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

Red Sea gravity currents cascade near-reef phytoplankton to the twilight zone

Helge Niemann1,4,*, Claudio Richter1, Henk M. Jonkers2, Mohammad I. Badran3

1Center for Tropical Marine Ecology, Fahrenheitstr. 6, 28359 Bremen, Germany
2Max Planck Institute for Marine Microbiology, Celsiusstr. 1, 28359 Bremen, Germany
3Marine Science Station, The University of Jordan and Yarmouk University, PO Box 195, Aqaba, Jordan
4Present address: Max Planck Institute for Marine Microbiology, Celsiusstr. 1, 28359 Bremen, Germany

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

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