Long term phosphate (2 µM) and nitrogen (20 µM urea + ammonium) enrichment of a patch reef at One Tree Island, Great Barrier Reef, caused >50% suppression of reef calcification. This is attributed primarily to the phosphate. It is suggested that this effect is involved, together with algal competition and the more usually accepted depression of temperature, in reducing the growth rate of reefs adjacent to upwellings. It is possible that the effect was more general during the first half of the Holocene transgression.