Some of the ocean’s top predators, such as tuna and sharks, are likely to feel the effects of rising carbon dioxide levels more heavily compared other marine species.
That’s just one of the results of a study published today in Proceedings of the National Academy of Science.
Over the past five years we’ve seen a significant increase in research on ocean acidification and warming seas, and their effect on marine life. I and my colleague Sean Connell looked at these studies to see if we could find any overarching patterns.
We found that overall, unfortunately, the news is not good for marine life, and if we do nothing to halt climate change we could lose habitats such as coral reefs and see the weakening of food chains which support our fisheries.
Acidifying and warming oceans
Humans have been adding carbon dioxide to the atmosphere largely through burning fossil fuels. Under a worst-case scenario, without doing anything to stop increasing emissions, we’d expect concentrations of carbon dioxide to reach around 1,000 parts per million by the end of the century.
This increase in greenhouse gases is “acidifying” the oceans. It’s happening now. Carbon dioxide concentrations have reached around 400 parts per million, compared with around 270 parts per million before the industrial revolution.
This extra carbon dioxide, when it dissolves into the seas, is reducing the pH of the oceans – that is, making them more acidic.
Many ocean creatures, particularly those that build habitats such as corals and shellfish, make skeletons out of calcium carbonate, which they get from ions dissolved in sea water.
When carbon dioxide dissolves in seawater, it makes these calcium carbonate ions harder for marine life to collect and turn into skeletons. It’s like a person going on a diet without calcium.
At first this results in marine life producing brittle skeletons, but can ultimately lead to the skeletons dissolving.
A calcium-free diet
Many studies have looked at what will happen to these lifeforms that produce skeletons, but we wanted to look at how rising carbon dioxide would affect the ocean at a broader scale.
We analysed more than 600 experiments on ocean acidification and warming seas.
Overall it seems warming temperatures and acidifying oceans will have a negative effect on species and ecosystems. This means reduced growth, abundance, and diversity of marine species.
We also found these results were mostly consistent across latitudes - they weren’t just limited to tropical oceans.
It’s likely that acidification will interact with warming to have a worse effect. For instance, if you would see a 20% reduction in calcification rates because of rising temperatures, and a 25% reduction in calcification because of acidification, then the combined reduction might be 60%. We see these effects regularly in the studies we looked at.
Of course not every species will show the same response. We expect some species to be able to acclimate or adapt to changes, particularly over longer time periods perhaps like a couple of decades. For example, a recent study on a coral living in a tropical lagoon found it has some capacity to adapt. We found that more generalist species like microorganisms seem to be doing particularly well under climate change, and also some fish species at the bottom of the food chain may show increases in their populations.
Changing whole ecosystems
Most worrying are not only the changes to individual species but also whole ecosystems.
We found that reef habitats are vulnerable: coral reefs, but also temperate reefs built by molluscs such as oysters and mussels. A lot of shallow temperate waters used to have oysters reefs, but there are few natural reefs remaining.
There are also cold-water reefs formed by other species of coral, which grow slowly over thousands of years in the cooler temperatures. In our analysis we found that acidification could cause these habitats to show reduced growth. These habitats are often located in deep waters and are very sensitive to human impacts.
We also found that these changes affect whole ocean food webs.
We found that warmer temperatures mean more phytoplankton - the tiny plant-like lifeforms that form the basis of many ocean food chains. This means more food for grazing species that feed on phytoplankton.
Warmer temperatures also mean faster metabolisms, which require more food. However this didn’t translate into higher growth rates in grazing species. That’s fatal because the next level up in the food chain (the species that eat the grazing animals) would have less food, but still need more food because of faster metabolisms.
This effect is expected to become stronger as you go up the food chain, so predatory species like tuna, sharks, and groupers will be the species that would feel the strongest effects.
These species are also threatened by overfishing, which adds another level of stress. Overfishing alters important food web interactions (e.g. top-down control of prey species) and may also reduce the gene pool of potentially strong individuals or species that could form the next generation of more resilient animals. And this is on top of other threats such as pollution and eutrophication.
Therein lies an opportunity. We cannot change climate change (or ocean acidification) in the short term. But if we can mitigate the effects of overfishing and other human stressors we can potentially buy some time for various species to adapt to climate change.
Species can genetically adapt to changes over geological timescales of thousands of years - as we can see from modern species’ survival over many ups and downs in the climate. But the changes we have wrought on the oceans will take place over decades - not even one generation of a long-lived sea turtle or shark.
With such fast changes, many species in the ocean will likely be unable to adapt.