Spatial distribution of ultraphytoplankton in the North East Atlantic (POMME study area) during Spring 2001

Inès HAMDI, Melilotus THYSSEN, Guy CANIAUX, Michel Jean DENIS

Abstract


Ultraphytoplankton (< 10 µm) abundances were determined during the spring cruise (23 March–13 April 2001) of the Programme Océan Multidisciplinaire Méso Echelle (POMME) in the Northeastern Atlantic between 39.0°–44.5°N and 16.6°–20.6°W. Sampling covered a grid of 81 stations regularly spaced. Four ultraphytoplankton cell groups, namely: Prochlorococcus, Synechococcus, picoeukaryotes and nanoeukaryotes, were resolved by flow cytometry on the basis of their fluorescence and scatter properties. The study area was divided into 6 zones as in Thyssen et al (2005) to account for latitudinal and longitudinal influences and to reduce the lack of synopticity induced by the sampling duration. The ultraphytoplankton vertical distribution was investigated down to 300 m depth in each zone. Results show that the Spring bloom was still ongoing over the study area, well advanced in the south, approximately half way in the north, indicating a mesotrophic regime. In each zone, the very large variability of average nitrate concentration within the 200-500 m layer may result from the impact of hydrodynamic structures, mixing nitrate-rich deep waters with not only partially nitrate-depleted surface waters, but also newly formed mode waters.
The Spring ultraphytoplankton distribution bore the fingerprint of the geostrophic current field and the mesoscale features identified in the study area, i.e., cyclonic and anticyclonic eddies and frontal structures.

Keywords


Ultraphytoplankton, flow cytometry, North Atlantic, eddies, POMME programme

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