Bioengineering slow flow habitats as refugia for coastal calcifiers from ocean acidification

Chair: Jessica Ericson

Catriona L. Hurd (1)*, Katherine Schmutter (2), Merinda Nash (3), Will Howard (4)

1 Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, TAS 7004, Australia, catriona.hurd@utas.edu.au
2 Fenner School of Environment and Society, The Australian National University, Canberra, ACT 2601, Australia
3 Research School of Physics and Engineering, Electronic Materials Engineering, The Australian National University, Canberra, ACT 2601, Australia
4University of Melbourne, Earth Sciences Department, Melbourne, Victoria, 3010, Australia

Background
Ocean acidification is the ongoing decline in the pH of surface waters, and the projected 0.4 unit pH decrease by 2100 is likely to negatively impact benthic coastal organisms that fabricate calcium carbonate ‘skeletons’. Coralline algae are considered the most vulnerable of all benthic calcifying organisms to ocean acidification, and calcifying invertebrates are also highly vulnerable. Research has focussed on identifying susceptible species but we also need to identify refuge habitats to enable their protection.

Methods
The susceptibility of coralline algae to ocean acidification depends on the pH at their surface, and this is regulated by the interplay between algal metabolism and water motion. Using pH microprobes, we show that algal metabolism causes local increases (photosynthesis, nitrate uptake) and decreases (calcification, respiration, ammonium uptake) in the pH at the algal surface; pH at the surface of a seaweed can thus fluctuate by ±0.5 units on a daily cycle.

Findings
Water motion sets the thickness of the diffusion boundary layer, and thick DBLs have been shown to ameliorate the negative effects of ocean acidification on coralline algae via their action of retaining metabolically-produced high-pH seawater at the algal surface, thereby preventing dissolution of calcium carbonate (Cornwall et al. 2014). Seawater velocities within beds of canopy forming seaweed, including members of the orders Laminariales and Fucales, are much reduced (75-95%) compared to the mainstream seawater velocity.

Conclusions
Slow-flow habitats may provide natural, low-cost refugia for coastal calcifiers from ocean acidification. We explore how bioengineering coastal habitats will locally mitigate the effects of ocean acidification, resulting in more resilient ecosystems with greater conservation value.