* Corresponding author. Tel.: #34 93-221-6416; fax: #34 93-221-7340; e-mail: cpedros@icm.csic.es. Deep-Sea Research I 46 (1999) 985}1019 Bacterioplankton and phytoplankton biomass and production during summer strati"cation in the northwestern Mediterranean Sea Carlos Pedro H s-Alio H *, Juan-Isidro Caldero H n-Paz, Nu H ria Guixa-Boixereu, Marta Estrada, Josep M. Gasol Departament de Biologia Marina i Oceanograxa, Institut de Cie % ncies del Mar, CSIC, Passeig Joan de Borbo & s/n, E-08039 Barcelona, Spain Received 22 January 1998; received in revised form 26 May 1998; accepted 17 July 1998 Abstract We examined bacterioplankton biomass and heterotrophic production (BHP) during sum- mer strati"cation in the northwestern Mediterranean in four successive strati"cation seasons (June}July of 1993}1996). Values of phytoplankton biomass and primary production were determined simultaneously so that the data sets for autotrophic and heterotrophic microbial plankton could be compared. Three standard stations were set along a transect from Barcelona to the channel between Mallorca and Menorca, representing coastally in#uenced shelf waters, frontal waters over the slope front, and open sea waters. Conversion factors from H-leucine incorporation to BHP were empirically determined and varied between 0.29 and 3.25 kg C mol. Bacterial biomass values were among the lowest found in any marine environment. BHP values (between 0.02 and 2.5 gCL d) were larger than those of low nutrient low chlorophyll areas such as the Sargasso Sea and lower than those from high nutrient low chlorophyll areas such as the equatorial Paci"c. Growth rates of bacterioplankton were highest at the slope front (0.20 d) and lowest at the open sea station (0.04 d). Phytoplankton growth rates were similar at the three stations ( & 0.50 d). Integrated values of bacterioplank- ton biomass, BHP and bacterial growth rates did not show signi"cant di!erences among years, but di!erences between the three stations were clearly signi"cant. Phytoplankton biomass, primary production, and phytoplankton growth rates did not show signi"cant di!erences either with year or with station. As a consequence the bacterioplankton to phytoplankton biomass (BB/BPHY) and production (BHP/PP) ratios varied from the coastal to the open sea stations. The BB/BPHY ratio was 0.98 at the coast and & 0.70 at the other two stations. These ratios are similar to those found in other oligotrophic marine environments. The BHP/PP ratio was 0.83 0967-0637/99/$ } see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 3 7 ( 9 8 ) 0 0 1 0 6 - X