Phase-shift dynamics and resilience of temperate reef states to urban stressors

In recent decades there has been a concerning trend of loss of kelp beds to sea urchin 'barrens' as a result of overgrazing by urchins. These two habitat states have been extensively studied across many systems worldwide, however the underlying dynamics of the transition to barrens, the recovery to kelp, and mechanisms of resilience of either habitat configuration are not fully appreciated. This thesis studies the transitions of kelp beds to sea urchin barrens and vice versa in relation to the stressors typical of urbanised coastal areas, namely enhanced levels of nutrients and sediment. At several sites in Port Phillip Bay (Victoria, Australia) adjacent the city of Melbourne, the role of sea urchins (Heliocidaris erythrogramma) in influencing the abundance of kelp (Ecklonia radiata) and other macroalgae was examined by assessing behavioural traits of the urchins and characteristics of both urchins and kelp in experiments considering both the 'bottom-up' force of nutrients and the 'top-down' force of grazing, together with the effects of sedimentation. It is revealed that the sea urchin is a major driver of community structure on rocky reefs in Port Phillip Bay. Sediment enhancement has no direct effect on intact kelp beds, while nutrient enhancement can play a beneficial role in strengthening the resilience of kelp beds by stimulating growth of kelp, but grazing of urchins overwhelms the positive effect of the nutrients. Densities exceeding 4 urchins m\\(^{-2}\\) increase the risk of overgrazing with a definite phase-shift to urchin barrens at densities of 8 urchins m\\(^{-2}\\)or greater. Conversely, kelp bed recovery will only occur when urchin densities are below 4 m\\(^{-2}\\). Nutrient enhancement does not change the thresholds in either direction. Assessments of sea urchin behaviour reveal that the availability of drift-kelp alters urchin foraging behaviour, so that if sufficient drift material is available, urchins will not actively overgraze attached algae. Considering that neither enhancement of nutrients nor sediments changes the likelihood of a phase-shift, sea urchins should be the focal point of any effort to protect existing kelp beds or effect the recovery of kelp on urchin barrens. It is therefore essential to identify the tipping points in urchin biomass/density that lead to shifts in either direction, and how to influence urchin foraging behaviour to prevent overgrazing of attached algae.