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Estuaries are among the most sensitive systems to climate change. Previous studies have suggested that the Sado Estuary (Portugal) has decreasing trends for water temperature, which is uncommon in a global warming scenario. However, no long-term analysis regarding water column conditions has been conducted on this estuary to date. Therefore, the main aim of this study is to understand if and how the properties of this estuary have changed, considering a 34-year trend analysis and an assessment of their main drivers. Water temperature significantly decreased (up to 0.04 °C/year) along with chlorophyll a. Salinity increased and nutrients displayed an overall decrease. These trends were potentially driven by higher influence of upwelled water combined with a reduction of the river flow. Sado appears to have a lower vulnerability to climate change than other estuaries as it does not show a high sensitivity to changes in its water column physicochemical properties., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Although regional studies and projections suggest the deterioration of estuaries as a consequence of climate change, it is still difficult to fully understand the importance of such changes in estuarine systems. This limitation is particularly important considering their high dynamism and the lack of temporally extended in situ databases with a good spatial coverage for these systems worldwide. Furthermore, contradictory patterns have been observed across the globe. Motivated by these issues, in this study we question the availability of in situ observational evidence of climate change in estuarine systems through a detailed meta-analysis of existing publications. A topic-related search considering the outputs of the Web of Science library was conducted in order to obtain a characterization of the existing studies on climate change in estuarine systems. Results confirmed that climate change has increasingly been studied since 2000 and that marine climate change constituted the focus of 9.69 % of those studies. From these, only 9.30 % encompassed estuarine studies and just 1.13 % used in situ observations from estuarine systems (i.e., 0.11 % of the total climate change publications). Reanalysis products were the most used tools to assess changes in estuarine systems and sea temperature was the most analyzed variable. These results highlight the need to further address such questions using in situ observational data and to implement long-term observatories to fully identify evidence of climate change in estuarine systems, supporting modelling approaches and promoting the development of effective mitigation plans., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 The Authors. Published by Elsevier B.V. All rights reserved.)