Turf proliferation depends on kelp loss and maintenance by sea urchin grazing

Temperate reef ecosystems are increasingly challenged by acute marine heatwaves and chronic overfishing which can lead to trophic cascades. While these disturbances differ in their impact and temporal expression, both can manifest as a collapse of kelp followed by proliferation of filamentous algal turfs. However, the importance of these disturbance types in turf proliferation remains unclear. Here, we investigate whether turf proliferation on Tasmanian reefs is driven primarily by acute kelp loss or chronic urchin grazing. Using standardised settlement tiles, we quantified short-term (2.5 mo) turf proliferation across 4 treatments: (1) intact kelp beds, (2) intact urchin barrens, (3) kelp beds with kelp removed, and (4) urchin barrens with urchins removed. Additionally, we tracked the longer-term (~12 mo) fate of these experimental patches to assess either kelp recovery or turf persistence. Turf proliferated in all treatments where kelp was absent or removed, with approximately 50-fold higher turf cover in disturbed kelp beds. This suggests that acute kelp loss is the predominant mechanism that allows turf proliferation in this system. However, the capacity of turf to persist was dependent on chronic urchin grazing. After 12 mo, our results revealed that at a site with lower urchin densities, kelp recovered within the patches with an associated decline in turf cover. At a site with higher urchin densities, kelp failed to recover, and turf continued to proliferate. Overall, when kelps are lost, competitive release allow turfs to establish, but their persistence appears dependent on chronically elevated levels of herbivory preventing kelp re-establishment.