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An estuary is a partly enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea.

Estuaries form a transition zone between river environments and maritime environments and are subject to both marine influences, such as tides, waves, and the influx of saline water; and riverine influences, such as flows of fresh water and sediment. The inflows of both sea water and fresh water provide high levels of nutrients in both the water column and sediment, making estuaries among the most productive natural habitats in the world.

Under the Water Framework Directive they are refered to as Transitional Water Bodies. In England these have been defined in this manner:

Transitional waters Upper estuarine regions that are tidal but freshwater were included as part of the lower reaches of river water bodies and typed accordingly. The land-ward limits of transitional water bodies were determined by the chloride content. The seaward limits of transitional waters utilised existing boundaries from the Urban Waste Water Treatment Directive (UWWTD) where appropriate. In some cases the use of these boundaries meant that some waters were predominantly characterised by fully marine and were further defined using available salinity data to indicate an appropriate seaward boundary for the transitional water. The bay closure lines for UWWTD were retained to define near shore coastal water bodies.

Saline lagoons A list of saline lagoons occurring in England was provided by English Nature (EN) and the Joint Nature Conservation Council (JNCC), and in Wales by the Countryside Council for Wales (CCW). Lagoons were identified in the original data sets as being either sluiced, percolation or isolated. Where a number of lagoons of the same type occurred in close proximity, these were clustered together, and the resulting cluster was used as the water body unit. Lagoons or lagoon- clusters of less than 5 ha were not counted as water bodies. Lagoons or lagoon-clusters of less than 50 ha were only identified as discrete water bodies if they fell within a Special Area of Conservation (SAC) or Special Protection Area (SPA).

The UK adopted Typology System B, using the following factors which were obligatory for transitional waters;

  • latitude and longitude: defined by the ecoregion, which for the UK are the Atlantic and North Sea ecoregions.
  • salinity:. Transitional waters have been defined as either polyhaline or mesohaline or just predominately polyhaline.
  • tidal range: differentiated using the agreed definitions in the Common Implementation Strategy 2.4 guidance i.e. micro-tidal < 1m, meso-tidal 1-5 m, and macro-tidal > 5m.

Optional factors used in defining types included for;

  • Transitional: current velocity, wave exposure, mixing characteristics, mean substratum composition and extent of intertidal area (depth)were identified as a specific type; and additional types were required to describe coastal waters; sea lochs and coastal lagoons.

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