Global warming: Too much plankton, not enough fish

Study warns of ‘top-down’ ecosystem changes

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New research suggests a potential upheaval in ocean ecosystems due to climate change. @bberwyn photo.

Staff Report

Global warming could lead to a “top-down” collapse of ocean ecosystems, researchers warned after showing how ocean acidification and warming are likely to cause a reduction in diversity and numbers of various key species that underpin marine ecosystems around the world.

The study, published in the journal Proceedings of the National Academy of Sciences, concluded there is only “limited scope” for acclimation. Very few species will escape the negative effects of increasing CO2, with an expected large reduction in species diversity and abundance across the globe. One exception will be microorganisms, which are expected to increase in number and diversity.

“This ‘simplification’ of our oceans will have profound consequences for our current way of life, particularly for coastal populations and those that rely on oceans for food and trade,” said Ivan Nagelkerken, an associate professor at the University of Adelaide, Australia.

The large-scale analysis of existing research, analyzed data from 632 experiments covering tropical oceans, Arctic water and a range of ecosystems from coral reefs through kelp forests to open oceans.“We know relatively little about how climate change will affect the marine environment,” said Sean Connell, a marine biologist at the University of Adelaide. “Until now, there has been almost total reliance on qualitative reviews and perspectives of potential global change. Where quantitative assessments exist, they typically focus on single stressors, single ecosystems or single species.

“This analysis combines the results of all these experiments to study the combined effects of multiple stressors on whole communities, including species interactions and different measures of responses to climate change.”

Primary production from the smallest plankton is expected to increase in the warmer waters but this often doesn’t translate into secondary production (the zooplankton and smaller fish) which shows decreased productivity under ocean acidification.

“With higher metabolic rates in the warmer water, and therefore a greater demand for food, there is a mismatch with less food available for carnivores … the bigger fish that fisheries industries are based around,” Nagelkerken said. “There will be a species collapse from the top of the food chain down.”

The analysis also showed that with warmer waters or increased acidification or both, there would be deleterious impacts on habitat-forming species for example coral, oysters and mussels. Any slight change in the health of habitats would have a broad impact on a wide range of species these reefs harbour.

Another finding was that acidification would lead to a decline in dimethylsulfide gas production by ocean plankton which helps cloud formation and therefore in controlling the Earth’s heat exchange.

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