Warmer oceans seen to downsize sea life, impact carbon sequestration

A new model predicts that marine microbes could shrink by up to 30 per cent in the future due to climate change, impacting bigger organisms that eat them including fish, potentially disrupting the food chain from the bottom up.

Accurately predicting warming impacts on marine life could improve ocean resource management. Image: Cyrille Humbert, CC BY-SA 3.0, via Flickr.

The future may be smaller for sea life, according to a new scientific model. Influenced by warming oceanic conditions, microbes and megafauna may not grow as large as they do now.

This shrinking effect, should it occur, could have wide repercussions: reduced food mass at the bottom of the food chain would affect fisheries, leaving less food for people, as well as mean less carbon sequestered in the sea, potentially making climate change worse.

Scientists say the ability to accurately predict these impacts could improve management of ocean resources. But researchers don’t agree on exactly why this sea life shrinkage is happening, and say a variety of factors may need to be considered to make accurate forecasts.

In the new study, researchers present a mathematical model that explains these size reductions as a response to lower oxygen levels in the ocean. Looking at rising temperature and reduced oxygen level forecasts for the next decades, researchers found that zooplankton and other microscopic species could be up to 30 per cent smaller, with impacts reverberating higher up the food chain.

There is a “temperature-size rule,” that describes the tendency for ectotherms (animals whose body temperature regulation depends on external sources) to reach smaller adult sizes under warmer conditions. With climate change escalating, the implications for marine life — and humanity — could be dire.

With climate change, as species are finding themselves in environments that are beyond their normal temperature range, oxygen limitation is one thing that can cause them problems. But it is not the only thing.

Brad Seibel, biological oceanographer, University of South Florida

But so far, direct experimental evidence for this rule mostly comes from organisms with a body mass of less than 1 gram, explained study lead author Curtis Deutsch, a climate scientist at Princeton University in the US

“Our mechanistic model tries to quantify that effect and use it to understand how much smaller things might get in the future,” explained Deutsch, who collaborated with ocean biologists and a paleobiologist to formulate the model. The results “suggest that larger fish [further up the food chain] will also be subject to shrinkage.”

The model is based on how marine ectotherms meet their metabolic demand for oxygen. The assumption is that their rate of metabolism rises in warmer water, with more oxygen needed to support those higher metabolic rates. But as water temperatures go up, the levels of dissolved oxygen go down. So if you need more oxygen, but the ocean is supplying less, then you may be in trouble, explained Deutsch.

One way to dodge that problem is to stay small. But that solution isn’t as simple as an organism growing only half as big to use half as much oxygen. As an animal gets larger, its outer surface area does not scale up equally with the increased number of cells needing oxygen. The new modelling equation predicts how much an organism must shrink, based on resting metabolic needs, under the limited-oxygen conditions expected as global warming worsens.

Additional support for this model can be gleaned from the fossil record, said Jonathan Payne, a paleobiologist at Stanford University in California and a senior author on the paper. Evidence from extinction events linked to global warming and ocean deoxygenation show that larger species were more likely to die off, while surviving species tended to be smaller.

And though a lot of information about the prehistoric world is missing, Payne said, there are enough proxy or fossil measurements to test the new equation’s calculations. It’s a “really neat tool for connecting the modern world with all the things that have happened in the history of animal life,” he noted.

The equation could also be incredibly useful now, said Rowan Lockwood, a conservation paleobiologist at Virginia’s College of William and Mary in the US, who was not involved in the study. “We spend a fair amount of time and energy trying to predict what the effects of global warming are going to be from a variety of perspectives. This model gives us a much broader and more applicable framework than what we’ve had before,” she pointed out.

In the northeastern United States, for example, sea scallops are shifting northward and into deeper water. It won’t be long before these migrating scallops move to where you can’t fish for them within the current permit system, the current dredging system, or with the boats now available, she explained. “So, for me, the coolest part of this [model] is that it gives tools to fisheries, managers, and harvesters, to predict where their harvest is going,” said Lockwood. And perhaps enough time to adapt management plans.

But this oxygen-limiting modelling approach doesn’t resolve the temperature-size rule for everyone. “With climate change, as species are finding themselves in environments that are beyond their normal temperature range, oxygen limitation is one thing that can cause them problems. But it is not the only thing,” explained Brad Seibel, a biological oceanographer at the University of South Florida, US, who collaborated with Deutsch on other studies.

The new study compared the way oxygen supply scales with the way resting metabolism scales, Seibel noted. “But that doesn’t test the equation against conditions where oxygen is actually limiting, because when you’re at rest, you’re not oxygen limited until the environmental oxygen declines to some very low critical level.” If the data were available, he suggests, it would be ideal to compare oxygen supply to the maximum metabolic rate.

Other studies indicate that species’ response to rising temperature is more complex than metabolic changes alone would suggest. What’s needed, some researchers say, is experimental evidence that accounts for species’ development rates, reproductive output, and impacts on overall fitness, tracked long term over many generations.

“It’s not a slam dunk, like experimental evidence,” Deutsch said of the model. But getting generational field data — especially for big fish — can be difficult and time consuming. The researcher is now pursuing additional indirect support for the model, looking at US and international fisheries data sets collected over many years.

If the new one-line equation helps explain some first-order things, such as the general mechanism underlying the temperature-size rule,  then we can start figuring out what other things matter to explain more details, noted Payne.

But the outlook appears daunting if this model’s predictions are even close to correct.

From Payne’s paleontological perspective, we face a challenging situation, but geology also tells us that things have been worse. After all, ancient records of shrinking species and mass extinctions weren’t affected by overfishing, pollution, land-use changes, and other human-caused impacts. “A lot of it is our own doing, which means we should also have the ability to undo at least some aspects of it,” he said.

This story was published with permission from Mongabay.com.

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