Collapse of kelp beds to turf-dominance is mediated by urchin overgrazing not water quality alone

Coastal water quality has declined worldwide due to increasing urbanisation of coastal environments and associated catchments. While many studies have examined singular urban stressors on the resilience of marine ecosystems, relatively few have examined interactions between such stressors on disturbance-recovery dynamics of kelp beds. We used multi-factorial manipulative experiments to simultaneously test the effects of sedimentation, nutrients, disturbance (clearance of canopy) and urchin grazing on kelp bed communities in Australia’s largest urbanized embayment (Port Phillip Bay, Victoria). Elevated nutrient solely, had negative indirect effects on cover of kelps, but positive direct effects on cover of turfing algae. However, nutrients and disturbance together, resulted in greater development of turfing algae at the expense of kelps and large fucoids. Furthermore, a transplant experiment clearly showed that reducing urchin density increased the survival and production of transplanted kelp at sites across a gradient of decreasing water quality. Following the urchin driven collapse of kelp beds, nutrification then favours turf-dominance which inhibits recovery of kelps. Increased sedimentation on its own and in combination with other stressors had no direct observable effect on turf or kelps, however the shift to turf-dominance with urchin enhancement was not as pronounced under elevated sediment loads.