Particle removal by coral reef communities: picoplankton is a major source of nitrogen

Removal and uptake of planktonic particulate organic matter by coral reef benthos is widely recognized as an important pathway for carbon and nutrients. We placed 3 natural assemblages of coral reef benthos, including 3 species of corals with associated sponges, ascidians, actinians, and bryozoans, in a long flume (24 x 0.4 x 0.3 m). Water was re-circulated at various speeds (5, 13, 22, and 32 cm s(-1)) over 6 h, and the disappearance of particles (pico-, nano-, microplankton and detrital particles) were measured using flow cytometry and microscopy. Control communities consisted of dead coral skeletons. Rates of removal of all particles were proportional to their concentrations. The first-order rate constant for the decrease in particle concentration ranged from 36 to 97 x 10(-6) m s(-1) (mean +/- SD = 63 +/- 16 x 10(-6) m s(-1)), with 71% of this variation explained by particle type. Water velocity had no significant effect on these rate constants. Living particles contributed 96% of the total nitrogen removal, with picoplankton (cells 0.2 to 2 mum) accounting for 92%. Overall, nitrogen removal from particles (8.8 to 10.3 mmol N m(-2) d(-1)) appears to be similar in magnitude to that of dissolved inorganic nitrogen; thus, picoplankton is a major source of nitrogen for these coral reef assemblages.