Physico-Chemical Characterization of Two Portuguese Coastal Systems: Ria de Alvor and Mira Estuary

Abstract

The present study aims to research the physico-chemical processes in two under-researched coastal systems located on the west and south Portuguese coast—Mira Estuary and Ria de Alvor—through the development and exploitation of dedicated coupled physical and water quality models. Both systems are highly dynamic, supporting a wide range of biological diversity; however, they are characterized by distinct environmental and oceanographic conditions, enhancing the importance of a comparative approach. In this context, the Delft3D modeling suite was implemented and successfully calibrated and validated for both systems, accurately reproducing their hydrodynamic, hydrographic, and chemical features. A broad characterization of Mira Estuary and Ria de Alvor was carried out. Results show that the tidal wave interacts differently with the different geomorphology of each estuary. The tidal wave amplitude decreases as it propagates upstream for both estuaries; however, the magnitude for Ria de Alvor is higher. A flood tidal dominance was found for Mira Estuary, allowing the transport of well-oxygenated water into the estuary, contributing to high residence times in the middle estuary and therefore to poor dissolved oxygen (DO) replenishment. Ria de Alvor shifts from ebb dominance at the central area to flood dominance upstream with low residence times, allowing an effective exchange of water properties. Results also reveal that the water temperature is the dominant driver of seasonal dissolved oxygen variations in both estuaries, with the lowest levels occurring during the late summer months when the water temperature is highest. In addition, pH is influenced by biological activity and freshwater inflow.

Ria de Alvor Suitability for Aquaculture: Future Challenges

“…The Ria de Alvor is a mesotidal shallow coastal lagoon located on the southern coast of Portugal (Algarve region) (Figure 1). The average depth of the lagoon is 2.3 m relative to the mean sea level (excluding intertidal areas); however, to ensure navigability to the recreational and fishing port of Alvor, the navigation channel is often dredged [17,18]. Semidiurnal tide constituents are the major hydrodynamic force [18,19], with a mean spring tidal range of 2.85 m [17].…”

“…The average depth of the lagoon is 2.3 m relative to the mean sea level (excluding intertidal areas); however, to ensure navigability to the recreational and fishing port of Alvor, the navigation channel is often dredged [17,18]. Semidiurnal tide constituents are the major hydrodynamic force [18,19], with a mean spring tidal range of 2.85 m [17]. The lagoon is mildly ebb-dominant in the central area, revealing a potential to export sediments and other properties to the ocean, and it is strongly flood-dominant upstream, which means that the upstream regions are prone to accretion and property retention [18].…”

“…Semidiurnal tide constituents are the major hydrodynamic force [18,19], with a mean spring tidal range of 2.85 m [17]. The lagoon is mildly ebb-dominant in the central area, revealing a potential to export sediments and other properties to the ocean, and it is strongly flood-dominant upstream, which means that the upstream regions are prone to accretion and property retention [18]. Additionally, the reduced freshwater inflow [18,19] associated with the tide-dominated hydrodynamic regime makes the Ria de Alvor a euhaline system (salinity range from 30 to 36) [17].…”

“…In this context, an application of the Delft3D model previously developed and calibrated by Picado et al [18] is explored, considering the hydrodynamic (FLOW) and the water quality (WAQ) modules. A single cartesian curvilinear orthogonal grid was used, allowing a coarser resolution at the offshore open boundary (approximately 300 m) and a finer resolution inside the lagoon (between 20 and 40 m).…”

“…A single cartesian curvilinear orthogonal grid was used, allowing a coarser resolution at the offshore open boundary (approximately 300 m) and a finer resolution inside the lagoon (between 20 and 40 m). A 2D depth-averaged approximation was considered since the Ria de Alvor is vertically homogeneous [18,19]. A time step of 30 s for the FLOW module and 5 min for WAQ was chosen to guarantee the stability and accuracy of numerical results.…”