Fluxes of bacterioplankton between a tidal estuary and the sea: returning to the “Outwelling Hypothesis”

The tidal dynamics of bacterioplankton communities at the outer part ofa shallow estuary (Ria de Aveiro, Portugal) were studied during 6 tidal cyclesat a fixed sampling site. Bacterial numbers (0.2–8.1 ×109 cells l−1), aminopeptidase activity(189–1662 nmol l−1 h−1),β-glucosidase activity (1.7–67.0 nmoll−1 h−1) and potential glucoseincorporation (0.48–3.99 nmoll−1 h−1) followed a consistent patternof increase during ebb and decrease during flood.Fluxes of bacterioplankton populations and associated heterotrophic activitiesbetween the estuary and the coastal area during a tidal cycle were estimatedfrom the water flux as derived from a two-dimensional vertically-integratednumerical model. The net fluxes estimated for a tidal cycle ranged from−26.0 to −2.5 to × 1016 bacterial cells. The nettidal fluxes of potential heterotrophic activities ranged from −10 to−80 mol h−1 for aminopeptidase, −0.33to −1.10 mol h−1 for β-glucosidase and−0.18 to +0.03 mol h−1 for glucoseincorporation. Net fluxes were generally negative in sign indicating thetransfer of phyto- and bacterioplankton, as well as potential capacities for thedegradation and recycling of organic matter, from the outer estuarinecompartment to the sea.