Recovery of intertidal areas and natural tidal dynamics

The Oyambre estuary is a small estuarine system (100 ha), located in northern Spain. It is formed by two branches: the eastern one, known as the Ría de la Rabia, which runs north-south, and the western one, known as the Ría del Capitán, which runs east-west. The two branches join at the estuarine mouth to form an arrow-shaped dune system which is an important tourist attraction in the area and a high valued leisure resource for the local people.

This estuary has been subject of an intense anthropic transformation over the last two centuries. This process began in the 19th century with the construction of one dike in the middle section of the western branch (Ría de Capitán) to desiccate the inner area for agriculture. In addition, around the same period a tidal mill was built in the eastern branch (Ría de la Rabia), partially restricting the tidal flow. The tidal mill collapsed in 1985, leading to the total restriction of the tidal flow and the transformation of the area in a low salinity lagoon. In 1955, a second dike was built in the Ría de Capitán closer to the estuary mouth, which made it possible to drain an area of around 16 hectares and create a eucalyptus plantation.

Around 1995 the external dike of the Ría de Capitán collapsed, allowing the entrance of sweater that killed the eucalyptus plantation and enhance the development of high marsh communities. Yet, the tidal regime was still highly restricted.

The total or partial tidal restriction that historically affected both estuarine branches led to the accumulation of sediments and the reduction of salinity, leading to the drying of the marsh vegetation and the appearance of an invasive alien species (IAS), the American shrub Baccharis halimifolia, which competes with native species of the high marsh (e.g. Juncus maritimus, Halimione portulacoides)

Baccharis halimifolia in the Oyambre estuary (before the restoration of the tidal dynamics)

The tidal restriction has also impacted some of the ecosystem services provided by the estuarine communities, such as the role that intertidal vegetated habitats played as carbon sinks and their capacity to adapt to sea level rise through sediment vertical accretion (as demonstrated within the EU LIFE ADAPTABLUES project led by IHCantabria).

In addition, the reduction of the tidal prism due to the presence of dikes and the transformation of intertidal areas for other uses had caused erosion problems in the beach and dune system, threatening one of the main tourist attractions of the area, and flooding problems in the inner La Rabia’s branch, where water accumulated and collapsed the road

The restoration of the natural tidal regime

In 2019, the Government of Cantabria, through the European project LIFE CONVIVE (led by IHCantabria), reduced significantly the height of the external dike of the Ría de Capitan, which allowed the entrance of seawater while allowing the local people to keep accessing the beach and dune system during the low tide. The recovery of the tidal regime contributed to the development of low marsh communities in previously unvegetated tidal flats and caused the death of the IAS Baccharis halimifolia at the inner section, where it still had a significant presence.