Blooms of benthic dinoflagellates of the genus Ostreopsis; an increasing and ecologically important phenomenon on temperate reefs in New Zealand and worldwide

Blooms of benthic dinoflagellates belonging to the tropical genus Ostreopsis are an increasingly common phenomenon in temperate regions worldwide. This is reflected in the rapid upsurge of publications on Ostreopsis from temperate regions since 2000. Relatively little is known about these blooms or their effects on other organisms. An unprecedented bloom of Ostreopsis siamensis occurred on shallow reefs in northern New Zealand in 2004 providing an opportunity to examine the dynamics of an O. siamensis bloom and its effect on community structuring sea urchins (Evechinus chloroticus). The bloom occurred following a period of calm sea conditions with warmer than average water temperatures. The cover of O. siamensis was highly ephemeral and strongly related to temporal and spatial variation in wave action. Blooms were most prevalent at sites protected from prevailing swells where O. siamensis covered 30–60% of the reef with the concentrations on macroalgae reaching 1.4 × 10⁶ cells g⁻¹ wet weight, some of the highest recorded worldwide. Surveys of the health of sea urchins in relation to the cover of O. siamensis suggested strong negative effects on this ecologically important herbivore and urchin densities declined by 56–60% at bloom sites over the study period. Further research is needed to examine the factors controlling the distribution and intensity of this new phenomenon, and into the ecological effects of such blooms on marine communities and the potential mechanisms responsible.