Temporal genomic monitoring of kelp reveals subtle temporal change despite exposure to an extreme marine heatwave in Western AustraliaExtreme Events

Jane Edgeloe¹, Thomas Wernberg^{1, 2}, Samuel Starko¹, Georgina Wood^{1, 3}, Antoine Minne¹, Sofie Vranken^{4, 1}, David Wheeler⁵

¹University of Western Australia, Western Australia - ²Norwegian Institute of Marine Research, Norway - ³Flinders University, Australia - ⁴Ghent University, Belgium - ⁵Orange Agricultural Institute, New South Wales

Extreme climatic events can have profound demographic and ecological impacts, yet their genetic consequences are less well understood, particularly in marine ecosystems. Marine heatwaves (MHWs) are one of the principal extreme climatic events that threaten marine species persistence and have had dramatic impacts on habitat forming species such as kelps. While some studies have empirically documented shifts in genetic diversity and population structure following MHWs, a lack of temporally replicated genetic data usually hinders proper assessment of MHW impacts. In this study, we leveraged rare historical samples to investigate temporal genetic changes in the kelp Ecklonia radiata (order Laminariales) at three locations in Western Australia over a 16-year period, encompassing a severe MHW in 2011. Despite the intensity of the MHW, overall genetic diversity and composition remained stable when comparing before (2006 and 2008) and after (2012, 2018, and 2022) the event, suggesting resilience in overall population genetic structure. However, subtle temporal changes were observed at some loci which may have experienced selection, including those associated with temperature, characterized by shifts in some allele frequencies and presence post-MHW. These changes were more pronounced at lower-latitude locations where the ecological impacts of the MHW were greatest, with diminishing evidence of selection at poleward locations. Our findings highlight that while MHWs may not always drastically alter overall genetic composition, they can drive locus-specific genetic shifts in response to extreme climatic stressors, reflecting selection or altered gene flow. This underscores the importance of temporal genetic monitoring to better understand population responses to environmental change.