Ocean acidification reverses the positive effects of seawater pH fluctuations on growth and photosynthesis of the habitat- forming kelp Ecklonia radiata

Chair: Gretchen Hofmann

Damon Britton(1), Christopher E Cornwall(1,2), Andy T Revil(3), Catriona Hurd(1), Craig R Johnson(1)

1 Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, TAS 7001, Australia
2 School of Earth and Environment & ARC Centre of Excellence for Coral Reef Studies, University of Western Australia, Crawley, WA 6009, Australia
3 CSIRO Oceans and Atmosphere, Hobart, TAS 7001, Australia

Background
Ocean acidification (OA) is the reduction in seawater pH due to the sustained absorption of human-released CO2 by the world’s oceans. By 2100, the average surface oceanic pH is predicted to decline by 0.4 units. However, in some temperate coastal systems kelp forests metabolically modify seawater pH via photosynthesis and respiration, resulting in daily pH fluctuations of up to ± 0.45 units. It is unknown how these fluctuations in pH influence the growth and photo-physiology of the kelp themselves, or how this would change with OA.

Methods
We measured pH in beds of the canopy-forming kelp Ecklonia radiata on the east coast of Tasmania using both bottle sampling and deployment of pH sensors (pHTempion and SeaPHOXs). In two laboratory experiments we tested the effects of fluctuations in pH on E. radiata growth and photo-physiology. One experiment consisted of a fluctuating pH (8.4 in the day, 7.8 at night) treatment and 3 static pH treatments (8.4, 8.1, 7.8), all within the diel range observed today. A second experiment consisted of the same treatments, but with pH reduced (acidified) by 0.3 units across all treatments.

Findings
In the experiments mimicking the most extreme pH fluctuations measured within beds of E. radiata, growth and photosynthetic rates of juvenile E. radiata were significantly greater under fluctuating pH than in any of the static pH treatments. However, when the mean pH of each treatment was reduced by 0.3 units, pH fluctuations had no effect on growth rates, and a negative effect on photo-physiology.

Conclusions
Currently daily pH fluctuations have a positive effect on photosynthesis and growth of E. radiata, but this effect could be negated or reversed in the future under OA. This will impact future the productivity, and perhaps the ecological dynamics, of habitats dominated by E. radiata.