Mechanisms of kelp bed resilience and recovery on urbanised coasts

Coastal water-quality has diminished worldwide due to increasing human development in coastal environments, in some areas resulting in loss and fragmentation of kelp bed habitat on temperate reefs. Despite increased recognition of degradation of kelp beds, there are limited studies relating the patterns to mechanisms of loss and resilience. We identified patterns of loss and fragmentation of kelp beds using archival aerial images of the heavily urbanised northern region of Port Phillip Bay (PPB) in Victoria. We then explored mechanisms underpinning patterns of kelp bed loss using multi-factorial manipulations to simultaneously test the single and combined effects of enhanced nutrients, sediments, disturbance (clearance of canopy), and urchin grazing. We show that sedimentation at double the ambient load had little impact on kelp bed communities, while elevated nutrients significantly reduced kelp cover by attracting increased sea urchin abundance, and increased cover of turfing-algae. When nutrients were elevated in combination with disturbance (manual removal of kelp), turfing-algae dominated the reef. When we examined urchin abundance in the different zones of kelp bed patches (edge, centre), we found urchin density was higher around the edge of the kelp beds. Thus, while the centre zone of the kelp patch conferred some resilience, as kelp patches get smaller, an increasing perimeter:area ratio results in accelerating decline of the kelp patch. Overall, intact kelp beds appear resilient to reduced water-quality per se (increased sediment, nutrients), but are vulnerable to nutrification once disturbed (chiefly due to urchin grazing) begins to remove kelp biomass, resulting in a shift to a turf-dominated state which significantly limits kelp recruitment. It is clear that overgrazing by sea urchins plays a key role in driving kelp loss in PPB, leading to proliferation of turfs. It follows that successful restoration of kelp beds within the highly urbanised environment of PPB is not currently limited by water-quality, however, restoration attempts must confront extreme abundances of overgrazing sea urchins if recovery of this key habitat is to occur. Given the scale of kelp loss in northern PPB, transplant of adult kelps as a source of spores is also a likely requisite for kelp bed restoration.