The study was conducted at Lough Hyne, a highly sheltered marine nature reserve in southwest Ireland (N 51°29′52′ W 9°17′46′), from 29 July to 16 September 2011. Owing to its biological and physical conditions, Lough Hyne is well suited to experimental work and is broadly representative of temperate, shallow-water Atlantic communities (see O'Gorman & Emmerson [38] and references therein). An experiment was established, with cages measuring 42 × 41 × 10 cm and a mesh width of approximately 7 mm. The cages were loaded with 5 kg of 1–2 cm gravel at the outset and situated in the shallow subtidal of a bay with weak current on the south shoreline of Lough Hyne. Two blocks were set up at a depth of 1 m and two blocks at a depth of 1.5 m at low tide (tidal range being approximately 1 m). The distance between any two cages was at least 2.5 m to reduce the likelihood of confounding factors. Two experimental factors were manipulated in a full-factorial design: (i) ‘bottom-up treatments’ in which fertilizer pellets were added to cages to yield an enriched nutrient level compared to cages with ambient levels; (ii) ‘top-down treatments’ in which the body mass of the top predator was altered by placing populations with small, medium and large average body mass into the cages, plus a control without predators. Each treatment was replicated four times in randomized blocks, yielding a total of 32 cages.