Direct measurement of bacterial productivity in stratified waters close to a front in the Irish Sea

Abstract

Previous work on a frontal system in the Irlsh Sea has led to the hypothesis that bacterial productivity is greater in summer in waters above the thermocline than in waters below. A direct method for testing this was used by enclosing 3 pm pore size filtered water samples from above the thermocline (4 m) and below the thermocline (60 m) in dialysis bags and incubating them in situ. Cell numbers and size were monitored over 48 h, and production and generation times for the whole bacterial community calculated by regression. Bacterial productivity was 12.7 pgC 1 -l d-I at 4 m and 3.0 pgC 1-I d-' at 60 m with generation times of 0.9 and 2.0 d respectively. Waters above the thermocline were also characterised by high turnover of glucose, high heterotrophic indices and high rates of phytoplankton exudate uptake by organisms < l pm. All these rates were lower below the thermocline by a factor between 4.2 and 7.8. Dialysis bags were remarkably durable, no growth occurred on the bag surfaces after 48 h, reproducibility between bags was good, and there was no indication of substrate limitation in the dialysis bags when compared to similar incubations in glass bottles. This may be a useful tool to the microbial ecologist, particularily as diffusion across the dialysis membrane enables a chemical connection between the enclosed microbial community and the surrounding water and because of this it may be a useful alternative to enclosure in glass bottles.