Abstract
Background: Ocean warming and marine heatwaves have contributed to a global decline in kelp canopy cover, threatening the persistence of important kelp forest ecosystems in many locations. Kelp populations at the warm edge of their range can become more resilient to extreme temperatures through local thermal adaptation, but such adaptation has not occurred in some regions. In this study, five populations of the giant kelp Macrocystis pyrifera from the northern and southern edges of the species’ range in Aotearoa New Zealand were exposed to simulated 22-day marine heatwaves. Juvenile sporophytes were exposed to either 18 °C or 22 °C heatwaves, or a 14 °C control. The experiment also assessed whether the long-term storage of gametophytes under red light, a technique that is commonly used to create germ banks, could affect sporophyte performance.
Results: It was found that the 22 °C heatwave treatment caused 64% mortality and a significant reduction in blade growth rates across all kelp cultures, whereas all cultures were comparatively resistant to the 18 °C treatment, with just 23% mortality during the heatwave. Chemical composition of sporophytes was altered in both heatwave treatments, with total carbon content, carbon:nitrogen ratio, and δ15N values increasing significantly. There was little evidence that long-term storage of kelp gametophytes under red light impacts sporophyte survival or growth. Although survival rates and chemical composition differed between some populations, there was no consistent evidence overall for significant differences in thermal tolerance between northern and southern M. pyrifera cultures.
Conclusions: Based on these findings, there is little evidence for local thermal adaptation among M. pyrifera sporophytes in New Zealand, and recent thermal history and acclimatisation may contribute more towards thermal tolerance than long-term adaptation. Genetic connectivity between kelp populations might also erode local adaptation. If marine heatwave intensities continue to increase as the oceans warm, the survival, growth, and recruitment of juvenile sporophytes are likely to be reduced in M. pyrifera populations throughout New Zealand, especially in regions such as Fiordland that are exposed to very high temperature anomalies and cumulative stress. We recommend that future studies of thermal tolerance in M. pyrifera focus on within-population variability in thermal resilience and assess the potential effectiveness of thermal priming for vulnerable populations.