Top-end predatory fish will starve

Marine ecologists from the University of Adelaide have released the world-first global analysis of marine responses to climbing human CO2 emissions and global warming. And they've painted a grim picture for future fisheries and ocean ecosystems based on predicted ocean acidification in 85 years time.

Published in the journal Proceedings of the National Academy of Sciences (PNAS), the marine ecologists from the University of Adelaide say the expected ocean acidification and warming is likely to produce a reduction in diversity and numbers of various key species that underpin marine ecosystems around the world.

"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," says Associate Professor Nagelkerken. "There will be a species
collapse from the top of the food chain down."

In other words,
there will be a lack of food for the top-end predators and it's
this very food source, the smaller fish like jack mackerel, that today's
Government is allowing to be targeted en masse by factory trawlers in
Australian waters for short-term gain.

Associate Professor Nagelkerken and fellow University of Adelaide marine ecologist Professor Sean Connell have conducted a ‘meta-analysis’ of the data from 632 published experiments covering tropical to artic waters, 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," says Professor Connell. "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."

"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," says Associate Professor Ivan
Nagelkerken, Australian Research Council (ARC) Future Fellow with the
University’s Environment Institute.

The researchers found that there would be "limited scope" for acclimation to warmer waters and acidification. 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.

From a total food web point of view, 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," says Associate Professor Nagelkerken. "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 (DMS) production by ocean plankton which helps cloud formation and therefore in controlling the Earth’s heat exchange.

The global marine-chain meltdown is forecast to happen in less than 100 years, when forecast huge rises in CO2 levels are expected to cause acidification of waters and dramatic changes in habitat and marine life.

You can listen to Associate Professor Ivan Nagelkerken from the University of Adelaide in this podcast at:

https://radio.adelaide.edu.au/forecasting-ocean-acidification/