The demise of intertidal southern bull kelp (Durvillaea spp.) following heatwaves, earthquakes and sediment stress – and the flourishing of alternative foundation speciesResistance, Resilience and Phase Shifts

Mads Thomsen¹

¹University of Canterbury, New Zealand

Southern bull kelp (Durvillaea spp.) are large long-lived seaweeds in the order fucales (rockweeds) that are competitively dominant on wave-exposed rocky coastlines in New Zealand. These foundation species modify environmental conditions, increase biodiversity and provide habitat for many ecologically and economic important species. Here I review recent impacts on bull kelp forests from the South Island of New Zealand following seismic uplift, heatwaves, sediment stress and interactions with other seaweeds, including invasive Japanese kelp (Undaria pinnatifida). In some places bull kelps are now locally extinct, in other places bull kelp have been decimated but survive in patches, and yet in some places healthy near-intact populations remains. In contrast to the many case studies that have demonstrated conversions of kelp forests to urchin barren or small ephemeral turf alga, lost bull kelp in NZ has largely been replaced by competitively inferior (alternative) foundation species, like mussels or relatively large perennial rockweed species. These successional trajectories imply more subtle changes to biodiversity, interaction networks, and ecosystem functioning compared to when kelp forests are converted to barrens and turf.