The response of microbial assemblages to instantaneous temperature change was measured in a seasonal study of the coastal waters of the western English Channel. On 18 occasions between November 1999 and December 2000, bacterial abundance was assessed and temperature responses determined from the incorporation of 3H leucine, measured in a temperature gradient from 5°C to 38°C. Q10 values varied, being close to 2 in spring and summer but were >3 in autumn. There was a seasonal pattern in the assemblage optimum temperature (Topt), which was out of phase with sea surface temperature. In July, highest 3H-leucine incorporation rates were measured at temperatures that were only 2.8°C greater than ambient sea surface temperature but in winter, Topt was ∼20°C higher than the ambient sea surface temperature. Sea surface temperatures for the adjacent English Channel and Celtic Sea for 1982–2014 have periodically been >3°C higher than climatological mean temperatures. This suggests that discrete periods of anomalously high temperatures might be close to, or exceed, temperatures at which maximum microbial assemblage activity occurs. The frequency and magnitude of marine heatwaves are likely to increase as a consequence of anthropogenic climate change and extreme temperatures may influence the role of bacterial assemblages in biogeochemical processes.