Impact of light and water flow on the responses of coralline algae to ocean acidification

Coralline algae are globally abundant components of benthic habitats and play foundational roles in coastal ecosystems such as temperate kelp forests and coral or rocky reefs, where they are often the dominant substrate cover. The emerging understanding of the sensitivity of coralline algae to ocean acidification (OA) has emphasised the need to determine the natural variability of parameters that could influence growth and reproduction of coralline algae in situ to improve experimentation as well as predictions of climate change outcomes for this group. To identify spatiotemporal variability of seawater pH inside a Macrocystis pyrifera kelp forest, typical for southern New Zealand with high coralline algae cover, long term (> 1 year) pH, irradiance and temperature data were collected. Seawater pH was highly variable (pHT 7.45–8.49) and exhibited strong diel and seasonal differences that were associated with metabolic activity of the surrounding macroalgal community and linked to peaks of photosynthesis and respiration. These results show that coralline algae inhabit a highly variable environment, with occasional pH minima lower than end of the century predictions for the open ocean. Coralline recruitment peaked between late autumn and winter, when irradiance reaching the benthic communities was up to 85 % lower than during the summer months. Despite the ecological importance of coralline algae in low light environments and their vulnerability to OA, there is limited understanding of how the interplay between irradiance and seawater pH influences coralline growth and reproduction. To determine these impacts, a 212-day experiment exposed coralline communities growing on artificial substrates to two pH levels (pHT 8.05 – present-day/ pHT 7.65 – OA scenario) and a gradient of daily light dose (0.35, 0.17 and 0.1 mol m-2 d-1), based on the light environment typical at depth during the months of peak recruitment. Coralline growth was highest in the intermediate irradiance treatments and growth was reduced in response to lowered seawater pH with net dissolution occurring in the lowest light treatments, while communities in the pHT 8.05 conditions continued to grow. Furthermore, recruitment resulting from these communities in the OA treatment was reduced by more than half. The reduction in recruitment in response to OA was amplified under reduced irradiance, and recruitment was near zero in the lowest light treatment at pHT 7.65. The decline in growth and recruitment was attributed to limited capacity to counteract the energetically costly OA induced dissolution. This shows that coralline communities may be increasingly at risk from OA in habitats where irradiance is low, such as cryptic habitats or at depth, where they represent the main substrate cover. It has been proposed that habitats of reduced water flow may act as refuges for coralline algae, due to the possibility to metabolically increase pH in the immediate surrounding of the tissue when water flow is reduced. In a 220-day experiment the combined impacts of three flow rates and two seawater pH treatments (pHT 8.05 – present day/ pHT 7.65 – OA scenario) on the growth and recruitment of coralline communities were examined. Under the present-day pH scenario, flow treatments had little effect, while in the OA-scenario flow-dependent responses were evident. While growth of coralline communities was heavily reduced in the lowest flow treatment, coralline growth did not exhibit the same decrease in response to decreased seawater pH in the two higher flow treatments. Furthermore, lowered seawater pH and reduced flow synergistically decreased the number of recruits successfully establishing in the treatments. Reductions in flow exacerbated rather than ameliorate the effects of OA on coralline algae. These results show that irradiance and water flow can exert strong influences on coralline algae under OA, with high susceptibility of coralline recruitment.