Sea urchin control of macroalgal communities across a productivity gradient

Opposing bottom-up ‘resource-driven’ and top-down ‘consumer-driven’ forces interact to shape the structure of ecosystems. While these counteracting forces are well recognised, debate remains regarding which is more influential across space and time. Here we explore bottom-up versus top-down control of macroalgal communities for temperate rocky reef communities in highly urbanised Port Phillip Bay (PPB), southeast Australia. Field surveys show macroalgal cover to paradoxically decline with increasing ‘bottom-up’ nutrient inputs while the abundance of grazing sea urchins (Heliocidaris erythrogramma) increased. The mechanisms underpinning this pattern were examined by constructing urchin-exclusion plots using octocoral (Erythropodium hicksoni) colonies that grow on urchin barren grounds and form natural barriers to grazing sea urchins. Octocoral plots were constructed by cutting 200 mm by 200 mm squares to expose bare reef substratum within the centre of octocoral colonies, which enabled efficient replication of urchin-exclusions on barren grounds across three distinct zones of anthropogenic nutrient input in PPB. Octocoral plots successfully excluded urchins across zones and, in the absence of grazing, macroalgal production increased with increasing nutrient concentration as expected. This novel opportunity to efficiently replicate urchin-exclusions on high-density barren grounds across different zones of ‘bottom-up’ forcing demonstrates that urchin overgrazing can keep pace with and overwhelm increasing macroalgal productivity. Our findings also highlight that impacts of grazing can be greatest where bottom-up forces enable large abundances of herbivores to accumulate, which is counter to perceptions that impacts of herbivores will be greatest where macroalgal productivity is low.