Abstract
The Minho and Lima are adjacent estuaries located in the north of Portugal, with high ecological and economic importance. To address gaps in knowledge about changes in nutrient patterns in adjacent estuaries subject to different freshwater inflows, a numerical model, Delft3D, was implemented and developed, using a single domain, which allowed physical communication between estuaries. Calibration and validation of the model was successfully performed. Three numerical simulations were carried out, in which only river flows were varied (1st corresponds to a baseline numerical run, the 2nd a flood scenario, and the 3rd a drought scenario). Under flooding conditions, similar patterns were verified in both estuaries, with high fluvial discharges showing to have a reduced impact on both estuarine dynamics. In this case the nutrients were not a limiting factor for the biota, both for summer and winter seasons, since there was no significant decrease in dissolved oxygen concentration. For the drought scenario, it was observed that the estuary with the lower inflow of freshwater (Lima) was the most affected, with a significant decrease in the concentration of nutrients and oxygen dissolved in the winter season (decrease of 2 mg O2/L). In conclusion, this work reveals that it is essential to continuously monitor dam-controlled estuarine systems, as a significant decrease in river discharge will cause significant changes in the variables analysed (O2, PO4, and NO3) and may cause loss of biodiversity.
Highlights
Estuarine systems are highly dynamic since their geomorphology and their physicochemical properties are constantly changing [1]
In addition to these aspects, other factors contribute to the alteration of estuarine dynamics and to the degradation of water quality, such as the construction of upstream river dams and the existence of punctual and diffuse discharges along the estuaries that release high concentrations of nutrients and contaminants, of which this movement may be altered by climate change through increased atmospheric deposition and surface run-off [28,29,30]
The model Delft3D was applied to Minho and Lima estuaries in order to study the impact of climate change, especially extreme river discharge events, on nutrient dynamics in two estuaries subject to different hydrological regimes
Summary
Estuarine systems are highly dynamic since their geomorphology (processes of erosion and sedimentation) and their physicochemical properties are constantly changing [1]. These physicochemical changes are strongly influenced by atmospheric and hydrodynamic processes (waves, tides, wind stress, freshwater discharges by rivers, and exchanges with the atmosphere) making the estuaries very productive systems that play a very important role in biogeochemical cycles, intercepting watershed-derived materials before delivering them to the marine environment [2], and influencing the sedimentation rate, zonation and species productivity, water temperature, and salinity of each zone [3] These very complex features of estuaries enable them to have a high primary production, due to nutrient fluxes from both rivers and coastal upwellings, which is very valuable since they have high metabolic rates and serve as important sites for water filtration and nutrient retention, transformation, and recycling [3,4,5,6]. To explain the existing and predictable biogeochemical processes in estuaries, numerical models are used nowadays since they show many important advantages [38,39,40,41], and they can serve as predictive or decision support tools [42]
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