Heterotrophic bacterial turnover along the 20°W meridian between 59°N and 37°N in July 1996

Heterotrophic bacteria were enumerated by flow cytometry in samples from 200 m depth profiles along the 20°W meridian between 59°N and 37°N during June and July 1996. Bacterial volume estimates made by size fractionation were used in the determination of heterotrophic bacterial biomass. Concentrations of heterotrophic bacteria in the surface mixed layer were close to 106 ml−1 along most of the transect, but decreased to about 0.2×106 ml−1 at the southern end; this corresponded with a biomass range between about 2 and 15 mg C m−3. Concentrations also decreased to 0.1–0.3×106 ml−1 below the top mixed layer. Production of heterotrophic bacteria was measured in samples from eight profiles by following the simultaneous incorporation of isotopically labelled thymidine and leucine; in the mixed layer it ranged between a maximum of 1.5–2 mg C m−3 d−1 in the region of a frontal system near 50°N and 0.2–0.25 mg C m−3 d−1 at the southern end of the transect, with bacterial growth rates generally about 0.1 d−1. The daily production of heterotrophic bacteria integrated for the euphotic layer ranged between about 5 and 15% of the daily 14C primary production of phytoplankton. Heterotrophic nanoplankton biomass reflected the bacterial biomass and was about half as large; it showed a strong correlation with the production of heterotrophic bacteria. At the station with the highest bacterial biomass, the rate of grazing of bacteria by heterotrophic nanoplankton was 5–6% d−1, but at all other stations it was too low (<5% d−1) to be measured reliably by the techniques used. The data suggest that the nanoplankton grazed at least half of the bacterial production.