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Serial dllution experiments were conducted on JGOFS-North Atlantic cruise of RV 'Meteor' M36/2 at a 20" W transect in June and July 1996 to assess the role of m~crozooplankton grazing and nitrogen supply in controlling phytoplankton stocks in the subtropical and temperate northeast Atlantic. Rates of m~crozooplankton grazing ranged from 0. sources in nitrate-rich waters of the muted layer at 54" N and at the nitracline at 47" N, whereas nitrogen regeneration dominated at the nitrate-depleted surface waters at 47" N. However, In the deep chlorophyll maxlma at 33" N and 40" Y phytoplankton growth was primarily maintained by n~t r o g e n regeneration, although external nitrogen was sufficiently available. The recycling efficiency of the microb~al community was d e f~n e d as the ratio of regenerated growth yield to herbivorous grazing loss. Efficiencies of -100":1 under post-bloom situations indicated tight coupling of predation, nitrogen supply and phytoplankton growth. We suggest that n~icrozooplankton grazing has a high potential for nitrogen supply and biomass control of phytoplankton communities during summer in the temperate and subtropical northeast Atlantic.

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