Bio-optical properties of the northern Red Sea and the Gulf of Eilat (Aqaba) during winter 1999

Underwater light field, chlorophyll-a concentration, phytoplankton absorption spectra, chlorophyll-a specific absorption coefficients, and phytoplankton diversity in the Gulf of Eilat (Aqaba) and northern Red Sea, were studied during a cruise on the RV 'Meteor' in winter - early spring, February-March 1999. The mean euphotic depth of the entire area was 86 ± 8 m. The vertical attenuation coefficient, K_d (PAR), averaged from the surface down to 100 m depth, was only 0.054 ± 0.006 m^{- 1}, as might be expected from the low chlorophyll-a concentrations (< 0.4 μg l^{- 1}). The spectral attenuation coefficients were similar in the entire area. The contribution of yellow substances to attenuation was negligible. Chlorophyll-a specific absorption coefficients for both the UV and PAR domains in the entire study area were representative of the dominant picophytoplankton. When mixing depth reached 600 m, on average each alga was exposed to 3% of surface light. Phytoplankton populations, as quantified by flow cytometry, were typical of oligotrophic oceans. In the Red Sea, the deep chlorophyll maximum developed at about 50-60 m, ∼ 1 × 10⁸ cells l^{- 1}, dominated by high concentrations of Prochlorococcus (∼ 75%), whereas in the Gulf of Eilat (Aqaba), ∼ 4 × 10⁷ cells l^{- 1}, eukaryotic algae (∼ 20%), cyanobacteria (Synechococcus) (∼ 50%), and Prochlorococcus (∼ 25%), were distributed throughout the water column. The cell size and pigment concentrations did not depend on depth distribution or geographic location. The northern Red Sea is permanently stratified, while at that time of year, the Gulf waters were deeply mixed. The results illustrate the common features of these two domains, while underscoring their differences. Even though they share the typical phytoplankton attributes of oligotrophic ocean provinces, mixing regimes still set them apart.