Phytoplankton distribution and production along a wide environmental gradient in the South-West Atlantic off Uruguay |
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Authors: | Danilo Calliari Ernesto Brugnoli Graciela Ferrari Denise Vizziano |
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Institution: | 1.Sección Oceanología, Facultad de Ciencias,Universidad de la República,Montevideo,Uruguay;2.Sven Lovén Centre for Marine Research-Kristineberg, Marine Ecology Department,G?teborgs University,Fiskeb?ckskil,Sweden;3.Dirección Nacional de Recursos Acuáticos,Montevideo,Uruguay |
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Abstract: | We investigated phytoplankton biomass, assemblage structure and production along an environmental gradient to evaluate if
chlorophyll-a (as proxy for biomass) and primary production peaked under conditions hypothesised to favour phytoplankton growth. During
Spring 2003, a wide area from shallow estuarine waters to the shelf slope off the Río de la Plata was sampled and routine
measurements included CTD profiles, nutrients, chlorophyll-a, phytoplankton composition and abundance, seston and organic matter loads, downwelling light and, at selected stations, production
versus irradiance experiments. Spatial differences in abiotic variables suggested distinct hydrographic zones that differed
in phytoplankton biomass and productivity. Chlorophyll-a was highest under estuarine influence and peaked at low salinity when strong stratification developed in the outer estuary,
and was minimum at the shelf break and slope. In that area, however, relatively high chlorophyll-a was associated to oceanographic fronts and to the occurrence of Sub Antarctic water within the photic depth range. Productivity
was maximum in shallow waters, but biomass-specific productivity peaked at the outer shelf in oceanographic fronts or in upwelled
Sub Antarctic waters. Over shelf and slope waters productivity and biomass were not tightly coupled, as indicated by situations
of high biomass and low productivity (Station 9), low biomass and high productivity (Station 10), or both high biomass and
productivity (Station 22). Ordination analysis of phytoplankton taxa suggested that assemblages changed gradually along the
environmental gradient and correlated to abiotic variables defining geographic zones. Overall results were consistent with
an interpretation that phytoplankton biomass and growth were modulated by light in estuarine and coastal waters, and by hydrographic
processes on the continental shelf and slope.
Handling editor: Luigi Naselli-Flores |
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Keywords: | Phytoplankton Chlorophyll Primary production Southwest Atlantic Río de la Plata Uruguay |
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