Interactive effects of ocean acidification and water flow on growth and recruitment of early successional coralline algal communities

ABSTRACTCoralline algae play crucial roles in near-shore ecosystems but are susceptible to ocean acidification (OA). It has been hypothesized that low water velocity, allowing localized photosynthesis-driven pH-increases in the coralline surface boundary layer, could buffer against the negative impacts of OA. To test how water motion affected the sensitivity of coralline algae to OA, coralline communities (from 2 m and 10 m depth) were grown for 220 days at two pH levels (present-day: pH 8.03, OA: pH 7.65) under differing inflow rates (400, 200 and 100 ml min–1) providing water velocities of 2.7, 5.9 and 7.8 cm s–1. Communities from both depths were grown together, photographed to assess growth, and the resulting recruitment was evaluated at the experiment’s conclusion. Low seawater pH reduced growth by c. 11% (highest flow), further decreased by >23% under the lowest flow. This reduction resulted in differential outcomes for the two depths, with skeletal net-dissolution under the combination of low flow and pH 7.65 for 10 m communities. Furthermore, there was a synergistic interaction between the effects of flow and pH, whereby the negative effect of OA strengthened under low flow, with recruitment halved at pH 7.65. This demonstrates that OA impacts can be modulated by the flow environment. Surprisingly, increased flow rates/water velocities reduced negative impacts of low pH, thus further challenging the notion that slow flow habitats offer protection from OA. The observed interactions between water flow and OA on early successional communities and their recruits may hold implications for the future of rocky reef systems dominated by these communities.KEYWORDS: Flow rateFuture oceanNew ZealandTemperate coralline communitiesWater motion ACKNOWLEDGEMENTSWe thank Brenton A. Twist for his help with culture maintenance and Kim Currie for her continuous support and expertise in seawater carbonate chemistry. Special thanks to the management committee of the East Otago Taiāpure for facilitating the field component of this research.DISCLOSURE STATEMENTNo potential conflict of interest was reported by the authors.Supplementary InformationSupplemental data for this article can be accessed online at https://doi.org/10.1080/00318884.2023.2272776Additional informationFundingThis work was funded by the Coastal Acidification, Rate, Impacts and Management (CARIM) research program (M.L), funded by the New Zealand Ministry of Business, Innovation and Employment, the CARIM Postgraduate Scholarship (to A.K.) and the Ministry for Primary Industries (MPI) New Zealand under the Biodiversity Research Programme (ZBD2014-07; W.A.N.). C.E.C. was supported by a Rutherford Discovery Fellowship from The Royal Society of New Zealand Te Apārangi (VUW-1701).