Top-down sea urchin overgrazing overwhelms bottom-up stimulation of kelp beds despite sediment enhancement

Biogenic marine habitats are threatened by a multitude of human stressors, and urbanised temperate coasts in particular are exposed to an intense and accumulating range of impacts. Here we use a 3-way factorial design to test the individual and combined effects of an approximate doubling of nutrients, sedimentation and sea urchin herbivory on rocky reef macroalgal bed structure over a 14-month field experiment within the urbanised setting of Australia's largest embayment (Port Phillip Bay, Victoria). Our aim was to examine whether these stressors would result in altered macroalgal bed structure (i.e. canopy and understorey components) when applied singly and in combination. The experiment was designed to evaluate the opposing influences of “top-down” (urchin overgrazing) and “bottom-up” (elevated nutrients) control of macroalgal bed structure in the presence and absence of elevated sedimentation. We show that kelp cover and abundance respond positively to nutrient enrichment (approximately twice ambient nutrient levels overall; with NH₃, NO_x and FRP being 2.6, 1.5 and 1.4 times higher respectively), but when combined with enhanced abundance of grazing sea urchins (at double ambient abundance), the local positive effect of nutrient enrichment was overwhelmed by the negative effect of increased herbivory. Fucoid cover showed no response to experimental treatments indicating greater resilience to urban stressors, however the trends in abundance of fucoids indicated the potential for positive nutrient and negative urchin effects manifesting beyond 14-months. Turf cover showed a very different trend, with no detectable response to nutrient enrichment, yet showed a positive response to urchin enhancement mediated by competitive release from kelp as a result of preferential urchin grazing of kelp. Enhanced sedimentation (twice ambient load), had no detectable effects on kelp, fucoids or other algal guilds, but appeared to negate the effect of urchin grazing when applied in combination with enhanced urchin abundance. Cover of non-turf understorey showed no response across all experimental treatments, indicating relatively high resilience of this guild. Collectively, our study demonstrates that ‘top-down’ effects of urchin grazing can overwhelm the positive ‘bottom-up’ effect of nutrient enhancement on kelp, and providing urchin abundance does not exceed the overgrazing threshold, that macroalgal community structure appears to demonstrate resilience to an approximate doubling in nutrient levels and/or sedimentation.