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 of a shallow estuary (Ria de Aveiro, Portugal) were studied during 6 tidal cycles at a fixed sampling site. Bacterial numbers (0.2-8.1 x 10(9) cells 1(-1)), aminopeptidase activity (189-1662 nmol I-1 h(-1)), beta-glucosidase activity (1.7-67.0 nmol I-1 h(-1)) and potential glucose incorporation (0.48-3.99 nmol I-1 h(-1)) followed a consistent pattern of increase during ebb and decrease during flood. Fluxes of bacterioplankton populations and associated heterotrophic activities between the estuary and the coastal area during a tidal cycle were estimated from the water flux as derived from a two-dimensional vertically-integrated numerical model. The net fluxes estimated for a tidal cycle ranged from -26.0 to -2.5 to x 10(16) bacterial cells. The net tidal fluxes of potential heterotrophic activities ranged from -10 to -80 mol h(-1) for aminopeptidase, -0.33 to -1.10 mol h(-1) for beta-glucosidase and -0.18 to +0.03 mol h(-1) for glucose incorporation. Net fluxes were generally negative in sign indicating the transfer of phyto- and bacterioplankton, as well as potential capacities for the degradation and recycling of organic matter, from the outer estuarine compartment to the sea.