Discharge of groundwater into the sea is widespread. Overlooking it may lead to serious misinterpretations of ecological data in studies of coastal pollution, of benthic zonation and productivity, and of the flux of dissolved substances within and between bottom sediments and overly~ng water. Data are presented indicating that in the Perth region of Western Australia, submarine groundwater discharge delivers several times as much nltrate to coastal waters a s does river runoff.

In the Abrolhos Islands, site of a high-latitude coral reef, the net rate of community calcification is high but growth of frame-building corals is reduced. Our studles suggest that the latitudinal limits of coral reef development are often set by competition of macroalgae with corals, and that macroalgae are favoured at these latitudes by high nutrient concentrations, moderate water temperatures (for macroalgae) and possibly by reduced grazing pressure, while coral growth is reduced by temperatures which, for reef corals, are low.

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Dissolved nutrient flux between a coral reef community and the overlying water displayed 3 d~fferent patterns: Type 1: Fluxes displayed no diel rhythm but were correlated with ambient dissolved nutrient concentrations. Type 2: F l u e s were not concentration-dependent and exhibited a diel rhythm, net uptake being hlghest for several hours around mid-day. Type 3: Fluxes displayed neither a measurable diel rhythm nor a correlation with dissolved nutrient concentrations. Our observations suggest that, as dissolved nutrient concentrations increase relative to community nutrient demand, nutrient uptake patterns will shift from Type 1 to Type 2 and subsequently to Type 3 in aquatic communities.