The direct chemical effect of our CO 2 emissions on our planet’s oceans gets far less attention than the indirect effects caused by global warming. But CO 2 lowers the seawater’s pH—known as “ocean acidification”—and this has been shown to be a serious problem for many species. Acidification makes it harder for critters with calcium carbonate shells to grow them, and it even changes the way fish behave.

The majority of studies that have looked at ocean acidification’s impact have fallen into two basic categories: laboratory experiments with carefully isolated conditions and species, and surveys of life at natural CO 2 seeps on the seafloor. Each category has drawbacks and advantages. Lab experiments are carefully controlled and can provide unambiguous results. Surveys at natural CO 2 seeps can integrate more processes, like species interactions or adaptation over generations. But it’s also true that seeps are surrounded by “normal” ecosystems that could be lending support.

Carbon offsets

A group of University of Adelaide researchers led by Silvan Goldenberg set out to help fill in the span between those two categories by designing controlled experiments that stepped up the ecological complexity. In 1,800 containers called “mesocosms,” the researchers combined species to form tiny ecosystems. The stars of the show were eight species of fish and shrimp, which swam among a rich supporting cast of over 90 other species—everything from algae and microbes to predators.

Some tanks had their pH reduced to 7.9, their temperature increased by almost 3 degrees Celsius, or both in order to match projected changes in the ocean by 2100 if we follow a path of high greenhouse gas emissions. The fish and shrimp were carefully tracked over the four-and-a-half month run of the experiment.

The results included some surprises. While experiments have found that fish lose the ability to properly respond to either scent or visual signals of prey or predators when pH drops, the fish in these experiments behaved normally as long as both scent and visual signals were there. And critically, since the algae and plants in these tanks grew faster with more CO 2 in the water, grazing fish and shrimp also did well as a result.

Warmer water temperature raised the metabolisms of these organisms, making them take greater risks in search of the extra food they needed—even if there were predators around. But at least in these tanks, the increased food availability was enough to allow the fish and shrimp to adapt and do quite well.

Ecosystem services

The researchers also looked at more than 100 previous studies of varying types to see how their results fit in. They found that the more complex the ecosystem studied, the less harmful the resulting impacts of ocean acidification were. It’s an illustration of the powerful effect biodiversity has on the stability of an ecosystem. When there are more species interacting, it’s easier for each species to make small changes that help them adjust to adverse conditions.

GEOMAR researcher Lennart Bach, who was not involved in the study, told Ars:

It became clear in the last couple of years that we have to go beyond studying ocean acidification effects on individual species toward ocean acidification studies of natural communities—or better, entire ecosystems. Such studies with natural communities are extremely difficult to do and detecting indirect effects is even harder in light of the enormous complexity of marine food webs. This study is a significant step forward... I think the key advance of their study is that they convincingly reveal how indirect effects within natural communities can modify ocean acidification impacts which we were expecting to see based on laboratory findings. So they were addressing one of the biggest uncertainties we are currently facing in ocean acidification research.

That said, the study’s results do not show that the effects of ocean acidification will disappear outside the laboratory. The researchers themselves emphasize that “While our findings provide a broader framework in which to consider ocean acidification—a field dominated by reports on negative effects—ecosystems as a whole still seem likely to experience losses in species and functional diversity.”

The habitats they studied were shallow-water communities with plants and algae that can actually take advantage of added CO 2 . But in other ecosystems—like coral reefs or the open ocean—the base of the food web itself is vulnerable to ocean acidification and warming. Also, species exhibiting the risky food-seeking behavior they observed in warmer water could be punished for it out in real habitats when lots of predators are around. And while the species used in the experiment could tolerate water that was warmer by 3 degrees Celsius, other species will find that makes their homes too warm.

The researchers’ take-home point is that studies of more complex ecosystems provide important information about how species will really be impacted by continuing ocean acidification. In some cases, that will even give us some welcome good news about how certain species will fare.

Nature Climate Change, 2018. DOI: 10.1038/s41558-018-0086-0 (About DOIs).