Partitioning of phytoplanktonic organic carbon production and bacterial production along a coastal-offshore gradient in the NE Atlantic during different hydrographic regimes

The rates of primary production of particulate (POC-pr) and dissolved organic carbon (DOC-pr), production of heterotrophic bacteria (BHP), and the abundance of autotrophic and heterotrophic picoplankton were estimated on 2 cruises in NE Atlantic coastal waters off the Iberian peninsula. Downwelling conditions prevailed during the spring cruise (1997), while a sequence of marked upwelling-relaxation-weak upwelling was found in late summer (1998). The standing stocks and activities of phytoplankton and bacteria displayed coastal-offshore gradients in both sampling periods. Whereas DOC-pr was similar on both cruises, POC-pr was generally higher in late summer, yielding lower values of percent extracellular release (PER): 6 ± 5% versus 9 ± 4% (mean ± SD) in spring. Short-term changes in hydrographic conditions strongly affected the relative release of photosynthate in late summer. PER tended to decrease with increasing production, with values below 5% at rates higher than 3 mg C m^-3 h^-1. Chlorophyll a (chl a) normalized DOC-pr (DOC^B-pr) also increased significantly the higher the contribution of picoplankton (< 3 pm) to total chl a concentration, confirming the importance of the size distribution of algae in the relative rates of extracellular release. Although BHP was significantly correlated with chl a, it bore no relationship to primary production rates, either dissolved or particulate, and released DOC was insufficient to meet estimates of bacterial demand of labile carbon, indicating that the activities of bacterioplankton and phytoplankton were uncoupled during the surveys. These results suggest that bacteria were mostly independent of algal DOC for growth in this coastal system.