MEPS 269:91-99 (2004)  -  doi:10.3354/meps269091

ABSTRACT: Cross-reef variations of sea water density, currents and phytoplankton pigments were investigated in the northern Gulf of Aqaba, Red Sea. Differential cooling of near- and offshore surface water during cold winter nights results in cross-shore gradients of density (σ-t) triggering gravity currents which cascade downslope along sandy valleys. Two 4-wk deployments of 3-D acoustic current meters showed that cross-shore currents near the sea bed (35 and 90 m) were 2- to 5-fold higher than long-shore currents, inversely related to surface flow and highly correlated with seasonally detrended diel variations in seawater temperature. Cross-shore flow varied during the course of the night, with offshore pulses of up to 15 cm s^-1 associated with sudden temperature drops of ~0.15°C. Spatial distribution of chl a pigments shows that phytoplankton enriched nearshore waters are entrained in the gravity currents to depths >300 m. Gravity currents may therefore provide an important and previously overlooked pathway of reef-borne material to the food-impoverished deeper strata of the Red Sea.

KEY WORDS: Gulf of Aqaba · Red Sea · Gravity currents · Density cascading · Advection · Phytoplankton · Chl a · Coral reef ecology