Algal blooms commonly occur along the South African west coast. In March 1994 a dense bloom
developed within St Helena Bay. Its subsequent decay caused near-shore hypoxia and elevated hydrogen
sulphide levels, leading to it being termed a ‘black tide’. The bloom caused immediate massive intertidal
mortalities (95% reduction of biomass), significant changes in community structure, and declines in
richness and diversity. Only four taxa were not diminished: ephemeral algae, the gastropod Afrolittorina
knysnaensis, the anemone Bunodactis reynaudi and the false limpet Siphonaria capensis. This limpet
species subsequently increased at impacted sites relative to controls, probably reflecting reduced
competition from other grazers. Ephemeral algae flourished during early recolonisation while grazers
were depleted, but declined as grazers re-established. Manipulative experiments demonstrated that
grazers do have this capability. Perennial algae also flourished, but lagged behind ephemerals, possibly
contributing to their decline. Predators took longest to recover. Differences in the responses of species
reflect differing tolerances, zonation patterns and biological interactions. Communities at control sites
remained unchanged over the four-year study, and those exposed to a low intensity of the bloom were
little affected, but communities exposed to its full intensity failed to recover fully within four years.
Increasing frequency of harmful algal blooms along the west coast of South Africa could, therefore,
significantly alter rocky