Estuaries – when rivers meet the sea

Estuaries cover a wide range of habitats from riverbank deciduous forests to submerged marine aquatic plant communities. The estuary includes everything that is affected by the river’s flow.

Estuaries can be divided into three zones:

  1. The estuary or innermost area, which is dominated by diverse vegetation
  2. A river bay strongly influenced by the run-off effect. The accumulation of suspended solids makes it difficult for vegetation to become established in this area
  3. An estuary with a distinct gradient from fresh to saltwater

From the Baltic Sea perspective, this text mainly considers this third marine zone influenced by the river currents, as well as its associated suspended solids. 

Estuaries are constantly changing environments

The environmental conditions of estuaries are affected by the varying flow of the river. Freshwater from the river mixes with saltwater pushing in from the sea at the river mouth. Occasionally river water carries accumulated suspended solids and nutrients from the catchment area far out to the sea.

Thus, estuaries spread slowly towards the sea, i.e. at a rate of up to tens of metres per year.

For the most part, water in the inner estuary remains salt-free, which is also reflected in the flora and fauna of the area. By contrast, in the outer parts of the estuary, the water characteristics are influenced by regional differences, as well as the salinity of the surrounding water. 

The plant communities of estuaries grow mainly under water

Some of the estuarine plant species live submerged, others have floating leaves or reach above the water surface. The species composition of an estuary varies according to the biogeographic location and the bottom sediment.

The suspended matter carried by water currents is generally very fine and soft. The first plant species to establish themselves in such soft sediments are rushes (Schoenoplectus spp.) and lake reeds (Phragmites australis). In some plant species, the shoots that reach for the water surface are produced by root-like rhizomes, which  help to bind the sediments in place, thus making it easier for other plants to settle also.

In the deeper parts of the estuary which are affected by freshwater, common species include the European white water-lily (Nymphaea alba) and the yellow water-lily (Nuphar lutea). As both salinity and exposure increase, the bottoms become dominated by pondweeds (Potamogeton spp., Stuckenia spp.) and water milfoils (Myriophyllum spp.). The lushest waters may also have large growths of loose floating vegetation.

 Marsh marigolds blossom by the river side
The riverbanks are often decorated by the bright yellow flowers of marsh marigolds.

Birds and invertebrates thrive in estuaries

Extensive estuaries are important habitats, especially for waterfowl and waders, where many birds of prey, as well as passerines, i.e. songbirds, also thrive. Mammals, such as muskrats, are also a common sight crawling around the muddy roots of aquatic plants.

A diverse community of invertebrates and insect larvae find their home in the bottom sediments and the shelter of the aquatic plants. Those parts of the community that are permanently submerged provide the perfect spawning and juvenile nursery grounds for many species of fish.

 Bulrushs grow by the river side
Many bird species nest in the sheltered reedbeds of estuaries.

Estuarine species

  • Rushes (Schoenoplectus spp.) 
  • Lake reed (Phragmites australis
  • European white water-lily (Nymphaea alba
  • Yellow water-lily (Nuphar lutea
  • Pondweeds (Potamogeton spp., Stuckenia spp.) 
  • Water milfoils (Myriophyllum spp.) 
  • Muskrat (Ondatra zibethicus)