Abstract
We estimated the bacterial production and losses to predators along an open ocean trophic gradient from coastal upwelling waters to oligotrophic waters in the Subtropical Northern Atlantic Ocean. Two zonal sections (21 and 26 degrees N) extending from the NW African shelf to the Open Atlantic Ocean at 26 degrees W were sampled during September-October 2002 (autumn), and May-June 2003 (spring). The main goal was to elucidate whether the impacts of bacterial losses were more important in upwelling rather than in offshore waters. Whereas temperature and salinity decreased and nutrient concentrations increased from offshore to the coastal upwelling regions, phytoplankton, ciliate and bacterial biomass followed a similar trend increasing towards the upwelling zone. In addition, heterotrophic nanoflagellate biomass, bacterial production and grazing rates on bacteria did not follow this pattern, although the highest activities were recorded at upwelling stations. However, at the stations not affected by upwelling the average impact on bacteria expressed as a percentage of bacterial production consumed by predators in autumn and spring (values that varied between 70% +/- 6% and 129% +/- 15%, respectively) was significantly higher than at the upwelling stations (where it ranged between 49% +/- 7% and 68% +/- 5%, respectively). Our results suggest that in the upwelling areas bacteria escape from predators and growth cannot be balanced by grazing, while it is at the oligotrophic open ocean stations when, on occasions, grazing can overcome bacterial production.