Hydrodynamics of a highly stratified small estuary and the influence of nearby river plumes

In situ measurements are key to depict the hydrodynamic patterns of estuarine regions. Whereas extended studies are available on major estuaries, there is still a lack of knowledge concerning smaller ones. In this study, the analyses of dedicated in situ campaigns are presented to unravel the hydrodynamic patterns of a small shallow-water estuary located in northern Portugal – the Ave Estuary – in view of a possible implementation of new ecosystem-based management measures to contain plastic pollution. The in situ campaigns performed, which monitored several parameters, such as water depth, salinity, temperature, turbidity and current velocity and bottom sediment grain-size, revealed a highly stratified estuary, with very strong vertical salinity gradients. The measured current velocities were low but, despite its low velocities and low river flows, this estuary revealed interesting hydrodynamic patterns associated with local and remote (oceanic) conditions. The currents were mostly generated by tides and estuarine circulation during low river flows and by river dominance during high river flows. However, the study demonstrated that the circulation is also influenced by the coastal conditions generated by the plume of a neighbouring estuary, which enhanced an oceanward transport in the estuarine surface layer and an upstream flow in the bottom layer, revealing the importance of considering coastal conditions to understand the circulation patterns of shallow-water small estuaries.