Response of Early Life‐Stages of Forest‐Forming Seaweeds From Warm‐Edge and Central Populations to Marine Heatwaves

Marine heatwaves (MHWs) are among the most important threats to forest-forming seaweeds and the ecological functions and services they underpin. Responses of seaweeds to MHWs are likely to vary across life-stages, along their distribution and among populations with differing genetic structure. The effects of MHWs on early life-stages and genetically distinct populations have rarely been tested experimentally. We used germlings and juveniles of the forest-forming fucoid <i>Phyllospora comosa</i> from four genetically distinct populations, two at the warm-edge (31° S) and two at the centre of its latitudinal distribution (34° S) in eastern Australia to examine their responses (i.e., survival, growth, photosynthetic efficiency, condition) to MHWs. Two MHW scenarios were tested: strong (23°C-23.5°C) and extreme (26°C), which would occur ~100 days and ~50 days per year, respectively, by 2100 under the RCP4.5 scenario (SSP2-4.5). Survival under the extreme MHW was generally low for all populations and life-stages (< 20% for juveniles; 20%-30% for germlings). The extreme MHW also negatively affected growth, photosynthesis and condition (e.g., loss of structural integrity in juveniles) over time. In contrast, responses of germlings and juveniles to the strong MHW did not differ from the control for either population. However, germlings from the warm-edge population grew longer under the extreme and strong MHWs than those from the central population. Our results indicate only minor differences in responses to MHWs between genetically distinct populations of <i>Phyllospora</i>. While some <i>Phyllospora</i> individuals within each population appear able to resist strong MHWs, they are likely to be significantly affected by extreme MHWs that will be more common in the future. Given observed variation in responses among individuals within populations, finding, selecting, breeding and introducing resistant individuals (i.e., survivors to the extreme MHW) may allow increasing the resilience of <i>Phyllospora</i> forests into the future.