Compartments of oxygen consumption in a tidal mesotrophic estuary (Ria de Aveiro,Portugal)

Oxygen consumption rates were determined, in parallel with primary production and bacterial biomass production, as an approach to the analysis of carbon cycling in the estuarine community of the Ria de Aveiro. The water column of the marine zone was the major contributor (64–99 %) to the total aerobic carbon remineralisation in which O₂ uptake rates averaged from 80 to 127 mg O₂.m^–2.h^–1, respectively at low tide and high tide. The planktonic consumption of O₂ varied from 0.010 to 0.041 mg O₂.L^–1.h^–1 with the highest values in the brackish zone. Small water column depths in this zone, however, reduced the integrated average consumption of the plankton, per unit of surface area, to 57 (LT) and 66 % (HT) of that observed in the marine zone. Benthic O₂ consumption rates, 5.1 to 22.0 mg O₂.m^–2.h^–1, were two to four times higher in the brackish zone when compared to the rates in the marine zone. It represented 1–31 % of the total surface integrated values in different areas and at different tides. From the ratios of the primary production and bacterial biomass production, on a per surface unit basis, it is concluded that, in late autumn, the Ria de Aveiro was mostly a heterotrophic system with a feeble recovery of primary production at HT in the marine zone and at LT in the brackish water zone.