Genomic changes revealed by repeated sampling of kelp exposed to an extreme marine heatwave

Extreme climatic events are increasingly recognised as potent evolutionary forces, imposing sudden pressures on natural populations. Among these, marine heatwaves (MHWs) are major drivers of ecological change, causing mass mortality, range shifts, and community reorganisation, particularly in habitat-forming kelps. While their ecological impacts are well documented, the genomic consequences remain less clear. Specifically, MHWs can not only induce demographic bottlenecks but also potentially drive selection on loci associated with thermal tolerance, shifting allele frequencies in affected populations. Here, we leveraged historical samples to investigate temporal genetic changes in the kelp Ecklonia radiata (order Laminariales) at three locations in Western Australia over 16 years, spanning the severe 2011 MHW. Overall genetic diversity and composition remained largely stable before (2006, 2008) and after (2012, 2018, 2022) the event, likely reflecting population resilience. However, a subset of loci, including known temperature-associated loci, showed shifts in allele frequencies following the event. Moreover, the strongest and most numerous changes occurred at warmer, lower-latitude sites where ecological impacts of the MHW were greatest, potentially consistent with MHW-driven natural selection. These results demonstrate that MHWs, while not dramatically altering genome-wide diversity, may alter allele frequences at specific loci linked to thermal response, highlighting their role as episodic yet powerful drivers of evolutionary change in marine foundation species.