Nitrogen-limited primary productivity of coral reef algal turfs: potential contribution of ammonium excreted by Diadema antillarum

Abstract

turfs account for the majority of the considerable primary productivity exhibited by coral reefs. Algal turfs grazed by the sea urchin Diadema antillarum Philippi are 2 to 10 times more productive per unit chlorophyll a than turfs not grazed by D. antillamm. We investigated one of several hypotheses to account for this effect: primary productivity of algal turfs is nutrient-limited and excretion by D. antillarum provides a nitrogen supplement to algal turfs. Algal turfs on natural settling plates were fertilized with ammonium, phosphorus, or both, for 17 d on a backreef in St Croix, US Virgin Islands. Primary productivity per unit chlorophyll a was significantly greater in the ammonium treatment than in the control treatment, although algal biomass in each treatment was not significantly different from the control. Biomass-specific ammonium excretion by D. antdarum was correlated with sea urchin size and respiration rates and averaged 115 pg N ind.-' h-' Excretion was significantly greater d.uring the day than at night. In the field, ammonium concentrations were significantly higher under D. anuarurn than ambient concentrations, indicating that diss~pation of sea urchin excretions was not immediate or complete. Calculations of the nitrogen required for the observed primary productivity of algal turfs indicate that, prior to the mass mortality, up to 19% could have been supplied by D, antillarurn excretions