Damon Britton1, Cayne Layton1, Elizabeth Brewer2, Andrew T. Revill2, Juan Diego Gaitán-Espitía3, Craig N. Mundy1, Catriona L. Hurd1
1Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, TAS, Australia, 2 CSIRO Oceans and Atmosphere, Hobart, TAS, Australia, 3 The Swire Institute of Marine Science and School of Biological Sciences, University of Hong Kong, Hong Kong, China
Correspondence: Damon Britton, Damon.Britton@utas.edu.au
Identifying the responses of kelps to ocean warming and marine heatwaves requires an understanding of the point at which temperatures become stressful for a given population. Moreover, as warming is not occurring in isolation, understanding the influence of co-occurring future-ocean drivers such as elevated CO2 on thermal performance is needed to accurately project how populations will respond in a future ocean. In temperate Australia, Ecklonia radiata is the most widespread and abundant habitat forming kelp. E. radiata forms extensive forests and persists in seawater temperatures ranging from ~ 7 – 26 °C. E. radiata is susceptible to warming and marine heatwaves at the warm edge of its distribution, yet little is known about the thermal vulnerability of cool edge populations, and whether thermal performance is influenced by elevated CO2. To address this gap, we measured performance (growth, net photosynthesis) of E. radiata to eight temperatures (6- 29 °C) under both ambient and elevated CO2 (RCP 8.5 emissions scenario). Responses were used to construct thermal performance curves (TPCs) to identify thermal optimum and critical upper limits for growth and photosynthesis. Thermal optima were found to be considerably lower than those previously reported from warm edge populations and highlight the potential adaptation to local temperature regimes. TPCs were used to model annual performance under climate change and marine heatwave scenarios using historical SST data and future warming projections. Results suggest susceptibility to marine heatwaves is greatest in late summer and early autumn and this susceptibility will be exacerbated in a future ocean.