Phytoplankton distribution and grazing near coral reefs

Abstract

Depletion of phytoplankton cells and pigments over coral reefs was studied in the Gulf of Aqaba, Red Sea, during 1994–1996. Phytoplankton abundance and chlorophyll (Chl) a concentrations were 15–65% lower near the reefs than in the adjacent open waters. The decrease in chlorophyll near the reef was typically associated with an increase in the concentration of its degradation products, the pheopigments. The steepest slope of these cross‐shore gradients occurred within 1–3 m above bottom. More than 50% of the variation in the extent of the chlorophyll gradients, but not of pheopigments, could be explained by the advection of water during 2 h preceding the transect and by the concentration of Chl a in the open water. No cross‐shore gradients were observed at a sandy‐bottom site without reef. Eukaryotic phytoplankton (<5 pm) contributed >70% of the total depleted carbon near the reef during winter, while the cyanobacterium Synechococcus (1 pm) contributed the largest share in summer. The proportions of different taxa in depleted fractions were similar to those in ambient waters, indicating no size selectivity. Direct measurements of phytoplankton removal rates were made in water passing through a unique 5‐m‐long perforated reef, dominated by herbivorous soft corals. The waters downstream of that reef were strongly depleted of phytoplankton (10 to >36%, or 32 to >100 ng Chl a liter−1). When converted to carbon fluxes, these rates greatly exceeded reported values of carbon input to coral reefs via zooplankton predation. Phytoplankton grazing is an important component of benthic‐pelagic coupling in coral reefs.