Species don’t live in isolation; they live in very tangled, complicated, interconnected webs. So studying them in isolation has only limited utility, much like studying cells cultured in a sterile petri dish. These laboratory studies can yield suggestive and promising results, but these results are not always applicable to how the cells behave in the context of an organism, much less as part of a species in an ecosystem.

The increased carbon dioxide that humanity has been relentlessly pumping into the air since the onset of the Industrial Revolution is acidifying the oceans. Studies done to determine what this ocean acidification will do to fish have mostly assessed the direct effect of elevated CO 2 levels on the fishes’ growth and physiology, but they have not taken into account any effects ocean acidification might have on food webs as a whole.

Scandinavian marine biologists have tried to rectify this situation by studying the effects of ocean acidification in 10 mesocosms—fiberglass tanks seeded with rocks, sediment, plankton, and other microorganisms—they set up off the west coast of Sweden. Five were controls; the other five got elevated CO 2 , set to mimic levels that the Intergovernmental Panel on Climate Change calculates could occur by the end of this century. Those estimates are about 760 μatm (short for micro-atmospheres) pCO 2 , compared to today’s 380 μatm pCO 2 ). Being Scandinavian, these researchers examined the survival of... herring.

While the phytoplankton in the mesocosms were blooming, in mid-April 2013, the researchers added fertilized herring eggs to all 10 of them. Significantly more herring larvae survived in the five with the elevated CO 2 levels than in the controls, especially during the critical first feeding period just after the eggs hatch. This was a surprise, given that high CO 2 levels had negative effects on herring larvae in laboratory studies.

The scientists deduced—and went on to demonstrate—that herring survival rates differ on the basis of food availability. It was already known that elevated CO 2 in the ocean stimulates some types of phytoplankton. This abundance of phytoplankton allows for an abundance of zooplankton—small crustaceans that eat the phytoplankton. This, it seems, spills over to the herring larvae that eat these crustaceans.

A similar effect, in which the plus of more food overcomes the minus of more CO 2 (and more acid), was also seen for mussels in the Western Baltic Sea.

The authors conclude: “Despite the positive turnout for the herring larvae under high CO 2 conditions, the findings of this study should not be extrapolated to imply a bright future for fish recruitment in an acidifying ocean.” Part of the issue is that there were no predators of herring larvae in any of the test environments—while less artificial than labs, these mesocosms are still designed and controlled.

We should also keep in mind that herring are just one species. Other species—Atlantic cod and silverside, corals, larger phytoplankton—are not projected to fare as well in a more acidic ocean, and biodiversity as a whole will almost certainly be diminished. Much like the authors of a similar study just done in the Great Barrier Reef, these scientist maintain that the primary utility of these community-level studies is to help us prepare for and mitigate the dramatic effects that climate change will have across all of the lifeforms sharing the oceans.

Nature Ecology and Evolution, 2018. DOI: 10.1038/s41559-018-0514-6 (About DOIs).