Monitoring water quality parameters and their ecological effects in transitional waters is usually performed through in situ sampling programs. These are expensive and time-consuming, and often do not represent the total area of interest. Remote sensing techniques offer enormous advantages by providing cost-effective systematic observations of a large water system. This study evaluates the potential of water quality monitoring using Sentinel-2 observations for the period 2018-2020 for the Sado estuary (Portugal), through an algorithm intercomparison exercise and time-series analysis of different water quality parameters (i.e., colored dissolved organic matter (CDOM), chlorophyll-a (Chl-a), suspended particulate matter (SPM), and turbidity). Results suggest that Sentinel-2 is useful for monitoring these parameters in a highly dynamic system, however, with challenges in retrieving accurate data for some of the variables, such as Chl-a. Spatio-temporal variability results were consistent with historical data, presenting the highest values of CDOM, Chl-a, SPM and turbidity during Spring and Summer. This work is the first study providing annual and seasonal coverage with high spatial resolution (10 m) for the Sado estuary, being a key contribution for the definition of effective monitoring programs. Moreover, the potential of remote sensing methodologies for continuous water quality monitoring in transitional systems under the scope of the European Water Framework Directive is briefly discussed.

The estuarine phytoplankton communities are known to respond rapidly to environmental changes, being considered an important water quality indicator; thus, it is crucial to fully understand its natural variability. The objective of the present study was to assess the spatial variability of both physicochemical variables and the phytoplankton community, to understand how such variability is influenced by seasonality and to evaluate how the anthropogenic sources affect such patterns. The Tagus estuary was used as a case study, since it is one of Europe’s largest estuaries, with high spatial and seasonal variations and a high level of human pressure associated with large urban and industrial areas. To achieve this goal, environmental parameters, nutrients concentration, bivalve biomass (filter feeders) and phytoplankton pigments were quantified in a single summer sampling campaign with high spatial resolution and in monthly campaigns in eight sampling stations through the Tagus estuary, in one year. In general, suspended particulate matter and nutrients decreased from the upper part of the estuary to the estuary mouth; however, relevant local inputs were also observed in more downstream locations, near outfalls of wastewater treatment plants (WWTP). The chlorophyll-a concentrations were higher over the southern intertidal mudflats, probably due to resuspension of microphytobenthos, associated with higher nutrient concentrations. Through a grazing indicator, it was observed that grazers are important drivers of variability of the phytoplankton community composition. All water bodies achieved “good” and “high” water quality classifications for both physicochemical and biological indicators, with the worst results reported for the water bodies located at the upper estuary. Therefore, this estuary presents a decreasing trend of nutrients and chlorophyll-a in the upstream–downstream direction, except for the estuary channels and the outfall in the northern margin, which lead to an increase in nutrient concentrations. However, these increases did not affect the water quality of the three analyzed water bodies, presenting at least good ecological status, considering the nutrient and chlorophyll-a indicators.