Estuaries, changing environments

When the mouth of a river - where fresh river water meets ocean saltwater - is tidal, it is known as an estuary. According to the hydrological definition of the term, the head of an estuary is located at the point where tidal influence is no longer felt, while its mouth is less clearly defined and depends on water salinity levels. At the junction between rivers and oceans, these unique environments are characterized by numerous natural environmental constraints that affect the biodiversity found in their waters.

Due to their unique situation between freshwater and saltwater, the salinity of estuaries is extremely variable and the water within them is particularly unsettled. “Whether on the Atlantic or Channel coasts, effects of the tide are felt very strongly in French estuaries,” explains Jeremy Lobry, research director at Irstea Bordeaux. “Changing tides and river flows create currents as well as variations in water levels and salinity throughout the day, putting significant pressure on the organisms that live in that environment.” Additionally, these powerful currents can be linked to high water turbidity, leaving sediments and elements carried by the river permanently suspended in the water. Estuaries are constantly changing environments due to the high variation in salinity and water levels combined with other seasonal fluctuations such as river flows and temperatures, causing the organisms that live within them to adapt continuously. Managing this type of complex and dynamic environment is a particularly difficult task.

“Estuaries are particularly susceptible to the combined impacts of global climate change. They are affected by changing climates, invasive species, changes to natural habitats and pollution, all of which combine to greater effect. Furthermore, the pressures caused by global climate change merge with natural environmental constraints. It is therefore very difficult for managers to measure the additional impact of anthropic stressors (i.e., human activity) in such a changeable environment,” concludes the researcher.

Varied and iconic biodiversity

Estuaries are home to a range of aquatic species, including plants, plankton, crustaceans and fish. Given the large environmental variation between the head and the mouth of estuaries, their biodiversity is unique. Marine species, freshwater species and migratory species are all found in estuaries, alongside species that live within the estuaries all the time.

“An estuary is characterized by a reduced number of species, dominated by a few particularly tolerant species such as the Sand Goby (a small groundfish), common and whiteleg shrimp, and certain species of zooplankton in many French estuaries. Other species such as salmon or lamprey are very symbolic or have a high cultural value in their region, some such as eels are protected as vulnerable species, and others such as sturgeon are endangered,” specifies the researcher.

Furthermore, estuaries fulfill a crucial ecological role for certain species by acting as a nursery. Marine fish, such as sole, anchovies or sea bass, reproduce at sea but grow up in estuaries, protected from predators and with plenty of food, before returning to the ocean. It is an unavoidable part of their life cycle, and the reproduction of these species (that also have a high economic value) depends upon it. For example, in 2018, sole had the second highest sale value 1 in French fisheries, behind monkfish but ahead of hake.

Irstea’s expertise central to estuary monitoring protocols

Irstea is one of the main observers for the largest estuary in western Europe, the Gironde estuary, and a key stakeholder in the management of French estuaries as a whole. Institute scientists have been gathering data on the biodiversity of the Gironde estuary since the end of the 1970s and therefore benefit from an exclusive perspective on its condition and how it has evolved. Specifically, they have been able to observe the many ecological changes that have occurred, probably as a result of significant environmental changes. For example, the 2000s were characterized by low rainfall, resulting in low river flows and a consequent increase in sea water in the estuaries. These climate events influenced estuary biodiversity, increasing the proportion of marine species such as anchovies in relation to estuary or diadromous species (capable of living in saltwater and freshwater) such as shad.

Irstea’s unique expertise has been strengthened over the last few years through its work to support the European Water Framework Directive (EWFD).2 “We were involved in defining observation protocols and tools to evaluate the ecological state of these ecosystems, particularly fish indicators for estuaries. We coordinated observations for most French estuaries and developed criteria to define good and bad estuary conditions. With our expertise, we are now capable of carrying out an integrated diagnosis of the ecological status of an estuary, which is the first step in implementing adapted management or restoration processes,” explains Lobry. The protocol developed by Irstea scientists is now statutory. As such, it is recommended by the water agencies that implement the monitoring and used by consultancy firms that take inventory.

Our researchers are also looking at the impact of water contamination on fish and are providing various support and expert services to fisheries. The measurements taken by the institute on the state of fish populations are used to review fishing regulations and restore various species.

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  1. According to the 2019 key figures from FranceAgriMer
  2. This directive defines the concept of “good water status,” which all member states of the European Union, including France, must work towards achieving. It defines targets for the conservation and restoration of water and aquatic environments, implemented by a measurement program.