Estuaries are made up of a number of different habitat types. They are defined as the part of a river valley extending downstream from the limit of brackish water and subject to the tide. Estuaries have typical characteristics including a gradient of salinity from freshwater in the river to increasingly marine conditions towards the open sea, input of sediment from the river and seawater coming into the estuary and variable shelter from wave action and tidal flows within different parts of the estuary, with extensive intertidal sediment flats and sediment-filled subtidal channels often developing in areas of low current flow.
The three main estuaries of Pen Llŷn a’r Sarnau are the Glaslyn Dwyryd, the Mawddach and the Dyfi. All three of these estuaries are classified as bar-built estuaries that characteristically have a sediment bar across their mouth and are partially drowned river valleys that have subsequently been inundated. Pen Llŷn a’r Sarnau has some of the best examples of this type of estuary in the UK.
The mosaic of habitats within estuaries supports a large variety of different wildlife communities. In the intertidal and subtidal sediments, communities of worms, crustaceans and molluscs vary depending on the type of sediment, the salinity gradient and degree of exposure of the sediment to wave action and tidal streams. Where there is rocky habitat, communities of green and brown seaweeds generally develop with some communities’ characteristic of the variable salinity conditions. Transitions from saltmarsh to brackish, maritime and freshwater communities support their own particular assemblages of plants and animals.
In addition to the many and varied wildlife communities living in or attached to sediment and rock substrate, estuaries also support an assemblage of mobile species. Estuaries can provide important nursery areas for fish species and also provide a means by which migratory fish species make the transition between the marine and freshwater environments.