PHYTOPLANKTON DISTRIBUTION AND GRAZING NEAR CORAL-REEFS

Depletion of phytoplankton cells and pigments over coral reefs was stu died in the Gulf of Aqaba, Red Sea, during 1994-1996. Phytoplankton ab undance and chlorophyll (Chl) a concentrations were 15-65% lower near the reefs than in the adjacent open waters. The decrease in chlorophyl l. near the reef was typically associated with an increase in the conc entration of its degradation products, the pheopigments. The steepest slope of these cross-shore gradients occurred within 1-3 m above botto m. More than 50% of the variation in the extent of the chlorophyll gra dients, but not of pheopigments, could be explained by the advection o f 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 s andy-bottom site without reef. Eukaryotic phytoplankton (<5 mu m) cont ributed >70% of; the total depleted carbon near the reef during winter , while the cyanobacterium Synechococcus (1 mu m) contributed the larg est share in summer. The proportions of different taxa in depleted fra ctions were similar to those in ambient waters, indicating no size sel ectivity. 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 s trongly depleted of phytoplankton (10 to >36%, or 32 to >100 ng Chl a liter(-1)). When converted to carbon fluxes, these rates greatly excee ded reported values of carbon input to coral reefs via zooplankton pre dation. Phytoplankton grazing is an important component of benthic-pel agic coupling in coral reefs.