MEPS 189:93-104 (1999)  -  doi:10.3354/meps189093

ABSTRACT: Serial dilution 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 microzooplankton grazing and nitrogen supply in controlling phytoplankton stocks in the subtropical and temperate northeast Atlantic. Rates of microzooplankton grazing ranged from 0.08 d^-1 at 54° N to 0.53 d^-1 at 40° N and mean growth rates of phytoplankton ranged from 0.19 d^-1 at 54° N to 0.75 d^-1 at 40° N. Both rates were positively related to seawater temperature, whereas the apparent growth yield of phytoplankton declined with increasing temperature from 0.19 µg chl a dm^-3 d^-1 at 54° N to 0.01 µg chl a dm^-3 d^-1 at 33° N. Complete nitrogen saturation of phytoplankton growth indicated light or non-nitrogenous limitation at the nitracline at 47° N and in the deep chlorophyll maximum at 33° N, whereas in the mixed layer at 47° N and 54° N the ambient nitrogen supply was sub-saturated and yielded 63 and 39% of nitrogen- saturated growth. Nitrogen supply of phytoplankton growth was dominated by external and cellular sources in nitrate-rich waters of the mixed 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 maxima at 33° N and 40° N phytoplankton growth was primarily maintained by nitrogen regeneration, although external nitrogen was sufficiently available. The recycling efficiency of the microbial community was defined as the ratio of regenerated growth yield to herbivorous grazing loss. Efficiencies of ~100% under post-bloom situations indicated tight coupling of predation, nitrogen supply and phytoplankton growth. We suggest that microzooplankton grazing has a high potential for nitrogen supply and biomass control of phytoplankton communities during summer in the temperate and subtropical northeast Atlantic.

KEY WORDS: Microbial food web · Nitrogen supply · Recycling efficiency · Northeast Atlantic