Building global change resilience: Concrete has the potential to ameliorate the negative effects of climate-driven ocean change on a newly-settled calcifying invertebrate.

Global climate change is driving sea level rise and increasingly frequent storm events, which are negatively impacting rapidly-growing coastal communities. To mitigate these impacts, coastal infrastructure must be further protected by upgrading hard defences. We propose that incorporating pH-buffering materials into these upgrades could safeguard marine organisms from the adverse effects of ocean acidification and ocean warming during the vulnerable transition from planktonic larvae to benthic juveniles. To test this, we examined the effects of ocean warming (24 or 27 °C), ocean acidification (pH 8.1, 7.9, 7.7), and substratum (concrete, greywacke, granite) in all combinations on the settlement success of an ecologically and commercially important sea urchin, Tripneustes gratilla. Low pH (7.9, 7.7) generally reduced the quantity and size of juveniles four weeks post-settlement, although this was partially ameliorated by increased temperature (24 vs. 27 °C). In the warmed and acidified treatments, settlement rates were lower on concrete than granite or greywacke, but two weeks post-settlement, juveniles on concrete were larger, and had longer spines and higher survival rates than on greywacke or granite, respectively. The benefits provided by concrete to newly-settled juveniles may be related to alkali chemicals leaching from concrete buffering low pH conditions in surrounding seawater and/or increased availability of bicarbonate in the boundary layers around its surface. Our results highlight the potential for pH-buffering materials to assist marine organisms in coping with the effects of changing ocean conditions, but further research is required to understand the generality and mechanism(s) driving the beneficial effects of concrete and to test pH-buffering materials in the field.