Tidal influence on water quality indicators in a temperate mesotidal estuary (Tagus Estuary, Portugal)

In mesotidal estuaries, the spatial and temporal variation of physicochemical and biological parameters, and thus water quality, is strongly influenced by tidal dynamics. Hence, to improve monitoring and management strategies, it is imperative to understand how tidal conditions affect water quality parameters and the overall water quality assessments. Multiple indexes and water quality indicators have been used to address water quality issues, monitoring and reporting, mainly in the scope of the Water Framework Directive (WFD). In this work, the tidal effect on nutrients, dissolved oxygen, suspended solids (SPM), Secchi depth, chlorophyll a and phytoplankton composition in a temperate mesotidal estuary was assessed. Moreover, the effect of tidal conditions on the water quality indicators and classification systems was also evaluated. This study was conducted at two stations (Barreiro and Alcântara) located in the Tagus Estuary (Lisbon, Portugal), presenting different anthropogenic pressures. Nutrient concentrations were the variables which yielded the highest differences in response to the tidal conditions, especially at Alcântara, where the influence of a nearby outfall was evident. Therefore, Alcântara also presented stronger classification variability in response to the tidal effect, especially in classifications with less salinity classes. Results indicate that low neap tide is the best tidal condition to assess the worst-case scenario due to the low dilution factor. However, it is important to highlight that all tidal conditions should be sampled to understand the overall condition of water bodies. The classification using mean as a metric (TRIX) was more susceptible to the presence of high nutrient concentrations in general, while the 90th percentile responded better to the presence of several high values in an area with lower nutrient concentrations.