Assessment of Climate Change Impacts on Water Quality in a Tidal Estuarine System Using a Three-Dimensional Model

Hydrodynamic Model Ensembles for Climate Change Projections in Estuarine Regions

In the current context of climate change, understanding the effects of the changing conditions on estuaries is of utmost importance to protect populations and ecosystems. Given the diversity of impacts depending on the region, there is a need for local and dedicated studies to understand and mitigate the risks. Numerical models can provide forecasts of extreme floods and sea-level rise (SLR). However, they can present inaccuracies. In this work, the ensemble technique was applied to improve the numerical modeling forecasting for estuaries by considering scenarios of extreme river flow discharges (EFDs) and SLR scenarios for 2050 and 2100. The simulations were performed for two different estuarine regions in northern Portugal, and the superensemble was constructed with the results of two different numerical models. The results differed per estuary, highlighting the importance of a local approach. For the Douro estuary dynamics, the results showed that for the EFD, the effects of the SLR were not noticeable, indicating that, in this estuary, the river component was more important than the maritime component. In contrast, the Minho estuary dynamics were found to be affected by the SLR along the whole estuarine region, indicating a maritime influence and a worsening of the flood conditions for future scenarios.

Saltwater Intrusion in the Upper Tagus Estuary during Droughts

Droughts reduce freshwater availability and have negative environmental, economic, and social impacts. In estuaries, the dynamics between the saltwater and the freshwater can be affected during droughts, which can impact several natural resources and economic sectors negatively. The Tagus estuary is one of the largest estuaries in Europe and supports diverse uses and activities that can be affected by the saltwater intrusion (e.g., agriculture). This study assesses the saltwater intrusion in the upper reaches of the Tagus estuary using a process-based model to explore different scenarios of freshwater discharge and sea level rise. For the river discharge and mean sea level rise scenarios analyzed, salinity can reach concentrations that are inadequate for irrigation when the mean Tagus river discharge is similar or lower than the ones observed during recent droughts (22–44 m3/s). Lower river discharges aggravate the consequences. Results also show that the salinity increases with the duration of the droughts. In contrast, the impact of a moderate sea level rise on salinity intrusion is modest when compared with the impact of low river discharges. These findings contribute to support the management of the agricultural activities in the upper Tagus estuary and the water resources in the Tagus river basin.

Coupling between Hydrodynamics and Chlorophyll a and Bacteria in a Temperate Estuary: A Box Model Approach

The spatial patterns of chlorophyll a and bacteria were assessed in a temperate Atlantic tidal estuary during seasonal surveys, as well as in consecutive summer spring and neap tides. A box model approach was used to better understand spatial and temporal dynamics of these key estuarine descriptors. The Lima estuary (NW Portugal) was divided into boxes controlled by salinity and freshwater discharge and balance equations were derived for each variable, enabling the calculation of horizontal and vertical fluxes of plankton and, therefore, production or consumption rates. Chlorophyll a tended to burst within the oligohaline zone, whereas higher counts of bacteria were found in the mesohaline stretch. Whenever the water column was stratified, similar tide-independent trends were found for chlorophyll a and bacterial fluxes, with net growth in the upper less saline boxes, and consumption beneath the halocline. In the non-stratified upper estuary, other controls emerged for chlorophyll a and bacteria, such as nitrogen and carbon inputs, respectively. The presented results show that, while tidal hydrodynamics influenced plankton variability, production/consumption rates resulted from the interaction of additional factors, namely estuarine geomorphological characteristics and nutrient inputs. In complex estuarine systems, the rather simple box model approach remains a useful tool in the task of understanding the coupling between hydrodynamics and the behavior of plankton, emerging as a contribution toward the management of estuarine systems.

Modelling climate change impacts on nutrients and primary production in coastal waters

There is high confidence that the anthropogenic increase of atmospheric greenhouse gases (GHGs) is causing modifications in the Earth's climate. Coastal waterbodies such as estuaries, bays and lagoons are among those most affected by the ongoing changes in climate. Being located at the land-sea interface, such waterbodies are subjected to the combined changes in the physical-chemical processes of atmosphere, upstream land and coastal waters. Particularly, climate change is expected to alter phytoplankton communities by changing their environmental drivers (especially climate-related), thus exacerbating the symptoms of eutrophication events, such as hypoxia, harmful algal blooms (HAB) and loss of habitat. A better understanding of the links between climate-related drivers and phytoplankton is therefore necessary for projecting climate change impacts on aquatic ecosystems. Here we present the case study of the Zero river basin in Italy, one of the main contributors of freshwater and nutrient to the salt-marsh Palude di Cona, a coastal waterbody belonging to the lagoon of Venice. To project the impacts of climate change on freshwater inputs, nutrient loadings and their effects on the phytoplankton community of the receiving waterbody, we formulated and applied an integrated modelling approach made of: climate simulations derived by coupling a General Circulation Model (GCM) and a Regional Climate Model (RCM) under alternative emission scenarios, the hydrological model Soil and Water Assessment Tool (SWAT) and the ecological model AQUATOX. Climate projections point out an increase of precipitations in the winter period and a decrease in the summer months, while temperature shows a significant increase over the whole year. Water discharge and nutrient loads simulated by SWAT show a tendency to increase (decrease) in the winter (summer) period. AQUATOX projects changes in the concentration of nutrients in the salt-marsh Palude di Cona, and variations in the biomass and species of the phytoplankton community.