Anna Kluibenschedl1, Christopher D. Hepburn1, Jaret Bilewitch2, Christopher E. Cornwall3, Miles Lamare1, Wendy Nelson2,4, Brenton Twist5, Daniel W. Pritchard1
1 Department of Marine Science, University of Otago, 310 Castle Street, Dunedin, 9016, New Zealand
2 National Institute of Water & Atmospheric Research Ltd (NIWA), Private Bag 14-901, Wellington, 6241, New Zealand
3 Marine Biology/School of Biological Sciences, Victoria University of Wellington, 7, Kelburn Parade, Wellington, 6012, New Zealand
4 School of Biological Sciences, University of Auckland, Private Bag 92019, Auckland, 1142, New Zealand
5 Department of Botany, University of British Columbia, Vancouver V6T 1Z4, BC, Canada
Coralline algae play key ecological roles in costal ecosystems globally and are known to occur down to the limits of the photic zone. Despite their demonstrated susceptibility to ocean acidification (OA) and importance in low light environments, there is limited understanding of the interplay between irradiance and OA on growth and reproduction of this important group. Early successional coralline communities (established in the field) were subjected to a gradient of daily light dose (0.35, 0.17 and 0.1 mol m−2 d−1) – a range based on in situ measurements – and 2 pH levels (present-day pH 8.03, OA pH 7.65) in a 212-day experiment to test the sensitivity these communities and subsequent recruitment to OA in low light environments.
Lowered seawater pH reduced growth of the parental coralline communities by > 60% across the light gradient, with a parabolic growth response to irradiance and no interactive effects. In contrast, the OA-driven reduction in new recruitment (56%) was amplified under reduced light with recruitment near zero in the lowest light treatment.
The present study shows that irradiance may play an important role in determining the outcomes of OA on coralline communities and for the first time, shows the increased vulnerability of coralline recruitment to OA under low light. Coralline algae are known to be the
deepest growing macroalgal group. Thus, OA could affect coralline communities in these low light zones disproportionately and therefore also alter depth distribution.