Coastal bacterial viability and production in the eastern English Channel: A case study during a Phaeocystis globosa bloom

Abstract

Heterotrophic bacterial standing stocks (total and viable cells) and production were determined in the coastal surface waters of the eastern English Channel, during different stages of a phytoplankton succession. Two coastal zones of variable freshwater influence were surveyed within the ‘coastal flow system’ (Wimereux and Somme Bay) where massive and recurrent Phaeocystis globosa blooms take place in spring. The proportion of intact (MEM+) cells, assessed by the LIVE/DEAD® BacLight™ (L/D) method, varied from 15 to 94% at the two coastal stations studied (median of 46%). MEM+ and total (DAPI) cell counts were significantly correlated over the study period, whereas the higher proportion of MEM+ cells did not correspond to an elevated bacterial cell production (BP). Low levels of living (potentially active) cells were nevertheless responsible for the high productivity levels within the bacterial community when the P. globosa bloom declined. Our study revealed that the bacterial carbon production/primary production ratios (BCP/PP) showed broad variations (7 to 111%) within each site, going from low values (7–16%) when the bloom was the most productive, to higher values (61–111%) at the end of the bloom. This suggested (i) a temporal uncoupling between bacteria and phytoplankton throughout the bloom duration and (ii) a drastic change of the amount of PP potentially processed by the bacterial community among high and low productive periods. The BCP increase after the decline of the P. globosa bloom implies that, at this time, a large part of the phytoplankton-derived organic matter (OM) was remineralised via the bacterial heterotrophic production. With respect to the L/D results, this bacterial remineralisation was due to a small yet productive total cell fraction.