PRIMARY RESEARCH PAPER Phytoplankton distribution and production along a wide environmental gradient in the South-West Atlantic off Uruguay Danilo Calliari Æ Ernesto Brugnoli Æ Graciela Ferrari Æ Denise Vizziano Received: 7 April 2008 / Revised: 18 September 2008 / Accepted: 20 September 2008 / Published online: 4 October 2008 Ó Springer Science+Business Media B.V. 2008 Abstract We investigated phytoplankton biomass, assemblage structure and production along an environ- mental 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 Rı ´o de la Plata was sampled and routine measurements included CTD profiles, nutrients, chlorophyll-a, phytoplankton com- position and abundance, seston and organic matter loads, downwelling light and, at selected stations, production versus irradiance experiments. Spatial dif- ferences in abiotic variables suggested distinct hydro- graphic 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. Produc- tivity 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 environ- mental gradient and correlated to abiotic variables defining geographic zones. Overall results were con- sistent 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. Keywords Phytoplankton  Chlorophyll  Primary production  Southwest Atlantic  Rı ´o de la Plata  Uruguay Introduction Production and fate of organic matter in marine eco- systems are modulated to a large extent by physical Handling editor: Luigi Naselli-Flores D. Calliari (&)  E. Brugnoli  D. Vizziano Seccio ´n Oceanologı ´a, Facultad de Ciencias, Universidad de la Repu ´blica, Igua ´ 4225, CP 11400 Montevideo, Uruguay e-mail: dcalliar@fcien.edu.uy; danilo.calliari@marecol.gu.se D. Calliari Sven Love ´n Centre for Marine Research-Kristineberg, Marine Ecology Department, Go ¨teborgs University, Kristineberg 566, 450 34 Fiskeba ¨ckskil, Sweden G. Ferrari Direccio ´n Nacional de Recursos Acua ´ticos, Constituyente 1497, CP 11200 Montevideo, Uruguay 123 Hydrobiologia (2009) 620:47–61 DOI 10.1007/s10750-008-9614-7