Data from: 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 Phyllospora
comosa 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 (23-23.5°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 Phyllospora. While some Phyllospora
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 Phyllospora forests into the future.