12 pages, 5 figures, 4 tables.-- Full-text version available Open Access at: http://www.icm.csic.es/bio/projects/icmicrobis/pdf/Vaqueetal.09.pdfNarrow annual ranges of temperature characterize polar waters. Consequently, small increases in temperature could significantly affect the metabolic processes of marine microorganisms.ud We investigated the response of bacterial heterotrophic production (BHP) and grazing rates to small temperature changes in 3 zones near the western Antarctic Peninsula —Bransfield and Gerlache Straits, and Bellingshausen Sea— during December 2002. We performed 8 grazing experimentsud with water samples collected from depths where chlorophyll a (chl a) concentration was maximum,ud and incubated the samples at ambient temperature and at –1, 1, 2 and 5°C. We expected thatud grazing would increase in parallel with BHP at increasing temperatures; however, temperature differentially affected these 2 microbial activities. Thus, grazing rates increased maximally at temperatures ≤ 2°C, except in 1 station in the Gerlache Strait, while BHP increased maximally at temperatures ≥ 2°C, except in 1 station in the Bellingshausen Sea. The percentage of grazed bacteria to BHP at theud highest experimental temperatures was low (56 ± 19%) in the Gerlache Strait, high (395 ± 137%) inud the Bransfield Strait and approximately balanced (97 ± 24%) in the Bellingshausen Sea. This suggests that differential microbial processes in each zone at increasing temperatures will also depend on the autochthonous community. The present study contributes to the understanding of the variability of polar biogeochemical fluxes, and may aid in predicting the response of microorganisms in future scenarios with local and seasonal changes in temperature.This study was supported by the Spanish MCyT grant REN2001-0588/ANT to D.V. We thank theud scientists, scientific support staff and crew of the RV ‘Hespérides’ for their help during the enjoyable TEMPANO cruise.Peer reviewe