Projecting kelp (Ecklonia radiata) gametophyte thermal adaptation and persistence under climate change

center dot Background and aims Kelp forests underpin temperate marine ecosystems but are declining due to ocean warming, causing loss of associated ecosystem services. Projections suggest significant future decline but often only consider the persistence of adult sporophytes. Kelps have a biphasic life cycle, and the haploid gametophyte can be more thermally tolerant than the sporophyte. Therefore, projections may be altered when considering the thermal tolerance of gametophytes. center dot Methods We undertook thermal tolerance experiments to quantify the effect of temperature on gametophyte survival, relative growth rate (RGR) and sex ratio for three genetically distinct populations of Ecklonia radiata gametophytes from comparatively high, mid-and low latitudes (43 degrees, 33 degrees and 30 degrees S). We then used these data to project the likely consequences of climate-induced thermal change on gametophyte persistence and performance across its eastern Australian range, using generalized additive and linear models. center dot Key results All populations were adapted to local temperatures and their thermal maximum was 2-3 degrees C above current maximum in situ temperatures. The lowest latitude population was most thermally tolerant (similar to 70 % survival up to 27 degrees C), while survival and RGR decreased beyond 25.5 and 20.5 degrees C for the mid-and low-latitude popu-lations, respectively. Sex ratios were skewed towards females with increased temperature in the low-and high -latitude populations. Spatially explicit model projections under future ocean warming (2050-centred) revealed a minimal decline in survival (0-30 %) across populations, relative to present-day predictions. RGRs were also projected to decline minimally (0-2 % d(-1)). center dot Conclusions Our results contrast with projections for the sporophyte stage of E. radiata, which suggest a 257-km range contraction concurrent with loss of the low-latitude population by 2100. Thermal adaptation in E. radiata gametophytes suggests this life stage is likely resilient to future ocean warming and is unlikely to be a bottleneck for the future persistence of kelp.