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Kelp forests are productive and biodiverse ecosystems with high ecological, cultural, and economic importance. However, the high sensitivity of kelp to water temperature means that these ecosystems are vulnerable to marine heatwaves (MHWs), especially at the equatorward edge of their range. To date, few laboratory studies have compared the effects of MHWs of different durations or intensities on kelp, and it is difficult to determine these effects from naturally occurring MHWs in the field. We exposed juvenile sporophytes of the giant kelp Macrocystis pyrifera from Wellington, Aotearoa New Zealand to simulated MHWs three or six weeks in duration, at temperatures of 18°C, 20°C, and 22°C, corresponding to 2, 4, and 6°C above local mean summer temperatures. While all MHW treatments reduced mean kelp growth rates by over 30% relative to 16°C controls, the 22°C treatments had much more severe and wide-ranging effects, including rapid blade erosion, reduced chlorophyll fluorescence, tissue bleaching, increased δ13C values, andmortality. Nonetheless, sporophytes had some ability to recover from heat stress; within the 18°C treatment, mean relative growth rates neared or exceeded those within the control treatment within three weeks after MHWs concluded. These results support the findings of previous studies which indicate that M. pyrifera sporophytes experience a key physiological tipping point around 20°C. Additionally, our findings suggest that juvenile M. pyrifera from the Wellington population could be relatively resilient toMHWs if temperatures remain at sub-lethal levels. However, if averageMHWintensities and durations continue to increase over time, survival and recruitment of juvenile kelp could be adversely affected, thus threatening the long-term persistence of giant kelp forests near the warm edge of their range in New Zealand. [ABSTRACT FROM AUTHOR]

Marine 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. [ABSTRACT FROM AUTHOR]