Plankton community response to sequential silicate and nitrate depletion during the 1989 North Atlantic spring bloom

Abstract The onset of the North Atlantic spring bloom is controlled by physical conditions and nutrient availability in the surface layer. The nutrient of interest is usually considered to be nitrate. In an intensive study of the 1989 bloom near 46°N, 18°W, dissolved silicate was rapidly depleted to a minimum before nitrate was depleted. This coincided with a shift in dominant phytoplankton from diatoms to small flagellates. The cyanobacteria, nanoplankton and some components of the microplankton community were followed over the course of two cruises through April and May. A true-color image analysis system, combined with epifluorescence microscopy, was used to yield rapid semi-automated measurements of cell abundances, population size spectra and trophic classification. Although few direct measurements of diatom abundance or biomass were made, the diatom bloom appeared rapidly and sank (or was grazed) over the course of 2–3 weeks. The diatoms were replaced by a community of abundant (10 4 ml −1 ), small (2–5 μm) phytoflagellates, presumed to be primarily prymnesiophytes, and chlorophyll levels remained high (2–4 μg l −1 ). Similarly abundant and small aplastidic (heterotrophic) flagellates coincided with the plastidic flagellates. Carbon standing stock budgets, as well as direct microscopic observations, indicated a large amount of detritus in the water column near the end of the diatom bloom that was not present 2 weeks later. The zooplankton community was dominated by protozoans and copepod nauplii, and mixotrophic ciliates increased significantly over the study period. The nutrient-controlled shift in phytoplankton community composition apparently reduced the flux of recently fixed carbon out of the surface and increased recycling and mixotrophy in the surface layer.