NANOPLANKTON AND PROTOZOAN MICROZOOPLANKTON DURING THE JGOFS NORTH-ATLANTIC BLOOM EXPERIMENT - 1989 AND 1990

Complex mesoscale eddy interactions are characteristic of the North At lantic, resulting in a mosaic of water masses with different physical, chemical and biological properties. Observations of protist assemblag es during spring 1989 and 1990 in the vicinity of 47-degrees-N 18-degr ees-W indicate that timing, composition, and further development of th e spring bloom community are highly variable between years. During 198 9 a microbial community, dominated by small photosynthetic nanoplankto n and protist grazers, was observed after the main diatom bloom in the transition zone between two cyclonic eddies. This community was chara cterized by a high ratio of 'protozoan' to 'phytoplankton' carbon, and dominance of the microzooplankton by mixotrophic ciliates. A nanodiat om/prymnesiophyte bloom was observed to replace the typical 'microdiat om' bloom in a front between a cyclonic and anticyclonic eddy during 1 990. After the demise of the diatoms, high standing stocks of nanophyt oplankton persisted until early June. In this post-diatom-bloom period , the 'protozoan' biomass was lower and the 'nanophytoplankton' stocks higher than in 1989. Very high stocks of heterotrophic nanodinoflagel lates were observed in 1990. The factors responsible for the developme nt of these quite different microbial food-webs in two consecutive yea rs and the consequences thereof for ecosystem function remain to be mo re fully explored.