Grazing during early spring in the Gulf of Aqaba and the Northern Red Sea

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 mum was studied by incubation experiments in the presence and absence of microzooplankton. Mesozooplankton grazing was studied by measuring per capita clearance rates of individual zooplankton with radioactively labelled food organisms and estimating in situ rates from abundance values. Protozoan grazing rates on heterotrophic bacteria and on algae < 6 mum 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.