Variability of physicochemical and biological parameters in the Sado estuary: integration of in situ observations and satellite data

Tese de mestrado em Ciências do Mar, Universidade de Lisboa, Faculdade de Ciências, 2020Os sistemas estuarinos desde sempre captaram a atenção do ser humano. Os estuários estão entre os ecossistemas mais produtivos do mundo, têm uma elevada biodiversidade, que é característica destes sistemas, e oferecem zonas mais protegidas, com enorme importância para funções de proteção, alimentação e berçário para várias espécies. São, portanto, áreas influenciadas pela atividade do Homem, que se habituou a explorar estes sistemas nas vertentes económicas e sociais, por exemplo, através da construção de portos, da atividade piscatória ou do turismo marítimo. Como consequência desta exploração, existe impacto da atividade antropogénica na dinâmica dos estuários, que deve ser devidamente avaliada. No entanto, a análise deste tipo de ambientes apresenta inúmeros desafios. Além do forçamento antropogénico, os estuários têm fortes interações com a atmosfera, condição que confere uma variação sazonal aos parâmetros físico-químicos da água. Adicionalmente, a mistura diária de água doce, vinda do rio, com água salgada, que entra pela foz, promove uma variação constante desses parâmetros e influencia a componente biológica da região. Estes fatores, bem como a recorrente interação com a costa, conferem uma condição natural complexa aos estuários. O presente estudo aborda este dinamismo e a variabilidade dos parâmetros físico-químicos no estuário do Sado, o segundo maior estuário de Portugal e um dos maiores da Europa. Devido à sua rica biodiversidade, produtividade e valor estético, o estuário do Sado foi definido como reserva natural em 1980, a fim de promover a sua preservação, evitando possíveis impactos antropogénicos nos seus processos e características naturais. Os primeiros estudos oceanográficos realizados no estuário do Sado decorreram nos anos 70. No entanto, após 40 anos, ainda é um desafio conseguir compreender na íntegra a dinâmica deste estuário e é crucial estudá-lo utilizando uma abordagem que integre observações in situ (com um regime de amostragem frequente, feito em profundidade e que acompanhe o ciclo de maré) com dados de satélite, para permitir uma análise temporal e espacial mais completa. Seguindo esse objetivo, foram realizadas campanhas in situ entre setembro de 2018 e setembro de 2019 no estuário do Sado, para recolher dados físico-químicos (temperatura, salinidade, fluorescência, turbidez, intensidade e direção das correntes) e biológicos (concentração de clorofila a). Essas campanhas foram realizadas com uma periodicidade mensal utilizando um CTD (Condutividade, Temperatura, Profundidade), um correntómetro e uma sonda multiparamétrica, e recolheram-se amostras de água para posterior quantificação de concentrações de clorofila a e matéria em suspensão (SPM). Adicionalmente, em novembro de 2018 e junho de 2019, a recolha de dados foi feita ao longo do ciclo de maré, em condições de marés vivas e de marés mortas. Os resultados indicaram que a região da embocadura do estuário é espacialmente homogénea, verificando-se, ocasionalmente, uma ligeira estratificação da coluna de água. A variabilidade dos parâmetros físico-químicos observada aparentou ser consequência de forçamentos antropogénicos e da variabilidade sazonal ou pontual das condições meteorológicas. Na região do estuário analisada, foi possível observar valores de salinidade tipicamente oceânicos, podendo concluir-se que o rio teve pouca influência na região mais exterior do estuário. Após comparados os resultados obtidos com os de estudos anteriores, pareceu ter existido uma diminuição da influência do rio na embocadura do estuário nos últimos anos, uma vez que se obtiveram valores de salinidade mais elevados ao longo do ano em análise, e uma maior amplitude térmica da água. A circulação no estuário aparentou ser feita através dos dois canais de navegação, sendo o canal sul, a via mais relevante de entrada e saída de água. Observou-se que a maré teve um papel determinante na direção da circulação no estuário. Ademais, a direção da corrente foi uniforme ao longo da coluna de água, contrariando estudos anteriores Com vista a melhorar o conhecimento sobre a dinâmica deste sistema estuarino, avaliou-se também a variabilidade intra-anual e interanual da temperatura da superfície do mar (TSM) e da concentração de clorofila a através de deteção remota por satélite. O primeiro desafio, foi o de perceber quais os sensores mais apropriados para um estudo em sistema estuarino, que apresentassem dados fiáveis e com elevada qualidade espacial e temporal. Estudou-se a viabilidade em utilizar a base de dados de TSM do Group for High Resolution Sea Surface Temperature (GHRSST) através da versão 4.1 da análise Multiscale Ultrahigh Resolution (MUR) disponibilizada pelo grupo. Estes dados foram validados com sucesso, por terem apresentado uma boa concordância com os valores de temperatura recolhidos in situ, principalmente na região do estuário mais próxima do oceano. No entanto, revelaram ter uma baixa resolução espacial. Ainda assim, foi percetível a sensibilidade deste produto em detetar variações sazonais ao longo do estuário. Foi feita uma análise deste parâmetro de junho de 2002 a setembro de 2019 e viu-se que anos de TSM mais baixas, parecem estar associados a anos de índice NAO positivo. Para analisar a concentração de clorofila a no estuário, utilizaram-se os produtos do Sentinel3 OLCI (Ocean and Land Colour Instrument) para serem validados com os dados in situ, e os produtos MERIS, para se alcançar uma maior cobertura temporal e fazer-se uma análise histórica da variação de clorofila a no estuário de 2002 a 2012. A análise da clorofila estimada a partir do Sentinel-3 deu uma indicação de que esses dados eram mais apropriados para a área de estudo, devido à resolução espacial do sensor e à boa aplicação a águas costeiras. Foi possível observar concentrações mais elevadas de clorofila a nos canais mais interiores do estuário. Adicionalmente, as concentrações máximas de clorofila a foram encontradas durante a primavera em todo o estuário. No entanto, a correlação entre os valores de satélite e os obtidos in situ não foi a ideal (R 2 = 0,33). Uma das fontes de erro associada aos dados do Sentinel-3 poderá ser a presença de matéria em suspensão que parece ter interferido na quantificação de clorofila a, principalmente durante a campanha de 8 de novembro de 2018. Dos resultados obtidos no presente estudo, foi também detetada uma tendência para um decréscimo da concentração de clorofila a na região, de 2002 a 2012. De 2002 a 2019, também a TSM tendeu a diminuir no estuário. No entanto, seria importante complementar o presente estudo com uma análise estatística que detetasse a significância das tendências observadas. Dada a complexidade dos ambientes costeiros, os algoritmos disponíveis (quer de correção atmosférica, quer de determinação de variáveis biogeoquímicas) ainda não são totalmente eficientes e outros componentes oticamente ativos (e.g., partículas em suspensão ou matéria orgânica dissolvida) podem interferir nas estimativas da concentração de clorofila a. No entanto, quando foram analisadas as bases de dados MERIS e Sentinel-3, utilizaram-se os algoritmos desenvolvidos para águas costeiras e foi possível observar que, com o tempo, houve um aumento da qualidade dos produtos de satélite disponíveis. Como tal, é importante que se continuem a desenvolver novos algoritmos direcionados para ambientes costeiros, utilizando como base os resultados obtidos pelos exercícios de validação de produtos de satélite já realizados por vários autores em diferentes áreas do planeta. O presente estudo permitiu uma caracterização da dinâmica do estuário do Sado na atualidade a partir das campanhas in situ e pareceu indicar que poderão ter existido algumas alterações na sua natural dinâmica nos últimos anos. No entanto, seria importante prolongar a análise in situ seguindo uma abordagem frequente acompanhando ciclos de maré completos. Posteriormente, mantendo-se a tendência dos resultados obtidos, seria importante perceber o que esteve na origem destas alterações e quais as consequências que estas poderão ter no futuro do ecossistema.Estuarine systems have always captured the attention of Man. Estuaries are among the most productive ecosystems in the world. They have a high biodiversity, that is characteristic of these systems, and offer sheltered areas, with enormous importance for protection, feeding and nursery functions for several species. Therefore, they are areas influenced by the activity of Man, that explores these systems economically and socially, through, for example, the construction of ports, fishing or maritime tourism. As a result of this exploration, an impact of the anthropogenic activity on the dynamics of the estuaries is verified, which must be properly assessed. However, the analysis of these types of environments presents numerous challenges. In addition to anthropogenic forcing, estuaries have strong interactions with the atmosphere, a condition that gives a seasonal variation to the physicochemical parameters of the water. Additionally, the daily mixture of fresh water, coming from the river, with salt-water, which enters through the estuary, promotes a constant variation of these parameters and influences the biological component of the region. These factors, as well as the continuous interaction with the coast, give estuaries a complex natural condition. The present study addresses this dynamism and the variability of the physicochemical parameters of the water in the Sado estuary, the second largest estuary in Portugal and one of the largest in Europe. Due to its rich biodiversity, productivity and aesthetic value, Sado estuary was defined as a natural reserve in 1980, in order to promote its preservation, avoiding possible anthropogenic impacts in its natural processes and characteristics. The first oceanographic studies carried out in the Sado estuary took place in the 70s. After 40 years, it is still challenging to understand the dynamics of this estuary and is crucial to integrate in situ observations (with frequent sampling, in depth and throughout the tidal cycle) with satellite data, to enable an extended temporal and spatial analysis. Following that objective, in situ campaigns were conducted between May 2018 and September 2019 in Sado estuary, to collect physicochemical (temperature, salinity, fluorescence, turbidity, intensity and direction of the currents) and biological (chlorophyll a concentration) data. These campaigns were conducted on a monthly basis using a CTD (Conductivity, Temperature, Depth), a current meter sensor and a multiparameter sonde, and water samples were collected for laboratory quantification of chlorophyll a concentrations and suspended particulate matter (SPM). Additionally, in November 2018 and June 2019, the data collection was made along the tidal cycle, in both spring and neap tide conditions. The results indicated that the outermost area of the estuary is spatially homogeneous, with occasional stratification of the water column. The observed variability of the physicochemical parameters appeared to be a consequence of anthropogenic forcing and of seasonal or occasional variations of the weather conditions. In the analyzed region of the estuary, it was possible to observe salinity values typically oceanic, so it is assumed that the river had a low influence in the outermost region of the estuary. When compared with the results of previous studies, it was possible to infer that there was a decrease in the influence of the river in the outermost region of the estuary in the past years, since higher salinity values were obtained throughout the year under analysis, and that is currently observed a greater thermal amplitude of the water. The circulation in the estuary appeared to be made in the two navigation channels, being the South channel the most relevant route for water exchange. It was observed that the tide played a determining role in the direction of the circulation in the estuary. In addition, and contrary to the results of previous studies, the direction of the current was uniform along the water column. In order to improve knowledge about the dynamics of this estuarine system, the intra-annual and interannual variability of the sea surface temperature (SST) and the chlorophyll a concentration through satellite remote sensing was studied. The feasibility of using the SST database of the Group for High Resolution Sea Surface Temperature (GHRSST) was studied through version 4.1 of the Multiscale Ultrahigh Resolution (MUR) analysis made available by the group. These data were successfully validated, as they showed a good agreement with the temperature values collected in situ, mainly in the region of the estuary closest to the ocean. However, they appeared to show a low spatial resolution. Even so, the sensitivity of the product in detecting seasonal variations along the estuary was noticeable. An analysis of this parameter was carried out from June 2002 to September 2019 and it was observed that years of lower SST values seemed to be associated with years of positive NAO index. To analyze the chlorophyll a concentration in the estuary, the products of the Sentinel-3 OLCI (Ocean and Land Color Instrument) were used for validation with the in situ data. MERIS products were also used to achieve a greater temporal coverage and to present a historical analysis of the variation of chlorophyll a in the estuary. The analysis of chlorophyll estimated from Sentinel-3 gave the indication that these data were more appropriate for the study area, due to the spatial resolution of the sensor and its better application to coastal waters. It was possible to observe higher concentrations of chlorophyll a in the innermost channels of the estuary. Additionally, the maximum concentrations of chlorophyll a were observed during spring throughout the estuary. However, the correlation between the satellite values and those obtained in situ was not ideal (R 2 = 0.33). One of the sources of error associated with the Sentinel-3 data may be the presence of suspended matter that appears to have interfered with the quantification of chlorophyll a, especially during the campaign of 8 November 2018. From the results obtained in the present study, it was also observed a trend towards a decrease in the concentration of chlorophyll a in the region, from 2002 to 2012 (MERIS data). From 2002 to 2019, the SST also tended to decrease in the estuary. However, it would be important to complement the present study with a statistical analysis that would detect the significance of the observed trends. Due to the complexity of coastal environments, the algorithms available (whether for atmospheric correction or for determining biogeochemical variables) are not yet fully efficient because other optically active components (e.g. Suspended Particulate Matter or Colored Dissolved Organic Matter) can interfere with the estimations of chlorophyll a concentrations. However, in the present work, when MERIS and Sentinel-3 databases were analyzed, algorithms developed for coastal waters were used and it was possible to observe that, over time, there was an increase in the quality of the available satellite products. It is important to continue to develop new algorithms for coastal environments, using as a basis the results obtained by the validation exercises of satellite products already carried out by various authors in different areas of the planet. The present study allowed to describe the dynamic of the Sado estuary, based on in situ campaigns and it was concluded that some changes in the natural behavior of the estuary may have occurred in the past years. However, it would be important to prolong the in situ analysis following a frequent sampling approach along full tidal cycles. Subsequently, if the trend of the results obtained is maintained, it would be important to understand the origin of these changes and what consequences could they have on the future of the ecosystem

Deriving water quality parameters using sentinel-2 imagery: A case study in the Sado Estuary, Portugal

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.Fil: Sent, Giulia. Universidade Nova de Lisboa; PortugalFil: Biguino, Beatriz. Universidade Nova de Lisboa; PortugalFil: Favareto, Luciane. Universidade Nova de Lisboa; PortugalFil: Cruz, Joana. Universidade Nova de Lisboa; PortugalFil: Sá, Carolina. Universidade Nova de Lisboa; PortugalFil: Dogliotti, Ana Inés. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; ArgentinaFil: Palma, Carla. Instituto Hidrográfico; PortugalFil: Brotas, Vanda. Universidade Nova de Lisboa; PortugalFil: Brito, Ana C.. Universidade Nova de Lisboa; Portuga

Evaluation of SMOS L4 Sea Surface Salinity Product in the Western Iberian Coast

Special issue Moving Forward on Remote Sensing of Sea Surface Salinity.-- 24 pages, 14 figures, supplementary materials https://www.mdpi.com/article/10.3390/rs14020423/s1.-- Data Availability Statement: Data sharing not applicableSalinity is one of the oldest parameters being measured in oceanography and one of the most important to study in the context of climate change. However, its quantification by satellite remote sensing has been a relatively recent achievement. Currently, after over ten years of data gathering, there are still many challenges in quantifying salinity from space, especially when it is intended for coastal environments study. That is mainly due to the spatial resolution of the available products. Recently, a new higher resolution (5 km) L4 SMOS sea surface salinity (SSS) product was developed by the Barcelona Expert Center (BEC). In this study, the quality of this product was tested along the Western Iberian Coast through its comparison with in situ observations and modelled salinity estimates (CMEMS IBI Ocean Reanalysis system). Moreover, several parameters such as the temperature and depth of in situ measurements were tested to identify the variables or processes that induced higher errors in the product or influenced its performance. Lastly, a seasonal and interannual analysis was conducted considering data between 2011 to 2019 to test the product as a potential tool for long-term studies. The results obtained in the present analysis showed a high potential of using the L4 BEC SSS SMOS product in extended temporal and spatial analyses along the Portuguese coast. A good correlation between the satellite and the in situ datasets was observed, and the satellite dataset showed lower errors in retrieving coastal salinities than the oceanic model. Overall, the distance to the coast and the closest rivers were the factors that most influenced the quality of the product. The present analysis showed that great progress has been made in deriving coastal salinity over the years and that the SMOS SSS product is a valuable contribution to worldwide climatological studies. In addition, these results reinforce the need to continue developing satellite remote sensing products as a global and cost-effective methodology for long-term studiesThis work was conducted within the framework of the project AQUIMAR—Marine Knowledge Supporting Aquaculture (MAR-02.01.01-FEAMP-0107), funded by the Mar 2020—Operational Program Mar2020. B.B. was funded by a grant from Mar2020 under AQUIMAR project and also by a PhD grant awarded by Fundação para a Ciência e a Tecnologia (FCT) within the scope of the MIT Portugal Program. A.C.B. was funded by FCT through the Scientific Employment Stimulus Programme (CEECIND/0095/2017). A.T. was funded by Project SARDINHA2020 (MAR-01.04.02-FEAMP-0009), funded by the Operational Program Mar2020. This work benefited from the Infrastructure CoastNet (http://geoportal.coastnet.pt, accessed on 30 September 2021), funded by FCT and the European Regional Development Fund (FEDER), through LISBOA2020 and ALENTEJO2020 regional operational programs, in the framework of the National Roadmap of Research Infrastructures of strategic relevance (PINFRA/22128/2016). This study also received further support from FCT through MARE’s strategic program (UID/MAR/04292/2019). This work represents a contribution to CSIC Thematic Interdisciplinary Platform PTI Teledetect, with the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S). This publication was also funded by the European Union’s Horizon 2020 Research and Innovation Programme under grant agreement N810139: Project Portugal Twinning for Innovation and Excellence in Marine Science and Earth Observation—PORTWIMSPeer reviewe

Variability of Currents and Water Column Structure in a Temperate Estuarine System (Sado Estuary, Portugal)

The circulation in estuaries promotes the transport of organisms, nutrients, oxygen and sediments. Simultaneously, the mixture of fresh and salt water leads to variations of the physicochemical and biological components of the region. Therefore, it is important to further understand the hydrodynamic patterns of an estuary as one of the bases to understand the whole dynamic of these systems, ecologically important regions that must be preserved. However, little is known about the hydrodynamics of some estuarine systems. In order to bridge the knowledge gap about the Sado estuary, sampling was conducted with the purpose of evaluating some circulation patterns of the estuary and classifying it according to the stratification of the water column. The campaigns were conducted to collect monthly data on the intensity and direction of the currents, and on the temperature and salinity of the water column, between September 2018 and September 2019. The data indicated that water circulation in the Sado estuary, occurred through the two main navigation channels (North and South), according to the tidal regime. Both the temperature and the salinity were homogeneous along the water column, revealing little stratification. The analysis suggests possible hydrodynamic changes of the estuary in the past years