Contrasting influence of sea urchins on attached and drift macroalgae

Sea urchins can exert a profound influence on temperate reefs, but the extent of their influence also varies considerably among geographic locations. Here, we describe the influence of the sea urchin Heliocidaris erythrogramma on attached macroalgae and drift macrophytes at a subtidal rocky reef in south-western Australia. In one experiment, we manipulated the density of H. crythrogramma in fenced plots; in a second, we manipulated the presence and absence of H. erythrogramma in unfenced plots. The manipulation of H. erythrogramma density did not result in changes in attached macroalgal biomass, but the treatment with the highest density of sea urchins had a greater drift macrophyte biomass. When H. erythrogramma was removed from unfenced plots, a slightly higher macroalgal biomass was observed in the austral summer, but this difference was not present a few months later. However, the drift biomass was 4 times higher in plots where sea urchins were not removed. The dominant macrophyte species in the drift were scarce or absent from the assemblage growing attached to the reef, suggesting that much of the drift originated elsewhere, either in adjacent seagrass beds or on reefs hundreds of metres to kilometres away. There was a disproportionate retention of drift kelp Ecklonia radiata by H. erythrogramma. Based on these results, we propose that spatial subsidies in the form of drift macroalgae result in a weak influence of H. erythrogramma on attached macroalgae.