RESUMO
BACKGROUND AND AIMS: Marine heatwaves (MHWs) are widely-recognised as pervasive drivers of ecosystem change, yet our understanding of how different MHW properties mediate ecological responses remains largely unexplored. Understanding MHW impacts on foundation species is particularly important, given their structural role in communities and ecosystems. METHODS: We simulated a series of realistic MHWs with different levels of intensity (Control: 14°C, Moderate: 18°C, and Extreme: 22°C) and duration (14 or 28 days) and examined responses of two habitat-forming kelp species in southwest United Kingdom (UK). Here, Laminaria digitata reaches its trailing edge and is undergoing a range contraction, whereas L. ochroleuca reaches its leading edge and is undergoing a range expansion. KEY RESULTS: For both species, sub-lethal stress responses induced by moderate intensity MHWs were exacerbated by longer duration. Extreme intensity MHWs caused dramatic declines in growth and photosynthetic performance, and elevated bleaching, which were again exacerbated by longer MHW duration. Stress responses were most pronounced in L. ochroleuca, where almost complete tissue necrosis was observed by the end of the long duration MHW. This was unexpected due to the greater thermal safety margins assumed with leading edge populations. It is likely that prolonged exposure to sub-lethal thermal stress exceeded a physiological tipping point for L. ochroleuca, presumably due to depletion of internal reserves. CONCLUSIONS: Overall, our study showed that exposure to MHW profiles projected to occur the region in the coming decades can have significant deleterious effects on foundation kelp species, regardless of their thermal affinities and location within respective latitudinal ranges, which would likely have consequences for entire communities and ecosystems.
