There is a crucial ecological zone between the river and the sea that often gets overlooked in environmental conservation discussions: estuaries. A recent study, conducted as part of the MAELSTROM project and published in the journal Estuarine, Coastal and Shelf Science, focuses on these environments. Authored by researchers from our project partners CIIMAR, Deltares, and CNR-ISMAR, the study investigates the hydrodynamic patterns of the Ave River Estuary in Portugal.

The importance of estuaries

Estuaries are transition zones between riverine and marine environments. Their unique characteristics make them crucial for various reasons: they are rich in biodiversity, hosting numerous species, including many migratory birds, and serve as nurseries for the larval stages of various fish species. In addition to providing numerous ecosystem services and being habitats for many species, estuaries play a vital role in biogeochemical cycles.

Their hydrodynamic patterns are essential because they determine the physicochemical characteristics of these environments, impacting all the life forms they support. This explains the scientific interest in these patterns. While several studies have characterized them, most have focused on the estuaries of larger rivers. The MAELSTROM research team has instead investigated the less studied Ave River Estuary, which flows into the Atlantic Ocean south of Vila do Conde.

The MAELSTROM study

The study is based on data collected through various in situ measurement campaigns conducted between 2021 and 2023. These campaigns aimed to thoroughly understand the environment where we would later implement the Bubble Barrier, a technology developed by our Dutch partner The Great Bubble Barrier to intercept riverine litter and prevent it from reaching the sea. During these campaigns, researchers monitored parameters such as water depth, salinity, temperature, turbidity, and current velocity. The measurements revealed a highly stratified estuary with strong vertical salinity gradients and very low currents. The currents were mainly generated by tides and estuarine circulation during low river flow periods and by river dominance during high river flows.

Additionally, it was observed that coastal conditions, influenced by the plume—a freshwater mass rich in particles and sediments extending from the river mouth into the sea—from the nearby Douro River Estuary play a significant role in the circulation of the Ave Estuary. The Douro plume enhanced seaward transport in the estuarine surface layer and upstream flow in the bottom layer. This underscores the importance of considering coastal conditions to understand the circulation patterns of small estuaries with low river flows.

Rich environments and complex dynamics

Overall, the data collected by our researchers highlight that it is not only local conditions, including anthropogenic structures, that influence the hydrodynamic patterns and environmental characteristics of the Ave River Estuary but also distal conditions such as winds, neighboring freshwater masses, and oceanographic processes (particularly upwelling and downwelling, referring to the vertical movement of water in the ocean). The relative strength of each process, and its variation in time, determines its influence on the estuary.

“This is why we intend to conduct future work, including new dedicated campaigns and the implementation of a hydrodynamic numerical model, to understand the effect of each factor on the hydrodynamics of the Ave River Estuary,” says Isabel Iglesias, CIIMAR associated researcher and lead author of the study.
“What might seem like a local study actually describes processes that can influence a wide range of estuaries. The observed mechanisms can have important implications for the ecological state of the estuary, affecting the exchange of nutrients, sediments, larvae, and pollutants. Moreover, they can be significantly impacted by climate change due to variations in coastal water properties,” concludes Dr. Iglesias. “This information is therefore fundamental for effective coastal planning, risk assessment, and supporting decision-making.”