Influence of freshwater inflow variability on the Douro estuary primary productivity: A modelling study

Changes in freshwater inflow patterns to estuaries are recurrent, due to climate change or direct human intervention. One of the most drastic human actions is dam construction, altering the amount, timing and variability of freshwater delivered to estuaries. Biological communities and their functions may be affected by these changes through several mechanisms, mainly associated to physical–chemical processes. In the Douro estuary, freshwater inflow is regulated by an electric power dam that represents its upstream limit. Freshwater discharge regime is very irregular, especially during low flow periods, when flow management is based on hydroelectricity needs. In the present study, a coupled hydrodynamic-biogeochemical model was calibrated and validated against phytoplankton biomass and production, nutrients and suspended matter data. Afterwards, it was used to analyze the influence of different flow regimes and magnitudes on estuarine biogeochemistry and primary production, a key process for the understanding of estuarine functioning. Results showed a parabolic relationship of phytoplankton biomass with flow, decreasing for both higher and lower river flow magnitudes, which may be explained by lower residence time and lower nutrient and biomass inputs, respectively. Considering flow variability, results suggest that stable flows enhance phytoplankton biomass and production in comparison to the highly variable regime presently observed in the estuary. Thus, river discharge regime is likely to have important consequences on phytoplankton production, which should be taken into account in the design of a management plan for the Douro estuary, in the vein of the Water Framework Directive.