Sydney will be the new Barrier Reef?’” Booth says. Sydney is merely acquiring a handful of tropical fish and coral, he says, “so, it ain’t the Barrier Reef by any means. Just a coral community starting, that’s all.”

Rapid die-off

In October 2003, while studying groupers at Otomi, Masuda noticed lots of tropical fishes that seemed out of place. Parts of southern Japan host tropical reefs, but Otomi sits at about 35° N, a zone typically occupied by seaweeds and associated fishes. The source of this anomaly was the Takahama nuclear power plant, only 2 kilometers away, which released warm water into the ocean after using it to cool reactors.

In 2004, Masuda began surveying Otomi and two other nearby sites, cataloging and counting fish. Then the Tōhoku earthquake and tsunami struck in 2011, precipitating the Fukushima Daiichi nuclear disaster. Japan stopped running all of its nuclear plants in response, including Takahama in 2012. As the warm discharge ceased, Otomi became an impromptu natural experiment in resiliency (SN: 12/5/14), and Masuda kept collecting data for the next five years. 

Soon, he started seeing dead and dying fish everywhere. “In normal marine environments, we scarcely see a dead fish,” says Masuda, since fish usually die by being eaten. But around Otomi, fish were succumbing en masse to the cold temperatures instead.

Neon damselfish (Pomacentrus coelestis) once congregated in Otomi’s warm water (left). But after a nearby nuclear power plant shut down, the waters cooled. Now, Japanese rockbass (Sebastes cheni) and the wrasse Halichoeres tenuispinis — typical species in temperate Japan — swim among sargassum seaweed (right).Reiji Masuda

Masuda was also surprised at how quickly Otomi shifted back to a temperate ecosystem. “Only two months after the die-out of tropical, poisonous sea urchins, temperate sea urchins appeared,” he says. “The sargassum seaweed bed recovered with some temperate fishes such as common wrasse and rockfish.”

Sneak peek

Otomi may provide a preview of some of the changes temperate reefs could experience as the global climate warms. After decades of warm water, Otomi still had no shelter-providing corals or large, tropical predators.

That lack of predators may have been behind Otomi’s high densities of tropical urchins, which had stripped the seabed clear of algae, obliterating access to food and shelter for many other species. There was nothing “to control their number and thus to maintain a healthy ecosystem,” he says. 

Masuda thinks it’s possible the die-offs were so severe and abrupt because of this poor ecosystem health. With species diversity lower than other tropical systems, the lack of redundancy can make the whole ecosystem more susceptible to stressors. In this case, that stress was a drop in temperature.

If there were many different species of urchin in the tropicalized reef, there’d be a higher chance that some could tolerate lower temperatures, Masuda points out. “This applies to fishes, too,” he says. “In healthy tropical ecosystems, there are many species — some should be relatively robust to temperature changes.”

Elsewhere in Japan, warming seas have already led to complete ecosystem shifts from kelp forests to coral, upending fisheries, Booth notes. 

As for Otomi, it may get another chance to be a natural experiment. In May 2017, the Takahama nuclear reactor turned back on, and Masuda has been diving and collecting data on the return of tropical fishes and urchins as the waters warm. Analyzing this much slower change, he says, “will be another fish to fry.”

About Jake Buehler

Jake Buehler is a freelance science writer, covering natural history, wildlife conservation and Earth's splendid biodiversity, from salamanders to sequoias. He has a master's degree in zoology from the University of Hawaii at Manoa.

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