Nanoplankton and protozoan microzooplankton during the JGOFS North Atlantic Bloom Experiment: 1989 and 1990

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Abstract
Complex mesoscale eddy interactions are characteristic of the North Atlantic, resulting in a mosaic of water masses with different physical, chemical and biological properties. Observations of protist assemblages during spring 1989 and 1990 in the vicinity of 47°N 18°W indicate that timing, composition, and further development of the spring bloom community are highly variable between years. During 1989 a microbial community, dominated by small photosynthetic nanoplankton and protist grazers, was observed after the main diatom bloom in the transition zone between two cyclonic eddies. This community was characterized by a high ratio of ‘protozoan’ to ‘phytoplankton’ carbon, and dominance of the microzooplankton by mixotrophic ciliates. A nanodiatom/prymnesiophyte bloom was observed to replace the typical ‘microdiatom’ bloom in a front between a cyclonic and anticyclonic eddy during 1990. After the demise of the diatoms, high standing stocks of nanophytoplankton 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 nanodinoflagellates were observed in 1990. The factors responsible for the development of these quite different microbial food-webs in two consecutive years and the consequences thereof for ecosystem function remain to be more fully explored.

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