Estuaries

Compiled by Katarzyna Roszkowska, Poland

 

1. European Unions Natural Marine Habitat types definitions: 1130 Estuaries

PAL.CLASS.: 13.2, 11.2

1.1. Description of the habitat 

Downstream part of a river valley, subject to the tide and extending from the limit of brackish waters. River estuaries are coastal inlets where, unlike 'large shallow inlets and bays' there is generally a substantial freshwater influence. The mixing of freshwater and sea water and the reduced current flows in the shelter of the estuary lead to deposition of fine sediments, often forming extensive intertidal sand and mud flats. Where tidal currents are faster than flood tides, most sediments deposit to form a delta at the mouth of the estuary. Baltic river mouths, considered as an estuary subtype, have brackish water and no regular tide, with large wetland vegetation (helophytic) and luxurious aquatic vegetation in shallow water areas.

1.2. Characteristic species

Plants: Benthic algal communities, Zostera beds e.g. Zostera noltii (Zosteretea) or vegetation of brackish water: Ruppia maritima (= R. rostellata (Ruppietea)); Spartina maritima (Spartinetea); Sarcocornia perennis (Arthrocnemetea). Both species of fresh water and brackish water can be found in Baltic river mouths (Carex spp., Myriophyllum spp., Phragmites australis, Potamogeton spp., Scirpus spp.).

Animals: Invertebrate benthic communities; important feeding areas for many birds.

1.3 Corresponding categories

German classification : "D2a Ästuare (Fließgewässermündungen mit Brackwassereinfluß u./od. Tidenhub...)

1.4 Associated habitats

An estuary forms an ecological unit with the surrounding terrestrial coastal habitat types. In terms of nature conservation, these different habitat types should not be separated, and this reality must be taken into account during the selection of sites.

1.5 References

Brunet, R. et al. (1993). Les mots de la géographie-dictionnaire critique. Ed. Reclus.

European Commission (2007). Guidelines for the establishment of the Natura 2000 network in the marine environment. Application of the Habitats and Birds Directives. Appendix I: Marine Habitat types definitions. Update of “Interpretation Manual of European Union Habitats” (EU interpretation manual).

Gillner, W. (1960). Vegetations- und Standortsuntersuchungen in den Strandwiesen der schwedischen Westküste. Acta Phytogeogr. Suec. 43:1-198.

2. Additional HELCOM Information

2.1. Description of the habitat

Baltic Estuaries are always transition zones, where riverine freshwater meets the brackish water of the sea. Due to wind induced backwater effects, which cause irregular tidal effects, estuaries are determined by an episodically moving mixed water body with high input of organic matter, often forming extensive intertidal sand and mud flats. Thus, they are always connected to the sea, but in many cases semi-enclosed. Mudflats, sandspits and/or barrier islands separate them from the sea. They can be of different shapes such as bay like river mouth areas, deltas or parts of an archipelago.

2.2. Distribution

Estuaries are present around the whole Baltic Sea area. Examples of river mouth areas in Denmark: Gudenĺen-Randers Fjord, Horsens Fjord, Vejle Fjord, Kolding Fjord Sweden: Bräkneĺn, Hagbyĺn, Virĺn, Loftaĺn, Finland: Porvoonjoki river mouth area, Kymijoki river mouth area, Merikarvianjoki, Aurajoki river mouth area, Russia/St Pe.: Neva estuary, Germany: Warnow estuary, Peene mouth area.

2.3. Importance (sub regional, Baltic-wide, global)

Baltic estuaries are considered to be of global importance. They are on one hand areas of high biological productivity in a brackish environment, but on the other hand also ecologically under stress, because of permanently changing physical conditions. They form important breeding, resting, and feeding sites for water birds such as kentish plover Charadrius alexandrinus, black tern Chlidonias niger, common tern Sterna hirundo or mute swan Cygnus cygnus. The vegetation in estuaries can be very rich/diverse, consisting of reeds, sedges and submerged plants. Estuaries are further considered to be very important spawning and nursery grounds for some marine fish species (e.g. herring) and numerous fresh water and migratory fish species. Especially large estuaries with restricted water exchange are playing a very important buffering role for nutrients transported from the drainage area downstream to the Baltic Sea.

2.4. Status of threat/decline

Human activities have caused various pressures in many estuaries, which in some cases resulted in severe damages. Generally, it can be considered that the natural values of the Estuaries in the Baltic Sea region are worsening. More over, the length of pristine shores of estuaries or estuaries in a natural or near natural state is steadily decreasing. The exact abundance or cover of threatened or declining estuaries is not known. Estuaries are under threat or decline in many HELCOM sub-regions. According to the HELCOM Red List of Biotopes and Biotope Complexes (HELCOM, 1998) the Baltic wide threat category is "Endangered".

2.5. Threat/decline factors

Eutrophication and pollution by drainage from farming and forestry and other sources (like traffic and industry) are serious threats and deterioration factors. Other pressures and threats are mainly caused by shipping, construction or enlargement of harbours and marinas in the river mouths. Changes in the water flow due to hydro-technical constructions, such as dams, cascades and river bank control, may also have adverse effects on estuaries. Other threats can be caused by introduction of non-indigenous, invasive species, unsustainable fishery and tourism as well as oil spills and construction of breakwaters. The environmental conditions in river mouths depend highly on inflows from local point sources as well as from the whole catchment area. They are therefore closely related to human activities on land.

2.6. Options for improvement

One of the main solutions to stop and reverse degradation of the estuaries is a genaral protection of this natural habitat type by law. Particularly still unregulated and natural river mouth areas need to become strictly protected sites. Further, programs and measures are needed to maintain or restore natural conditions along the whole course of the rivers, which e.g. allow natural erosion and temporary flooding of river banks. A drastic reduction of nutrient and pollution loads in the catchment area of estuarian rivers would help to improve the environmental situation of the whole Baltic marine area. The introduction of ecologically sound fishing and farming methods is essential in order to reach a more favourable conservation status of the natural habitat type. Additional protective measures would be: restriction of the construction of new port facilities and marinas, regulation of fisheries, prevention of an unregulated growing of tourismn and harmful recreational activities. As for all natural habitat types an inventory and a monitoring and assessment programme (also for human activities) is obligatory for EU Member States. They are further obliged to take all appropriate steps to avoid further deterioration of estuaries. This includes the obligation to protect this natural habitat type within the Natura 2000 network, and thus to designate as many SACs as necessary to guarantee its favourable conservation status. Member States have to follow Article 6 (3) of the Habitats Directive. Plans and projects which are not directly connected with or necessary to the management of a Natura 2000 site but likely to have a significant effect thereon, either individually or in combination with other plans or projects, shall be subject to appropriate assessment of its implications.

2.7. References

Warzocha, J. (2007). 1130 River Mouth (Estuaries). Annex 1 to the Polish report to EC on the results of monitoring and inspection of the preservation status of natural habitat types as well as species of plants and animals mentioned in the annexes to the Habitats Directive.  Marine Fisheries Institute in Gdynia, 4 pp.

HELCOM (1998). Red List of Marine and Coastal Biotopes and Biotope Complexes of the Baltic Sea, Belt Sea and Kattegat - Including a comprehensive description and classification system for all Baltic Marine and Coastal Biotopes. HELCOM-Baltic Sea Environment Proceedings 75, Helsinki Commission. 115 pp. 

Golubkov, S. (2007). Assessment of the state of biological diversity in the Eastern Gulf of Finland applying BSRP SGEH priority indicators, Lead Laboratory on Biodiversity Zoological Institute RAS, St.Petersburg, Russia, 18 pp.

Pastuszak, M., Witek, Z., Nagel, K., Wielgat, M., Grelowski, A. (2005). Role of the Oder  estuary (southern Baltic) in transformation of the riverine material. Journal of Marine Systems Vol. 57, Issue 1-2 pp. 30-54.