Floating tubes test sea-life sensitivity

Ocean labs probe effects of ocean acidification on ecosystems.

Researchers suspend 20-metre-tall sacs in a Swedish fjord to enclose entire ecosystems for study.

Global warming is not the only worrying consequence of rising carbon emissions. As levels of carbon dioxide increase in the atmosphere, more of the gas dissolves into the oceans, making the water more acidic. Marine scientists fear that the conditions will disrupt ecosystems by, for example, inhibiting some organisms’ ability to build shells. Yet the effects are unclear: in small-scale laboratory tests, certain species have proved surprisingly resilient, and some even flourish.

Marine biologist Ulf Riebesell says that these results tell only part of the story: scientists need to scale up and examine whole ecosystems. Lab studies of isolated species ignore variables such as competition, predation and disease, he says. Even minor effects of acidification on the fitness of individual species — especially small photo­synthetic organisms such as phytoplankton — can upset food chains, eventually harming larger species. “If you only focus on the lab results, you are being misled,” he says.

Riebesell and his colleagues at GEOMAR Helmholtz Centre for Ocean Research in Kiel, Germany, have developed innovative experimental environments — 20-metre-tall sacs suspended in the ocean, which enclose entire ecosystems and allow the effects of elevated CO2 to be measured. The first results, published this year, suggest that some plankton thrive in acidic environments and can wreak havoc on food chains1. Another experiment will end in July, and preliminary evidence suggests that conches and sea urchins are vulnerable to acidification.

The project is inspired by analogues on land, in which swathes of forest are bathed in extra CO2 to study the effects on plant life (see Nature 496, 405–406; 2013). For the sea, Riebesell and his colleagues constructed ‘meso­cosms’ — floating cylinders of thin plastic that function like giant test tubes2. When first put into the water, the sacs are left open at the top and bottom, allowing hundreds of small species to enter. After several days, they are closed and acidified water is pumped in (see ‘Sea lab’). Over weeks or months, researchers measure how the ecosystems inside fare in comparison with those in untreated sacs.

Realizing this simple idea has been challenging. The scientists began in 2006 with a prototype, free-floating in the Baltic Sea, that floated too well: currents carried it along much faster than expected, and the scientists had to chase it in a research ship. After only two days they reached Swedish waters, for which they had no research permits. When they tried to recover the meso­cosm, it broke.

The team conducted its first successful experiment in 2010, using a lighter design that was moored in place in the Norwegian Arctic archipelago of Svalbard. The researchers found that, compared with the controls, the acidic mesocosms produced less dimethyl sulphide3 — a gas that helps to form clouds, which reflect sunlight and can counteract climate warming. Riebesell is not sure what causes the change; he thinks that the plankton in the mesocosm might be making less of the gas, or the acidic water could be affecting its stability.

Picophytoplankton, the smallest photo­synthetic organisms, turned out to grow better in the acidic mesocosms1. But at the same time, diatoms — larger algae that are among the most important producers of ocean biomass — suffered. The change could mean that more nutrients are cycled among the picophyto­plankton rather than reaching larger animals such as fish. Indeed, preliminary results from the latest experimental run indicate that larvae of sea urchins and Strombidae conches are barely surviving in the acidic mesocosms. However, the scientists think that food quality may not be the main reason for their demise; pathogens and problems making shells could also have a role.

Adina Paytan, an oceanographer at the University of California, Santa Cruz, says that Riebesell’s work “fills an important niche between lab work and field studies” and has “advanced the field considerably”. She takes a different systems approach to acidification, studying ‘natural mesocosms’: under­water springs off Mexico that enrich zones in CO2.

Riebesell says that these regionsare a good lab for studying immobile seagrasses, but not organisms that can move freely. Paytan notes that there are problems with Riebesell’s mesocosms: for example, the plastic walls filter out some ultraviolet light, removing a natural stressor for photosynthetic organisms. And the tubes are impermeable, so nutrients in the water become exhausted, and experiments last only a few months. Nevertheless, Paytan says, “we still learn a great lot from these experiments”.

This year’s run, in the Swedish Gullmar Fjord, uses five control mesocosms and five in which acidity is boosted to levels associated with the atmospheric CO2 concentrations predicted for the year 2100. The experiment will end next month after a 6-month run — the longest yet — during which the researchers have monitored a natural plankton bloom.

Riebesell and his team seem comfortable with using their mesocosms as a hybrid between a controlled laboratory environment and a natural one. They have introduced fish eggs into the ecosystems for the first time, and Matias Scheinin, a marine biologist at GEOMAR, is using the sacs to explore natural selection. By tracking the abundance of individual strains of diatoms — which can undergo hundreds of generations in a few months — he hopes to identify those that flourish in acidic environ­ments. He will screen them for the genes responsible, to investigate rates and mechanisms of adaptation.

Oceans have gone through major acidification events during climate change in the ancient past. By accelerating evolution, Scheinin wants to get a glimpse of their future. “I have some hope that evolution can help marine life deal with acidification,” he says. “It’s not the first time it has had to go through it.”

Hristio Boytchev., 2013. Floating tubes test sea-life sensitivity. Nature 498: 420–421. Article.

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