Bull kelp (Durvillaea amatheiae) in southeastern Australia has compromised adaptive capacity to future ocean warming.

Temperate Australian kelp forests are highly vulnerable to range contractions because of ongoing ocean warming and extremes. The current mainland distribution of the bull kelp Durvillaea amatheiae is confined to the far southeastern corner of the continent where warming is occurring two to four times faster than the global average. Extant populations will, thus, require sufficient genetic adaptive capacity to survive these extremes. We examined neutral and adaptive genomic diversity to assess the adaptive capacity and selective pressure on these range edge populations. We have revealed that prevailing ocean currents and dispersal-restrictive life-history traits have resulted in isolated, inbred populations across the southeastern range, particularly at the northern edge. Temperature-related candidate loci exhibited signatures of selection along thermal gradients, with many of these alleles fixed across the species range, suggesting little remaining adaptive capacity, particularly among the warmest populations. Combined, these elements suggest D. amatheiae may be susceptible to range contractions in the face of ongoing climate change.