Diel ammonium fluxes of two caribbean reef corals and two macroalgae under low nutrient conditions

Two hermatypic south Florida corals, Dichocoenia stokesii and Porites Porites, and two algae, Dictyota cervicornis and Halimeda incrassata were used to study diel ammonium fluxes under non- and mildly enriched ambient conditions. A sensitive fluorometric technique was used to rapidly and accurately measure nanomolar concentrations of ammonium at ecologically relevant concentrations from 25-2,250 nM. Previous studies typically monitored uptake from enriched ambient mediums (5,000-100,000 nM). A unique response from the hermatypic coral Porites porites was a net, positive ammonium flux (3.54 nmol NH4 cm-2 h-1). Otherwise, the other three ammonium sink species generated net negative fluxes from -0.29 ± -1.09 nmol NH4 cm-2 h-1. Ammonium depletion trends showed that over time D. stokesii, H. incrassata, and D. cervicornis took up ammonium until the ambient medium became limiting and uptake rates approached zero at the lowest measured concentrations of 76, 45, and 26 nM, respectively. There was inter- and intraspecific variation of ammonium uptake rates from various initial ammonium concentrations (Pm and KS) were calculated for sink organisms and found to be highly variable between individuals in a species. Half-saturation constant (KS) ranges values for D. cervicornis and H. incrassata were near or below literature values for other tropical algae, while KS value ranges for D. stokesii were slightly higher than values reported for other hermatypic corals. D. stokesii, H. incrassata, and D. cervicornis had KS value ranges of 2,919-18,656 nM, 413-208,333 nM, and 508-2,215 nM, respectively.