Temporal and spatial patterns of intertidal sediment-water nutrient and oxygen fluxes in the Douro River estuary, Portugal

The Douro River watershed drains 17% of the Iberian Peninsula; the present average freshwater discharge into the estuary is 488 m 3 s -1 . The Douro River estuary receives largely untreated sewage from about 1 million inhabitants of Greater Porto. A major portion of the intertidal sediments is currently scheduled for removal by dredging. In order to assess their role in nutrient dynamics and productivity, sediment-water fluxes of ammonium, inorganic phosphorus, nitrate, nitrite, silicate and oxygen were determined quarterly in muddy and sandy intertidal sediments of the lower estuary. Fluxes were estimated with both light and dark incubations. The range of net nutrient uptake rates from all sites was as follows: ammonium 28 to 978 µmol m -2 h -1 ; nitrate 40 to 810 µmol m -2 h -1 ; nitrite 1 to 128 µmol m -2 h -1 ; dissolved orthophosphate 1 to 152 µmol m -2 h -1 and silicate 69 to 369 µmol m -2 h -1 . Stepwise multiple regression analysis revealed strong coupling between the concentrations of most nutrients in the water column and the corresponding flux into the sediment. Mean net nutrient effluxes to the overlying water were observed in only 6 cases out of a total of 120, and only at the muddy sediment site. Net oxygen production (light incubation) ranged from 58 to 618 mgO2 m -2 h -1 , while oxygen consumption (dark incubation) ranged from 11 to 64 mgO2 m -2 h -1 . Primary production correlated significantly with chlorophyll a concentrations in the top (0.5 cm) sediment layer at the sandy sites, but showed no correlation at the muddy site. All sam- pling sites were net daily sources of oxygen to the water. The net productivity of the intertidal flats and their capacity for water-column inorganic nutrient removal assumes particular importance in the Douro River estuary, a eutrophic system with a permanently heterotrophic water column.