Planktic foraminiferal production stimulated by chlorophyll redistributionand entrainment of nutrients

During September and October 1996 planktic foraminifers and pteropods were sampled from the upper 2500 m of the water column in the BIOTRANS area (47 degreesN, 20 degreesW), eastern North Atlantic, as part of the JGOFS progra m. Hydrography, chlorophyll fluorescence, and nutrient content were recorde d at high spatial and temporal resolution providing detailed information ab out the transition time between summer and fall. At the beginning of the cr uise a shallow pycnocline was present and oligotrophic conditions prevailed . Over the course of the cruise, the mixed layer depth increased and surfac e water temperature decreased by 1.5 degreesC. Both chlorophyll-a dispersed in the upper 50 m by vertical mixing and chlorophyll-a concentrations at t he sea surface increased. The nitracline shoaled and nutrient enriched wate rs were entrained into the mixed layer. Planktic foraminifers and pteropods closely reflected the changes in the hydrography by increased growth rates and changes in species composition. Three main groups of planktic foramini feral species were recognized: (1) a temperate and low-productivity group d ominated by Neogloboquadrina incompta characterized the shallow mixed layer depths. (2) A temperate and high-productivity group dominated by Globigeri na bulloides characterized the period with wind-induced dispersal of chloro phyll-a and entrainment of nutrient-enriched waters. (3) A warm water group containing Globigerinoides sacculifer, Orbulina universa, Globigerinoides ruber (white), and Globigerinella siphonifera was most common during the fi rst days of sampling. Synchronous with the hydrographic change from summer to fall, planktic foraminiferal and pteropod growth was stimulated by redis tribution of chlorophyll-a and entrainment of nutrient-enriched waters into the mixed layer. In addition, the seasonal change in the eastern North Atl antic resulted in a transition of the epipelagic faunal composition and an increased calcareous particle flux, which could be used to trace seasonalit y in fossil assemblages and allow for better paleoceanographic interpretati on of the boreal Atlantic. (C) 2000 Elsevier Science Ltd. All rights reserv ed.