The Vertical Distribution of Zooplankton in Relation to Habitat Zones in the Area of the Atlantis II Deep, Central Red Sea

Abstract

The vertical distribution of mesozooplankton was studied in the central Red Sea to determine its relationship to discrete habitat zones defined according to density, O2 and nutrient concentrations, seston, and phytoplankton profiles. A disproportionately large amount of the 300 .mu.m net plankton occurs both day and night in the epipelagic zone, which occupies the upper water layer above the abrupt thermohalocline at .apprx. 100 m. In the lower bathypelagic zone, a layer between 1050 and .gtoreq. 1850 m, the numerical abundance and biomass of the zooplankton (0.2 specimens and .ltoreq. 0.05 mg wet weight m-3) correspond to values encountered in other oligotrophic regions in tropical oceans below a depth of 4000-6000 m. The subsurface zooplankton maximum is situated in the core of the O2 minimum layer, which is located in the mesopelagic zone (100-750 m). Minimum O2 values at about 400 m delimit the upper and lower interzonal (mesopelagic) assemblages. The unusual plankton distribution, the predominance of a few species and the ontogenetic segregation of the interzonal species are related to the abnormally high temperature (.gtoreq. 21.5.degree. C) and salinity (.gtoreq. 40.5.permill. S) in the subsurface waters, coupled with a depleted O2 content in the mesopelagic zone and lack of food in the bathypelagic zone. Vertical transport of organic matter is achieved by the ladder of migrations involving a few interzonal species that descend into the upper bathypelagic zone but remain above about 1100 m. Transport of organic debris below this depth by sinking is ineffective because its decay is too rapid.