Ocean acidification Alters Benthic Community Composition and Metabolism in situ on Coral Reefs

Chair: Ulf Riebesell

Sam H.C. Noonan(1)*, Anna Kluibenschedl.(2), Katharina E. Fabricius(1)

1 Australian Institute of Marine Science, Townsville, QLD, 4810, Australia
2 Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven, D-27568, Germany

Background
There is wide consensus that ocean acidification will lead to a decline in many calcifying taxa and a proliferation of non-calcifying autotrophs. This is of particular concern for coral reefs due to the fundamental ecological roles provided by benthic calcifiers, namely the scleractinian corals and crustose coralline algae. Coupled with the predicted changes in community composition will be changes in rates of community metabolism. Communities altered by OA are predicted to have reduced calcification rates and perhaps enhanced primary production. However, empirical data with the scope to support these predictions is largely lacking.

Methods
Settlement tiles (n = 90) were deployed along CO 2 gradients at two CO2 seep and two control sites in Milne Bay, Papua New Guinea. The benthic community composition of the tiles was assessed after five and 13 months. At the 13 month census, rates of community gross photosynthesis, respiration and light and dark calcification were also measured on the tiles.

Findings
Increasing CO 2 led to a shift in algal community composition on the tiles. Crustose coralline algae dominated at the control sites, but they were progressively replaced by non-calcifying algae (especially cyanobacteria and macroalgae) as CO2 gradually increased. Patterns in the invertebrate communities were comparably weaker, although there were some declines in the ascidians, foraminifera and polychaetes.

Community changes resulted in altered rates of metabolism: by pH T 7.8, gross photosynthesis and respiration increased by ~10 and 20%, respectively. Dark decalcification significantly increased with CO2 exposure, while light calcification remained unchanged. Once photosynthesis and calcification rates were extrapolated to 24-hour light/dark cycles, no differences were detected along the CO2 gradients.

Conclusions
With ocean acidification steadily progressing, we are likely to see shifts in coral reef benthic communities in the coming decades. The new communities will have altered rates of metabolism compared to the reefs of today.