| Summary: | We present surface estimates of bacterial respiration, bacterial heterotrophic production (BHP), and bacterial growth efficiency (BGE), and their relationship with nutrient availability, along a trophic gradient from coastal upwelling waters to the open-ocean waters of the eastern North Atlantic. Bacterial respiration generally ranged between 10 and 30 μg C L_1 d-1 and was relatively unaffected by nutrient enrichment. In contrast, BHP showed higher variability (more than one order-of-magnitude range) and was affected by carbon and/or phosphorus additions in different regions. Empirical bacterial carbon-to-leucine (Leu) conversion factors (CFs) (range, 0.02-1.29 kg C mol Leu-1) decreased from the coast to the open ocean, largely influencing the BHP estimates in oligotrophic waters. We found high percentages of Leu respiration in oceanic waters (average 68% of Leu taken up by bacteria), possibly related to the low CFs found offshore. Empirical CFs were highly correlated to BGE (Pearson correlation coefficient r = 0.86, n = 12, p < 0.0004, log-log transformed), which varied between 1% in offshore waters and 56% in the upwelling waters. Empirical CFs could be critical not only for accurately constraining BHP, but probably also for predicting BGE in oceanic waters. © 2007, by the American Society of Limnology and Oceanography, Inc. Peer Reviewed
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