Stark warning on health of oceans

By Tim Radford

A combination of acidification and global warming could be turning oceans toxic and threatens to destabilise the balance of marine life, say US marine scientists.

LONDON, 4 May 2017 – Ocean acidification and global warming between them could severely damage the health of the oceans.

They could block the biological process that delivers nitrogen in the seawater to nourish micro-organisms. They could spark growth among the invertebrates but cause stress higher up the food web to destabilise the balance of marine life. And they could even create conditions that would make great stretches of oceans toxic.

Toxic oceans

The first two are possibilities based on laboratory experiments and warn of what could happen as the world warms, the climates change and the chemistry of the oceans continues to become more acidic. But the third may already be happening.

Marine scientists from Stony Brook University in New York state report in the Proceedings of the National Academy of Sciences that they looked at ocean temperature data and the growth of two of the most toxic algae in the North Atlantic and North Pacific oceans.

They have found that, since 1982, areas of these oceans have warmed and become more hospitable to Alexandrium and Dinophysis, two genera of micro-organism with species that manufacture neurotoxins that can cause paralytic and diarrhoetic shellfish poisoning in humans.

“The distribution, frequency and intensity of algal blooms have increased across the globe and this study links this expansion to ocean warming in some regions of the
North Atlantic and North Pacific oceans”

“Toxic or harmful algal blooms are not a new phenomenon, although many people may know them by other names such as red tides. These events can sicken or kill people who consume toxin-contaminated shellfish, and can damage marine ecosystems by killing fish and other marine life,” says Christopher Gobler, professor of marine science at Stony Brook.

“The distribution, frequency and intensity of these events have increased across the globe and this study links this expansion to ocean warming in some regions of the North Atlantic and North Pacific oceans.”

The global warming connection to increasing incidence of what biologists call “harmful algal blooms” has yet to be established. The Stony Brook study so far only suggests that any increase could be linked to climate change. More research is necessary: it is one thing to experiment in a laboratory aquarium, another to be confident about what actually happens on the high seas.

But the message from the laboratories is not encouraging. Chinese and US researchers report in Science journal that, according to their experiments, it became more difficult for a type of cyanobacteria called Trichodesmium to “fix” nitrogen in seawater that had been made more acid by carbon dioxide.

Marine algae, like plants on land, must have a source of nitrogen to build the tissue that larger animals eat and digest. The cyanobacteria tested in the laboratory tank are believed to be responsible for about half of all marine nitrogen fixation.

“This is one of the major sources of nitrogen for other organisms in the open ocean,” says Sven Kranz, an ecologist at Florida State University and a co-author. “If Trichodesmium responds negatively to the environmental changes forced upon the ocean by fossil fuel burning, it could have a large effect on our food web.”

The conclusion is provisional: like all such findings, it will be tested by other researchers in other ways. But even if the cyanobacteria find a way to go on fixing nitrogen in increasingly acidic oceans, there will be other marine hazards in a warming world.

Scientists at the University of Adelaide in Australia report in Global Change Biology that high carbon dioxide levels of the kind expected at the end of the century could boost production at some levels in the marine food chain. But since the oceans would increasingly warm at the same time as becoming more acidic, marine animals would also feel more stress.

Experimental ecosystems

The scientists made 12 experimental ecosystems in the laboratory – representing different habitats – with photosynthesising plants, grazing invertebrates such as shrimps, and fish that ate shrimps. Then they exposed them to the conditions likely by 2100. The results do not promise more productive fisheries.

“Elevated carbon dioxide concentrations boosted plant growth; more plant food meant more small invertebrates, and more small invertebrates, in turn, allowed the fish to grow faster,” says Silvan Goldenberg, one of the Adelaide authors.

“However, ocean warming cancelled this benefit of elevated carbon dioxide by causing stress to the animals, making them less efficient feeders and preventing the extra energy produced by the plants from travelling through the food web to the fish.

“At the same time, fish were getting hungrier at higher temperatures and started to decimate their prey, the small invertebrates.” – Climate News Network

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