Zooplankton grazing on bacterio-and phytoplankton was studied in the Gulf of Aqaba and the Northern Red Sea during Meteor Cruise Me 44-2 in February-March 1999. Protozoan grazing on bacterioplankton and autotrophic ultraplankton was studied by the Landry dilution method. Microzooplankton grazing on phytoplankton > 6 µm was studied by incubation experiments in the presence and absence of microzooplankton. Mesozooplankton grazing was studied by measuring per capita clearance rates of individual

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... oplankton with radioactively labelled food organisms and estimating in situ rates from abundance values. Protozoan grazing rates on heterotrophic bacteria and on algae < 6 µm were high (bacteria: 0.7 to 1.1 d -1 , ultraphytoplankton: 0.7 to 1.3 d -1 ), while grazing rates on Synechococcus spp. were surprisingly low and undetectable in some experiments. Mesozooplankton grazing was weak, cumulative grazing rates being ca. 2 orders of magnitude smaller than the grazing rates by protozoans. Among mesozooplankton, appendicularians specialised on smaller food items and calanoid copepods on larger ones. KEY WORDS: Phytoplankton · Protozoa · Bacteria · Zooplankton · Grazing · Red Sea · Gulf of Aqaba Resale or republication not permitted without written consent of the publisher