A NEW study has taken science out of the laboratory and put it back in the ocean to reveal the complex response of marine ecosystems to increasing global levels of carbon dioxide.
Published today in the journal Nature Climate Change, the investigation used natural phenomena known as seeps - localised areas of high carbon dioxide concentration near underwater volcanoes - to study the impact of ocean acidification on fish and plants.
“We found that high levels of carbon dioxide had not just a direct effect on fish behaviours, but also an indirect effect through changing their habitat,” said lead author Associate Professor Ivan Nagelkerken from the University of Adelaide.
The study focused on small territorial fish at two study sites: the Goby (Gobius bucchichi) in the Mediterranean, and the Triple Fin (Forsterygion lapillum) near New Zealand. Each fish species has a limited range of range of movement such that they remain either over the seeps (with high carbon dioxide), or in non-seep areas (with normal carbon dioxide) for their entire lives.
Each seep was around up to the size of eight tennis courts side-by-side, and characterised by carbon dioxide bubbling from vents on the sea floor.
Professor Nagelkerken said there were double and even triple the number of fish in areas of high carbon dioxide.
These increased fish numbers were associated with a loss of kelp seaweed and a predominantly algal plant profile over the seeps.
“Higher levels of carbon dioxide act as a preferential source of nutrition for low-lying turf algae,” explained Professor Nagelkerken.
“The algae grow faster and overgrow kelp, so instead of high 3D structure you end up with a low lying habitat.”
The scientists believe that altered plant life reduces the capacity for larger ambush predators like grouper to operate.
“In the Mediterranean the main predator is a kind of grouper, and that species was completely absent from the vents,” said Professor Nagelkerken.
Without the predators, smaller fish can thrive.
“They’re almost like ‘rats of the ocean’, these species that are not very sensitive to habitat change,” suggested Professor Nagelkerken.
“Their densities go up with higher levels of carbon dioxide in the water.”
While carbon dioxide in the seep areas comes from volcanic activity, carbon dioxide also dissolves into oceans from the atmosphere. And it’s a direct relationship: rising atmospheric carbon dioxide means rising oceanic carbon dioxide and increasing acidity.
"Using these carbon dioxide seeps, we've been able to get a unique preview of what the future ocean will look like under current projections for the end of the century – and it's not good,” said Professor Nagelkerken.
Team Leader at the Coral Ecophysiology Processes Group at UTS Sydney, Associate Professor David Suggett agrees the data offer a rare and valuable perspective.
“This is really the first paper that demonstrates that some fish populations can benefit from more acid waters, whereby ‘negative’ impacts of ocean acidification on fish behaviour appear substantially outweighed by corresponding changes to habitat structure,” he said.
“The study provides a really strong case for seeking out and understanding other naturally acidic sites to place us in the best possible position to predict and manage climate change.”
Study lead author Professor Nagelkerken also believes there are implications for primary industries management.
"Our research has given us a greater understanding of increasing carbon dioxide emissions as a driver of ecological change and what this might mean for future marine biodiversity and fisheries production," he said.