A few years ago, marine ecologist Kristy J. Kroeker visited an area of shallow waters near Ischia, Italy, that serves as a natural laboratory for what the oceans could look like in a world with higher carbon dioxide levels. CO2 gas upwells from volcanic vents in these waters like champagne bubbles, turning some patches of water more acidic—much as human CO2 emissions into the atmosphere make their way to the seas and sour the oceans.
Scientists, by exposing marine creatures to water of various acidities in the lab, have long known that higher acidity could harm certain species. But Kroeker, then a Stanford University doctoral student, wanted to know more: How would the altered chemistry affect whole marine communities consisting of multiple species? She and colleagues, equipped with scuba gear, collected creatures from areas of the water with different acidities and counted the creatures to assess biodiversity in each pH condition. They found that higher acidity hadn’t harmed all life uniformly but had made communities of organisms less diverse. Shell-building species such as clams and snails were all but absent, with other, simpler organisms replacing them (Proc. Natl. Acad. Sci. USA 2011, DOI: 10.1073/pnas.1107789108).
Kroeker is not alone in trying to broaden the study of acidification’s effects to marine communities and ecosystems as well as to other conditions that disturb oceanic equilibriums. A growing body of work confirms that ocean acidification and its effects don’t occur in isolation. Other environmental stressors, especially global warming and the deoxygenation of waters, can interact with acidification, in many cases enhancing its effects, scientists are finding. And impacts on one species don’t stay confined; they can spill over to the rest of the ecosystem, disrupting the food web, altering the balance of resources, and throwing off biogeochemical cycles.
These findings, enabled in part by increased ocean acidification research funding and the development of novel techniques, add complexity to a field that is already multifaceted, given the size of the world’s oceans and their internal variability. The findings also highlight the potential harms from human-sourced CO2 emissions, researchers argue. The work, they say, could inform lawmakers, scientists, and others as they decide how to protect marine life, particularly in local areas with unique combinations of stressors and biodiversity.
Both ocean acidification and global warming are being fueled by human-sourced CO2 emissions. But ocean acidification, the younger field, has garnered widespread attention only in the past 10 to 15 years. About one-fourth of emitted CO2 ends up in the oceans, where much of it converts to carbonic acid. Researchers suggest that human actions have already increased average ocean acidity by 30%, a decrease of 0.1 pH unit, since preindustrial times and are currently acidifying the waters at the fastest rate in 300 million years (Science 2012, DOI: 10.1126/science.1208277).
In the field’s early stages, to test how future acidity changes would affect marine life, scientists studied marine species one at a time, usually in the laboratory by exposing them to acidified seawater. They found that higher acidity waters harmed many species or altered their physiological processes. For example, calcifiers such as corals struggle to create or maintain their calcium carbonate skeletons as acidification reduces carbonate ion availability.
David I. Kline, a marine biologist with Scripps Institution of Oceanography, explains that these initial findings didn’t tell the whole story, but they established that acidification warranted further attention. “Like any new field, you’ve got to start simple and with experiments that are going to give you good, replicable data,” says Kline, who notes that funding in the field’s early days wouldn’t have often supported complex, multidisciplinary experiments.
That progression echoes what happened with acid rain research, which initially also focused on effects on individual species. Later studies on ponds with whole communities of life yielded more sophisticated results.
As early findings on acidification’s deleterious impacts began to paint an ominous picture for some ocean species, officials from various institutions—including the National Science Foundation and the European Union—responded with more research funding, Kline says. “The field has really pushed forward,” he explains. “As funding has improved, we’ve been able to do multiple-stressor, multiple-species, whole-ecosystem studies.”
Some studies involving multiple stressors’ effects have yielded troubling findings. For example, researchers led by Laura H. Parker, a marine biologist at the University of Western Sydney, in Australia, showed that a species of oysters suffers reduced fertilization and impaired embryonic development in lower pH conditions. These effects worsen when temperature increases to levels that scientists project will be prevalent by 2100 (Global Change Biol. 2009, DOI: 10.1111/j.1365-2486.2009.01895.x).
Scientists have also raised concerns about hypoxia, or low oxygenation of water. Hypoxic zones have expanded because of agricultural and industrial runoff as well as global warming, which, by heating oxygen-rich surface waters, reduces their ability to mix with lower-oxygen waters below. Hypoxia, warming, and acidification interact in multiple ways, including by altering physiological processes in marine organisms (J. Fish Biol. 2010, DOI: 10.1111/j.1095-8649.2010.02783.x).
Together, the “big three” stressors of global change—acidification, warming, and hypoxia—will pose major challenges to ocean life, marine scientists say in a new review paper (Mar. Pollut. Bull. 2013, DOI: 10.1016/j.marpolbul.2013.07.022). Paleobiological evidence such as fossils and sediment cores, as well as models of past ocean and climate conditions, lend support to this idea by suggesting that combinations of those stressors helped drive at least three of the biggest mass extinctions. Now, humans are making all three major stressors worse through large emissions of CO2 and localized conditions such as industrial pollution, overfishing, and nutrient runoff, researchers say.
Not all species will suffer from all of these stressors. For instance, some organisms, such as fleshy macroalgae (seaweed), may thrive in higher CO2 conditions, which boost photosynthesis. Even some species of coral, the poster child of the changing ocean environment, could be more resilient to acidification under warmer conditions (Nat. Clim. Change 2012, DOI: 10.1038/nclimate1473).
Still, when acidification—alone or in combination with other stressors—harms or helps even one species, the whole ecosystem could change, researchers warn. Of particular concern are coral reefs, which house some of the world’s most diverse communities and provide coastal protection, tourism opportunities, and a home for fish and other organisms. Excessive warmth can cause them to bleach, or lose their color, usually by shedding symbiotic, colorful algae that live within them. Acidification could further harm most reef-building corals by reducing their skeleton-calcifying abilities. These factors together could make corals more vulnerable to other stressors and disease. If corals were to die and reefs break down, the ecosystem’s species composition in turn would become markedly less diverse.
Some reefs face peril as a result. In one study, Kline, then a postdoctoral fellow at the University of Queensland, in Australia; Sophie G. Dove, a coral researcher at Queensland; and colleagues put colonies of corals and other organisms into mesocosms—experimental ponds or tanks that replicate key aspects of the real world—and exposed them to various pH and temperature scenarios (Proc. Natl. Acad. Sci. USA 2013, DOI: 10.1073/pnas.1302701110). Even in a low CO2 emissions scenario with a 2 °C temperature increase, some varieties of coral died and their communities became overrun by algae, Kline and coworkers found.
That doesn’t mean that all coral reefs are doomed, Kline says. “But it may mean we lose a lot of species associated with reefs,” he says. “And reefs will likely function in very different ways than they do today.”
Kline has helped pioneer a technique to study acidification’s effects on ecosystems or multiple species in the real world while controlling the carbonate chemistry of the water being studied. The studies, known as Free Ocean CO2 Enrichment (FOCE), involve lowering enclosures onto the seafloor. The species being studied are contained in the enclosure’s experimental chamber. Pumps in the device inject CO2-enriched water from an external source to keep the chamber’s pH at desired levels while leaving the surrounding water largely unaffected.
FOCE was developed by the Monterey Bay Aquarium Research Institute, in California. As James P. Barry, a marine biologist at the institute, explains, “We’ll try to take whole ecosystems and put them in there and see if over a period of time we see shifts in communities related to ocean acidification.” Kline has adapted FOCE for studying coral communities on shallow seafloors (Sci. Rep. 2012, DOI: 10.1038/srep00413).
In addition to mesocosm and FOCE studies, CO2 vent studies also hold promise for studying multiple-species or ecosystem impacts. Vent studies have been done before, Kroeker explains, “but not in context of environmental change or ocean acidification until we started to realize that this was a global problem.” Although the CO2-enriched water patches where vents are found may differ from what future oceans will look like, “these could be useful for looking at ecosystem effects of acidification,” she says.
Kroeker’s 2011 study on the Italy vents got her started. She and colleagues later tried another experiment there. They cleared entire plots of the water floor in areas of differing pHs and let those areas repopulate. Plots with lower pH water recovered with a smaller number and less diversity of species than areas with acidity at the current global average (Proc. Natl. Acad. Sci. USA 2013, DOI: 10.1073/pnas.1216464110).
Some organisms could adapt or acclimate to ocean acidification. In one study, sea-urchin larvae were exposed to current and lower pH waters. Many larvae had genetic traits helping them resist lower pH, and they were the most likely to survive, marine biologist Melissa H. Pespeni of Stanford University and coworkers found (Proc. Natl. Acad. Sci. USA 2013, DOI: 10.1073/pnas.1220673110).
But studies of adaptation and acclimation potential for higher forms of sea life will be neither easy nor straightforward, Kline says. Still, with ocean health worsening, “if we don’t push ourselves to push boundaries, we cannot get the right information in time” to policymakers, says Piero Calosi, a marine ecophysiologist with Plymouth University, in England. “It’s a fundamental step we have to face.”
The mere potential to adapt or acclimate is no guarantee it will actually happen, however. For many species, the rate of CO2 rise is likely to be too fast for adaptation or acclimation, says Alex D. Rogers, a conservation biologist with the University of Oxford. That may be especially true for species that lack genetic diversity, have long life spans, or are immobile, such as corals.
Another challenge that lies ahead for researchers is to get better at coordinating their work on multiple stressors and ecosystem impacts. Coordinated research efforts should aim to “derive a set of unifying principles to help identify which sensitive species and ecosystems to protect in the face of rising ocean acidity,” write Hans Pörtner, a marine ecophysiologist at the Alfred Wegener Institute, in Germany, and Sam Dupont, an evolutionary biologist at the University of Gothenburg, in Sweden (Nature 2013, DOI: 10.1038/498429a).
Ultimately, researchers won’t fully understand how ocean acidification alters marine life until it happens to a greater extent. That shouldn’t justify waiting to act, says Scott C. Doney, an ocean geochemist with Woods Hole Oceanographic Institution, in Massachusetts. Researchers have suggested a suite of actions, including CO2 emissions reductions, sustainable fisheries management, and cuts in local pollution.
“It would seem prudent, given what we’ve already seen, to try to reduce the risk to natural ecosystems and to try to reduce risks for commercial species that people depend upon,” Doney says, “instead of either waiting until the science is fully complete—whatever that even means—or waiting until we’re starting to see collapses in ecosystems.”