Bacterioplankton and phytoplankton biomass and production during summer stratification in the northwestern Mediterranean Sea

We examined bacterioplankton biomass and heterotrophic production (BHP) dur ing summer stratification in the northwestern Mediterranean in four success ive stratification seasons (June-July of 1993-1996). Values of phytoplankto n biomass and primary production were determined simultaneously so that the data sets for autotrophic and heterotrophic microbial plankton could be co mpared. Three standard stations were set along a transect from Barcelona to the channel between Mallorca and Menorca, representing coastally influence d shelf waters, frontal waters over the slope front, and open sea waters. C onversion factors from H-3-leucine incorporation to BHP were empirically de termined and varied between 0.29 and 3.25 kg C mol(-1). Bacterial biomass v alues were among the lowest found in any marine environment. BHP values (be tween 0.02 and 2.5 mu g C L-1 d(-1)) were larger than those of low nutrient low chlorophyll areas such as the Sargasso Sea and lower than those from h igh nutrient low chlorophyll areas such as the equatorial Pacific. Growth r ates of bacterioplankton were highest at the slope front (0.20 d(-1)) and l owest at the open sea station (0.04 d(-1)). Phytoplankton growth rates were similar at the three stations (similar to 0.50 d(-1)). Integrated values o f bacterioplankton biomass, BHP and bacterial growth rates did not show sig nificant differences among years, but differences between the three station s were clearly significant. Phytoplankton biomass, primary production, and phytoplankton growth rates did not show significant differences either with year or with station. As a consequence the bacterioplankton to phytoplankt on 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 simi lar to 0.70 at the other two stations. These ratios are similar to those fo und in other oligotrophic marine environments. The BHP/PP ratio was 0.83 at the coast, 0.36 at the slope and 0.09 at the open sea station. The last va lue is also similar to values found in other oligotrophic marine environmen ts. Vertical distribution of these ratios was also examined. The comparison of microbial parameters at the three stations indicates a di fferent kind of relationship between bacterioplankton and phytoplankton in oligotrophic open sea waters and in coastal, nutrient-richer waters. Accord ing to such parameters and to the values of the BB/BPHY and BHP/PP ratios, open waters in the northwestern Mediterranean (despite their relatively sho rt distance from the shore) were intermediate between the extremely oligotr ophic waters of the eastern Mediterranean or the Sarsasso Sea and the more productive waters of the equatorial Pacific. (C) 1999 Elsevier Science Ltd. All rights reserved.