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Marine Ecology Progress Series

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MEPS 334:47-61 (2007)  -  doi:10.3354/meps334047

Phytoplankton growth conditions during autumn and winter in the Irminger Sea, North Atlantic

Ben A. Ward1,*, Joanna J. Waniek2

1National Oceanography Centre, Southampton, School of Ocean and Earth Science, University of Southampton, Waterfront Campus, European Way, Southampton SO14 3ZH, UK
2Institut für Ostseeforschung Warnemünde, Seestraße 15, 18119 Rostock, Germany

ABSTRACT: Physical and chemical properties of the water column, along with meteorological conditions were examined for their relationship with phytoplankton biomass in the Irminger Sea during late autumn and early winter. Data were collected during 2 cruises to the region in November and December 2001 and November 2002. Phytoplankton biomass was approximated by (chl a) concentrations within the water column. When examined during autumn and winter alone, the Irminger Sea was suitably described as one biogeochemical region responding to varying meteorological forcing. Hydrographic differences within the region were not observed to have a significant effect on phytoplankton growth during this period. Strong correlations with latitude were seen in chl a concentrations, physical conditions (including mixed layer depth) and meteorological forcing (including net heat flux). Variability in autumn/winter phytoplankton growth conditions appears to be driven by light limitation modulated by meteorological forcing. The temporal and spatial scales of locations sampled in 2001 represent a progression in the physical and biological conditions from late autumn to early winter. Along this ‘virtual transect’, a baseline value of approximately 0.1 mg m–3 is seen in the mean chl a concentrations within the mixed layer. We postulate that convection provides a mechanism for reduction of net losses of phytoplankton, by helping to keep phytoplankton within the mixed layer. Under such conditions, a deeper and therefore more accurate estimation of the critical depth would be valid. Evidence of the extended maintenance of phytoplankton within the mixed layer is presented in the form of the relative dominances of different phytoplankton groups.

KEY WORDS: Phytoplankton · Winter production · Physical forcing · Light limitation · Meteorology · Irminger Sea

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