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.

Bivalves accumulate toxins produced by microalgae, thus becoming harmful for humans. However, little information is available about their toxicity to the bivalve itself. In the present work, the physiological stress and damage after the ingestion of toxic dinoflagellate species (Gymnodinium catenatum) and a diatom species (Skeletonema marinoi, which is non-toxic to humans but may be to grazers) in the oyster Magallana angulata are evaluated against a control treatment fed with the chlorophyte Tetraselmis sp. Oysters were exposed for two hours to a concentration of 4 × 10⁴ cells/L of G. catenatum and 2 × 10⁷ cells/L of S. marinoi. The biomarkers superoxide dismutase (SOD), catalase (CAT), glutathione S-Transferase, total Ubiquitin (Ubi) and Acetylcholinesterase (AchE) were assessed. The exposure of M. angulata to G. catenatum lead to a reduction in SOD and AchE activity and ubiquitin concentrations when compared to the control treatment. Moreover, it increased CAT activity in the adductor muscle, and maintained its activity in the other tissues tested. This may be related to the combination of reduced metabolism with the deployment of detoxification processes. S. marinoi also lead to a decrease in all biomarkers tested in the gills and digestive glands. Therefore, both species tested caused physiological alterations in M. angulata after two hours of exposure.

Despite the burgeoning number of non-indigenous species (NIS) in coastal ecosystems worldwide, the quantification of their direct impacts on native communities remains largely unexplored. This is particularly true for feeding competition among sympatric filter-feeding bivalves. This study aimed to fill a knowledge gap regarding potential trophic competition between native and non-indigenous bivalves by focusing on three species that co-occur in Portuguese estuarine systems: the native cockle ( Cerastoderma edule ) and the non-indigenous Portuguese oyster ( Magallana angulata ) and Manila clam ( Ruditapes philippinarum ). The specific objectives were to (i) estimate their capture efficiency (≈ particle retention efficiency; CE), (ii) assess their clearance rates (CR), and (iii) provide science-based support for suitable management measures regarding NIS. Experiments were conducted under both field and laboratory conditions using the natural seston present in seawater. CE was higher for the six larger particle-size classes (8–14 μm) measured (range: 4–14 μm; size resolution: 1 μm), regardless of species. Although individual CRs did not differ significantly among species, the CR per gram of ash-free dry body tissue weight was significantly higher for the native cockle, suggesting that NIS do not have a competitive advantage in clearing suspended particles. However, the Manila clam might limit food-source availability to the native species because their ecological niches overlap.

Phytoplankton communities are the main marine producers of O2 and have a key role in removing CO2 from the atmosphere. They form the base of most marine trophic chains, supplying about 50% of the global organic carbon. In this context, it is key to assess the structure of phytoplankton community, given that a change in its composition may have significant implications in the trophic relationships. Moreover, climate-related changes, such as changes in temperature and salinity, are also likely to affect the structure of the community. Several approaches have been used to evaluate the composition of the phytoplankton community (e.g. microscopy, pigment and molecular analysis). Apart from other limitations, microscopy is not appropriate for identifying and quantifying groups composed of small cells (pico- and nano-phytoplankton) and molecular analysis is costly and still has restrictions in terms of community screening. This study is focused on pigment-based chemotaxonomy, i.e. the classification of organisms based on pigments measured through high performance liquid chromatography (HPLC). The classification is performed using a matrix factorization method, named CHEMTAX, that relies on the use of ratios between the accessory pigments (used as biomarkers of phytoplankton) and chlorophyll a (Chl a; index of phytoplankton biomass) to reconstruct the community composition. This requires knowledge on the pigment:Chl a ratios for each group and should be adjusted to the study area, given that these ratios vary according to the existing species and the environmental conditions, such as light and nutrients. The ratios applied are commonly retrieved from the available literature and the number of groups assessed are based on the user evaluation of the pigment information. Therefore, a few approximations can affect the results obtained by this technique. The aim of this study is to optimize the chemotaxonomic methodologies for the characterization of the phytoplankton communities applying this technique to known conditions. That is, by using monocultures pigment composition to determine the input ratios; and mixtures of the same cultures to assess CHEMTAX accuracy in reconstructing the mixtures composition. This way, impacts of varying the number of pigments incorporated in the input matrix and the groups assessed could be evaluated. Statistical analyses (e.g. PERMANOVA) of these outputs will indicate if differences found between approaches are significant and which of the approaches agree better with the expected values. Results will also allow to evaluate the consequences for the results when the pigments and groups introduced in the analysis of CHEMTAX are varied.