Tolerance of three temperate macroalgal taxa to marine heatwaves of differing durations and intensities is not modulated by irradiance

ABSTRACTMarine heatwaves (MHWs) are recognized as principal drivers of ocean change. However, interactions with other important local and global drivers such as ocean acidification, eutrophication and sedimentation, and the physiological mechanisms via which MHWs act on species are poorly understood. Additionally, past studies have often focused on ecologically dominant taxa such as kelps, fucoids and coral, potentially leading to a lack of understanding of direct impacts on associated species. To this end, we examined the individual and interactive impacts of MHW duration (14 and 28 days) and intensity (20.5°C and 23°C) as well as irradiance (2.3 and 4.4 mol m–2 d–1) on standardized growth of Caulerpa brownii, Corallina berteroi and Phymatolithopsis complex, three temperate macroalgal taxa that are important spaceholders in their native ecosystem in Te Whanganui a Tara Wellington, Aotearoa New Zealand. All three taxa exhibited no declines in growth in response to the exposure to longer and more intense MHWs. We posit that the simulated MHW scenarios were tolerated due to large thermal safety margins which can be expected from the range centre/leading edge position of the sampled populations, even though some of these scenarios far exceeded the intensities encountered in this region previously. Interactions between intensity, duration and irradiance were limited to an antagonistic irradiance-duration interaction in the non-calcareous green macroalga Ca. brownii only. Together, the displayed tolerance of the study species to the simulated scenarios implies limited impacts of strong MHWs four weeks or shorter in duration on ecosystem functioning in New Zealand kelp forest systems if brown algal canopies remain intact.KEYWORDS: CaulerpaClimate changeCoralline algaeKelp forestMarine heatwave ACKNOWLEDGMENTSWe thank R. D’Archino for guiding the molecular identification of coralline algae. ECK and CEC were supported by a Rutherford Discovery Fellowship from The Royal Society of New Zealand Te Apārangi (no. RDF-VUW1701), Coastal People Southern Skies Centre of Research Excellence, and Victoria University of Wellington University Research Grant awarded to CEC.AUTHOR CONTRIBUTIONSECK, YSS, IMB, AT and CEC designed the research. ECK wrote the paper. YSS ran the experiment with assistance from all other authors. YSS and ECK conducted molecular species identification. ECK performed the statistical analysis. All authors edited the manuscript, or provided intellectual input, and agreed to its submission.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/10.1080/00318884.2023.2267411Additional informationFundingThis work was supported by the Victoria University of Wellington University Research Grant; Coastal People Southern Skies Centre of Research Excellence; Royal Society of New Zealand Te Apārangi [RDF-VUW1701].