A comparative study of the bacterioplankton abundance and functional activity was carried out in July and August 1999 in Sevastopol Bay (SB; Black Sea, Ukraine), which is warm-temperate and under considerable anthropogenic influence, and in the coastal water of the shoreline near Aberystwyth (Ab; Cardigan Bay, Wales, UK) that is cold-temperate and relatively clean. The chosen index for the investigation was the cell-specific, instantaneous rate of heat production (scalar heat flux) because it reflects the kinetics and thermodynamics of metabolism. The measurement of the native samples to secure this index was the extensive heat flow rate using an improved microcalorimetric method. It was found that in the SB ecosystem, the average in situ bacterial abundance (A), biomass turnover rate (K), production (P) and cell-specific heat flux (H) were-significantly higher than at Ab, with a tendency for values to be more variable (2.13 x 10(6) +/- 1.30 x 10(6) cells cm(-3) (A), 0.05 +/- 0.02 h(-1) (K), 1.48 +/- 0.53 mg C m(-3) h(-1) (P), 34.51 +/- 23.5 fW per cell (H) in SB versus 0.96 x 10(6) +/- 0.15 x 10(6) (A), 0.015 (K), 0.25 (P), 22.31 +/- 5.84 (H) in Ab, in the same units). The enhanced bacterial activity was partly due to the higher temperature conditions in SB (24degreesC versus 17.7 degreesC in Ab). With the exception of the mean heat flux (19.3 fW per cell in SB versus 22.3 fW per cell in Ab), however, the SB data corrected to the average in situ temperature in Ab remained higher. The daily entropy production of the bacterioplankton communities, calculated on a volume-specific basis, was greater in the more eutrophic and polluted waters (16.0 J m(-3) K-1 per day in SB versus 6.6 J m(-3) K-1 per day in Ab and 17.0 J m(-3) K-1 per day at the polluted versus 15.0 J m(-3) K-1 per day at the unpolluted stations in SB, respectively).