Modelling Salt Intrusion and Estuarine Plumes under Climate Change Scenarios in Two Transitional Ecosystems from the NW Atlantic Coast

Abstract

The Minho and Lima estuaries, separated by approximately 20 km, are two important transitional systems from the NW Atlantic Portuguese coast. Considering their contrasting characteristics, the present work aims to investigate climate change impacts in salt intrusion and estuarine plumes interaction, through an integrated hydrological characterization. The three-dimensional numerical model Delft3D was implemented to explore the hydrological response of these two systems taking into account their interaction. The model was applied to investigate two different climate change scenarios (RCP 4.5 and RCP 8.5), where physical variables such as sea level and river outflow were varied following summer and winter conditions. The numerical results revealed typically estuarine hydrological patterns, although some different features were found for each estuary. The head and mouth of the estuaries may be the zones most affected by climate change. In the first zone, the salinity may increase in the future in response to freshwater reduction from the Minho and Lima rivers. Considering the cumulative impact of the sea level rise, the saline intrusion may increase in the future: in the Minho estuary it reaches 14 km for the Present scenario during the dry season, penetrating further under climate change conditions (15 km for the RCP 4.5 scenario, and 17 km for the RCP 8.5 scenario); in the Lima estuary, the salt propagates further inland, reaching 17 km for the Present scenario, 18 km for the RCP 4.5 scenario, and a maximum of 20 km for the RCP 8.5 scenario. In all scenarios, during winter conditions, the estuarine plumes are larger and wider, being predicted that in the future they will reduce their dimensions during the winter period, and remain unchanged during summertime. Concisely, this modelling study provided more expertise about the physical patterns of these two estuaries and their interaction, as well as to anticipate the climate change impacts in their dynamics.

A numerical study of the mixing and stratification alterations in estuaries due to climate change using the potential energy anomaly

“…These results are in line with the results found by Liu and Liu, 2014;Duvall et al, 2022;Pereira et al, 2022 andCosta et al, 2023, where the increase of salinity intrusion in the estuaries is reported due to sea level rise. It is important to point out that the salinity and density in the Suances estuary are strongly related and the changes in temperature will be minimal as shown in Table 3.…”

“…This phenomenon will decrease the freshwater input to the estuary and increase the tidal excursion along the estuary, producing a displacement of the river/estuarine front upstream in line with the results observed by Ma et al, 2023. As shown in Figure 8, the starting point of the stratification zone moves upstream of the estuary in the projected scenarios as seen in the Minho estuary in Pereira et al, 2022 and the Wu river in Liu and Liu, 2014, where salinity intrusion extends more than 2.000 m and 900 m in the projected scenarios, respectively. In the Minho and Wu estuaries, sea level rise is the main cause of increased salinity intrusion.…”

“…Due to climate change, the stratification zone will be modified as observed in other estuaries worldwide (Leal Filho et al, 2022;Costa et al, 2023). On the one hand, sea level rise will pull the stratification zones back inland from the estuary as seen in the various studies analyzed in the region surrounding the study area, such as the NW Atlantic coast of Portugal (Pereira et al, 2022) or the UK coast (Robins et al, 2016).…”

“…Buoyancy input in the form of freshwater and saltwater provides a stratifying influence on estuaries and adjacent coastal waters, driving low-density surface water toward the ocean and high-density bottom water toward the river, respectively (Simpson et al, 1990). Due to the modification in estuarine forcings by climate change, the mixing and stratification conditions may be modified by increasing the intrusion of marine waters into the estuary, and, subsequently, generating a change in their biodiversity (Duvall et al, 2022;Garnier et al, 2022;Pereira et al, 2022).…”

A Dynamic Estuarine Classification of the Vertical Structure Based on the Water Column Density Slope and the Potential Energy Anomaly

“…The sea level rise will increase the depths, reducing the tidal marshal zones, making mixing and stratification processes in these areas more likely to occur. The reason behind these causes is the fact that sea level rise will increase the water column, and therefore create more pronounced stratification patterns back inland from the estuary as seen in the various studies analyzed in the region surrounding the study area, such as the NW Atlantic coast of Portugal [66,67] or the UK coast [68]. This is especially noticeable in the intertidal zones, where in the 2020 scenario there were fully mixed zones that have now been replaced by weakly stratified zones.…”