Sponges and ascidians control removal of particulate organic nitrogen from coral reef water

We studied removal rates of plankton and total particulate organic nitrogen (PON) by benthic reef communities from the overlying water in a large experimental flume. The flume was filled with mixtures of coral and coral rubble, and biomass of plankton was measured as water was recirculated over the experimental benthic community. All planktonic particle types, picoplankton, nanoplankton, microplankton, and total PON, decreased in concentration at rates proportional to their biomass. The mean first-order rate constant for the decrease in particle concentration was 96 6 61 3 1026 m s21, corresponding to PON uptake of 10 mmol N m22 d21. Synechococcus sp. and heterotrophic bacteria were the major sources of PON. Particulate organic nitrogen removed by rubble and live coral assemblages was directly related to sponge and ascidian biomass (number and area) on the coral and coral rubble. Uptake of PON was about the same as the previously measured uptake of dissolved inorganic nitrogen into these coral reef communities, making it an important flux of nitrogen into the reef. Particle feeding by coral reef benthos is considered an important pathway for carbon and nutrients in coral reef ecosystems. Coral reef benthos captures zooplankton and large diatoms (Glynn 1973; Sebens et al. 1996). More recently, picoplankton and bacteria are considered significant sources of carbon and nutrients: (1) Buss and Jackson (1979) demonstrated ‘‘reef-fouling’’ communities remove picoplankton; (2) Moriarty et al. (1985) argued there must be intense grazing of bacteria by reef communities to remove the rapidly 1 To whom correspondence should be addressed. Present address: Institut de Ciències del Mar, Centre Mediterrani d’Investigacions Marines i Ambientals, Consejo Superior de Investigaciones Cientı́ficas, Pg. Marı́tim 37-49, 08003 Barcelona, Spain (mribes@ cmima.csic.es).