Nutrient uptake by coral-reef microatolls

Abstract

We measured rates of ammonium and phosphate uptake into 12 microatolls at One Tree reef lagoon, Australia, on 14 occasions over a range of seasonal and meteorological conditions from 1993 to 1995. Nine of the microatolls were spiked with ammonia and/or phosphate every low tide (2.5–4 h) during low- (15-month) and high- (13-month) loading periods. The remaining three microatolls that were not spiked with nutrient served as reference conditions. Ammonium concentrations were elevated from an average background of 0.7 to 11 µM NH 4 during the low-loading period and 36 µM NH 4 during the high-loading period, resulting in a loading to the benthos of 3.5 and 18 mmol NH 4 m –2 low tide –1. Phosphate concentrations were elevated from an average background of 0.2 to 2.3 and 5.1 µM PO 4 during low- and high-loading periods, respectively, resulting in a loading of 0.66 and 3.9 mmol PO 4 m –2 low tide –1. Ammonium and phosphate concentrations decreased significantly over low-tides, and uptake rates were proportional to concentrations (first-order). The average uptake-rate constant, S (m s –1), for ammonium and phosphate did not differ between the two loading periods but was highly variable. Averaged over both loading periods, S for ammonium was 129±74×10 –6 m s –1and S for phosphate was 67±39×10 –6 m s –1. At background nutrient concentrations, estimated nutrient-uptake rates were 7.8 mmol NH 4 m –2 day –1 and 1.2 mmol PO 4 m –2 day –1. Excretion rates—calculated from the mean difference in uptake rates measured in reference and nutrient-enriched microatolls—were estimated to be 4.3 mmol NH 4 m –2 day –1 and 0.9 mmol PO 4 m –2 day –1. We reason and suggest that nutrient uptake rates in these microatolls were close to mass-transfer limited rates. We conclude that nutrient uptake into coral reefs can be highly dynamic, varying 10-fold spatially and temporally.