Seasonal and diurnal water quality and ecological dynamics along a salinity gradient (Mira channel, Aveiro lagoon, Portugal)

The patterns of water quality and ecological dynamics in estuaries vary at different time scales. Characterizing this variability at short (diurnal and seasonal) time scales is a pre-requisite to investigate the effects of long-term climate changes. Seasonal scales depend mostly on the natural climatic variability, while diurnal scales are generally associated with the tidal cycle and wind events. In order to study the role of these superimposed scales in the Aveiro lagoon, an integrated approach, which combines data acquisition and numerical modelling, was followed. The analysis was conducted with the three-dimensional, fully coupled hydrodynamic and ecological numerical model, ECO-SELFE, extended to account for the oxygen cycle. For data acquisition three field campaigns were performed along a salinity gradient (Mira channel), covering different seasonal situations: March 2009, September 2009 and January 2010. These campaigns, each lasting 24 hours, included the measurement of physical, chemical and biological parameters (river flow, water levels, currents velocities, water temperature, salinity, dissolved oxygen, nutrients and chlorophyll a). Results show the ability of the model to represent both the spatial and temporal patterns observed for the different variables. Chlorophyll a and nutrients tend to present larger concentrations upstream than downstream, with the largest values observed in spring. The distribution patterns are significantly influenced by the tidal dynamics.