Kelp in hot water? Investigating the responses of Aotearoa New Zealand’s giant kelp (Macrocystis pyrifera) populations to simulated marine heatwavesExtreme Events

Imogen Bunting¹, Laura Bornemann Santamaria¹, Neill Barr², Sarah J. Bury², Matthew Desmond^{3, 4}, Christopher Hepburn^{3, 4}, Yun Yi Kok², Erik C. Krieger^{5, 1}, Duong Le^{3, 4}, Nathan J. Kenny^{3, 4}, Roberta D’Archino², Christopher E. Cornwall^{1, 4}

¹Victoria University of Wellington Te Herenga Waka, New Zealand, ²National Institute of Water and Atmosphere Research Taihoro Nukurangi, New Zealand, ³University of Otago, New Zealand, ⁴Coastal People Southern Skies Centre of Research Excellence, New Zealand, ⁵King Abdullah University of Science and Technology, Saudi Arabia

Kelp forests are highly biodiverse ecosystems that support culturally and commercially important fish species and contribute to numerous ecosystem services. However, kelp forests are in decline globally, with ocean warming identified as a key driver. Marine heatwaves, which are predicted to increase in intensity and frequency as the oceans warm, can cause substantial losses of kelp canopy as water temperatures exceed critical thermal thresholds for kelp. Juvenile sporophytes of the giant kelp (Macrocystis pyrifera) from across the species’ latitudinal range in Aotearoa New Zealand were exposed to simulated marine heatwaves in a series of laboratory experiments.
It was found that M. pyrifera sporophytes were resistant and resilient to temperatures of up to 20°C over a period of 21 to 42 days, demonstrated by high survivorship and minor, short-term reductions in growth. However, at higher temperatures, sporophytes had higher mortality rates, rapid blade erosion, and declines in pigment content and chlorophyll fluorescence (effective quantum yield), indicating photosynthetic impairment. Importantly, it was found that light limitation could exacerbate the impacts of marine heatwaves, especially under high water velocity, with some treatments experiencing 100% mortality in these conditions. This is a key concern in coastal habitats, where localised sediment pollution from modified catchments can reduce light availability. Additionally, there is little evidence for differences in thermal tolerance between M. pyrifera populations throughout New Zealand. These findings indicate that giant kelp populations throughout New Zealand are likely to be adversely impacted as marine heatwaves continue to increase in intensity, and this could be exacerbated by other stressors. We suggest that restoration programmes could improve the thermal resilience of M. pyrifera populations by selecting planting sites with optimal environmental conditions and managing local sediment pollution.