Plankton of the Red Sea and the Arabian Gulf

Abstract

Though presenting some similarities, the Red Sea and the Arabian Gulf differ much in their configuration, hydrography and plankton populations. Both the recruitment of pelagic organisms into the Red Sea from the Gulf of Aden and their subsequent northward diffusion within the Red Sea basin, and the fluctuations in biomass and primary productivity are governed by the circulation pattern, which is itself dependent on the periodical Monsoon wind systems. The species diversity of Red Sea plankton is reduced relative to the Indian Ocean, but much higher than that of the Arabian Gulf. An indigenous assemblage of dinoflagellates, tintinnids, copepods and chaetognath species, however, appears to be well adapted to the conditions of this sea. Their wide distributions do not depend on the seasonal inflow. As a rule both the primary productivity and the zooplankton biomass are higher during the NE monsoon. The southern Red Sea is more productive than the northern with the two zones being separated by a low-productivity discontinuity zone at about 20° to 25°N, which is the zone of wind convergence in summer. More than 95% of the zooplankton biomass occurs in the upper 1000m and several endemic species have been reported. The Arabian Gulf is a vast, relatively shallow lagoon connected to the Gulf of Oman through the narrow strait of Hormuz. Circulation is anti-clockwise. Along the Iranian coast salinity rises from 36.6% near the strait to 40.6% in the NE Gulf and even higher along the Arabian coast. The Indian Ocean bathypelagic species as well as the dinoflagellate “Schattenarten” are completely missing from the Gulf. The species diversity is much poorer than that of either the Gulf of Oman or the Red Sea. An indigenous community, however, has developed which at first sight appears to be uniformly distributed especially along the Iranian coast, which is dominated by copepods and myodocopid ostracods. Some horizontal zonation is observable. Both nutrient and biomass concentrations are higher in mid-basin than in coastal waters. The Shatt-el-Arab to Kuwait area where estuarine components are not uncommon, and cladocerans replace ostracods in importance, is more productive and more diversified than the Trucial coast.