A schema of the nitrogen cycle off Port Hacking, New South Wales

Abstract

The Port Hacking 100 m Station represents a continental shelf situation where periodic enrichment occurs from below without the classical upwelling sequence, but with profound biological effects. The Station receives irregular influxes of low-salinity, high-nitrate slope water intruding onto the continental shelf. Vertical mixing distributes nitrate into the upper illuminated depths and an increase in algal growth ensues. The resulting algal population is generally confined to the layer above the pycnocline. It is probable that nutrient exhaustion limits the ultimate algal population which then becomes subject to attrition by grazing and sinking. The nitrogen introduced as nitrate in the slope water intrusions moves offshore as biological nitrogen. The latter remineralizes at subsurface depths giving an effectively closed nitrogen cycle.