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Data describing post-recruit (adult) fish assemblages and the abundance of recruiting cryptobenthic species associated with 28 artificial reefs of different sizes and supporting different densities of transplanted kelp (Ecklonia radiata) off the west coast of Maria island. Adult assemblages were assessed using diver-based visual surveys conducted at three times (early: spring 2015, middle: autumn: 2016, late: spring 2016). At each time, 2 surveys were conducted, and the results were averaged. Recruiting cryptobenthic fishes were assessed using SMURF (standard Monitoring Units for the Recruitment of Fishes) collectors that were deployed for a six-week period in the centre of each reef on four consecutive occasions (November 2015 to March 2016).
Shifts from productive kelp beds to impoverished sea urchin barrens occur globally and represent a wholesale change to the ecology of sub-tidal temperate reefs. Although the theory of shifts between alternative stable states is well advanced, there are few field studies detailing the dynamics of these kinds of transitions. In this study, sea urchin herbivory (a ‘top-down’ driver of ecosystems) was manipulated over 12 months to estimate (1) the sea urchin density at which kelp beds collapse to sea urchin barrens, and (2) the minimum sea urchin density required to maintain urchin barrens on experimental reefs in the urbanised Port Phillip Bay, Australia. In parallel, the role of one of the ‘bottom-up’ drivers of ecosystem structure was examined by (3) manipulating local nutrient levels and thus attempting to alter primary production on the experimental reefs. It was found that densities of 8 or more urchins m-2 (≥ 427 g m-2 biomass) lead to complete overgrazing of kelp beds while kelp bed recovery occurred when densities were reduced to ≤ 4 urchins m-2 (≤ 213 g m-2 biomass). This experiment provided further insight into the dynamics of transition between urchin barrens and kelp beds by exploring possible tipping-points which in this system can be found between 4 and 8 urchins m-2 (213 and 427 g m-2 respectively). Local enhancement of nutrient loading did not change the urchin density required for overgrazing or kelp bed recovery, as algal growth was not affected by nutrient enhancement.