The microbial loop in the Red Sea. dynamics of pelagic bacteria and heterotrophic nanoflagellates

Growth and grazing experiments with free-liv~ng bacteria and nano-and microzooplankton were conducted in February-March 1987 in the central Red Sea and the Gulf of Aden using metabolic inhibitors. Results demonstrate the existence of a dynamic 'microbial loop' which dominates the pelagic system in terms of biomass and production. In the epipelagial, bacterial abundance ranged from 5.2 to 8.8 X 105 cells ml-l, and heterotrophic nanoflagellate (HNF) abundance from 0.6 to 1.2 X lo3 cells ml-l. Bacterial growth rates varied between 0 014 and 0.097 h-', grazing rates between 0.010 and 0.108 h-' Population grazlng rates corresponded to 2.0 to 5.4 x 104 bacteria consumed ml-' h-' Bacterial biomass, growth and grazing rates were higher in the central Red Sea than in the Gulf of Aden.