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Why ocean acidification matters to zooplankton, a coral’s primary food source

Why ocean acidification matters to zooplankton, a coral’s primary food source


Zooplankton are a diverse and colourful community of microscopic animals that float in the waters of our oceans. They are the bread and butter for many larger animals including corals and fish. In a Nature Climate Change study released today, a team of scientists led by the Australian Institute of Marine Science (AIMS) have revealed that zooplankton living in coral reefs severely decrease in abundance under ocean acidification conditions.

The findings are based on pioneering field experiments at unique coral reefs that serve as natural laboratories, mimicking future ocean acidification conditions. Here, the scientists made some startling observations. Zooplankton experienced a threefold reduction in biomass at the ocean acidification sites compared to sites representing ‘normal’ oceanic conditions. This finding dramatically contrasts with conclusions drawn from previous laboratory-based studies, which found zooplankton to be rather tolerant to ocean acidification. 

The amount of zooplankton caught over reefs with high carbon dioxide levels (right 2 containers) is vastly reduced compared to that caught over normal reefs (left containers). Image credit: Katharina Fabricius

“Zooplankton are critical to marine ecosystems. They’re the insects of the sea and provide food for many marine animals, from small fish to large whales. They’re also an important component of the coral reef food web, providing food for the corals themselves. Less zooplankton in coral reefs under future ocean acidification conditions will have far reaching implications for not just the reefs, but the entire ocean food web,” said the study’s lead author, Ms Joy Smith of the Australian Institute of Marine Science.

Researchers also examined potential causes for the observed decline, and found changes to the zooplankton’s home environment to be a likely contributor to the loss in abundance.

AIMS Research Scientist and senior author, Dr Katharina Fabricius said: “Under ocean acidification, coral communities shift from delicately branching corals to massive bouldering corals. This shift reduces the structural complexity of the reef environment. As a consequence, these reefs fail to meet the habitat needs of many types of zooplankton.”

 

A structurally diverse coral reef landscape provides good habitat for zooplankton (right). Coral reefs in areas with high carbon dioxide tend to have low structural diversity and are poor habitat for zooplankton (left).  Image credit: Katharina Fabricius

The study “Ocean acidification reduces demersal zooplankton that reside in tropical coral reefs” is a joint research project of AIMS, Alfred Wegener Institute for Polar and Marine Research, University of Bremen, and Plymouth University. Co-funding for this research was provided by the Great Barrier Reef Foundation’s Resilient Reefs Program in collaboration with the Australian Government, the BIOACID Program of the German Government, and the Mares Programs of the European Union.