Temporal Variability of the Nutrient Environment on Davies Reef in the Central Great Barrier Reef, Australia

Abstract

Nutrient concentrations were measured at a fixed station in the lagoon of Davies Reef, the central Great Barrier Reef, Australia, monthly between January and December 1990. The day-to-day variation in nutrient concentration inside and outside the lagoon was determined in May. The average dissolved inorganic nitrogen (DIN) (sum of N03 , N02 , and NH4 ) concentration for the pooled data in each of 11 monthly observations ranged between 0.34 and 1.84 uM. There was only a two-fold variation in average P04 concentration. Average Si04 concentration showed a magnitude of variation similar to that of DIN concentration. DIN concentration was significantly higher in the lagoon than in the surrounding open water, suggesting that the significant DIN flux results from internal sources. There were no clear inside-outside differences in P04 and Si04 concentrations. N:P ratios in the lagoon were usually < 16: 1, suggesting the possibility of N limitation. However, the ratios became> 16: 1 under low wind conditions, when the exchange of lagoon water was restricted and the contribution of the internal DIN flux to the nutrient budget increased. Si was usually available in excess of the growth requirement of diatoms (Si: N > 1: 1), but occasional decreases in relative Si availability resulted in occurrence of a high number of dinoflagellates.