Saline lagoons

Saline lagoons are bodies of brackish to hyper-saline water that are partially connected with the sea. Some exchange of seawater occurs with the sea through over-topping an impermeable barrier, by percolation of sea water through the sediment or via man made sluices. Tidal range is greatly reduced or non-existent in the lagoon, and so there is little exposure of bed sediment. Saline lagoons occur in low-lying, often soft sediment coastlines, generally south and east England.

36 species are particularly associated with saline lagoons. 25 are confined to them: four plants, 20 invertebrates and one bird – the avocet. These are some of the rarest plant and animal communities in the UK, and some species are endemic to only one or two lagoons.

There are only about 360 saline lagoons covering 5,200 ha in the UK and they are listed as a priority Natura 2000 community.

There is potential to create this habitat from sand and gravel extraction and other surface mining activities adjacent to the sea. They quickly develop an outstanding value to wildlife. Their extremely high conservation importance makes saline lagoons a high priority when planning restoration of coastal mineral sites.

Examples of sites already created by mineral extraction include:

Killingholme – clay extraction, Humberside
Cliffe Pools – clay extraction, North Kent,
Snettisham Pits – gravel extraction, Norfolk.

With good design, new saline lagoons can be of outstanding value for breeding, passage and wintering birds, together with for specialist flora and fauna. However, location should be carefully considered, to avoid conflict with existing valuable habitats, such as shallow and/or seasonally dry brackish pools. Complement these rather than destroy them.

Areas of low-lying coast are likely to be affected by sea level rise in the foreseeable future. Many of the existing lagoons are vulnerable to the coastal erosion that may result. However, managed re-alignment of coastal defences to counter the effects of sea level rise can create opportunities for lagoon creation and coastal mineral extraction sites can be built into such plans.

Coastal mineral sites are attractive for disposal of dredgings. This can present a specific opportunity to create saline lagoon habitat with control over its design. The dredging material can be used over a wider area to create pools with a range of water depths, to an average of 1m. Bunds will aid water retention and salinity control.

Design

Lagoons require regular, but not free, exchange of seawater, either by percolation through a porous barrier, such as shingle, by over-topping during high tides, or through a channel.

The most species-rich lagoons have a salinity between 20 and 35 parts per 1000, are less than one metre deep and have a large edge to area ratio. However, certain specialist lagoon species (Baltic stonewort, bearded stonewort and bird’s-nest stonewort) prefer lower salinities.

A freshwater input, either from rainfall, surface or sub-surface movement or by managed input is needed. Careful hydrological assessment is needed to ascertain existing supply before creating a new one (e.g. by adding surface channels). Lagoon species diversity is often highest where there is a cline in salinity across the lagoon. Hyper-saline lagoons occur where evaporation exceeds freshwater input. These lagoons usually have lower species diversity, but support specialist species that occur nowhere else.

A tidal exchange of up to 40% of the water volume produces conditions suitable for high species diversity. Constructed lagoons usually incorporate sluices or weirs to balance salinities and control lagoon water levels. The location and design of these structures is critical to the lagoon’s success.

Constructed lagoons, particularly those in, or at the outflow of even small catchments, are liable to receive high volumes of fresh rainfall-derived water. Suitable sluices need to be incorporated remove this quickly to keep salinities within the tolerances of the specialist species. Fine tuning of levels is important to provide suitable shallow feeding areas for nesting birds such as avocets, so sluices should be used that can accommodate this.

For sufficient tidal exchange, inlet structures should allow seawater ingress at a level a little below that of neap high tide.

Habitat design

Landforming should provide:

Extensive areas of lagoon bed less than one metre deep.
Deeper sumps that hold water where water level management cannot prevent levels from dropping, and buffer the effects of evaporation on salinity.
Submerged reefs, set across the prevailing wind to limit wind fetch - which can cause erosion, and to reduce turbidity.
Shallow margins to provide areas for birds to feed.
A number of low islands that facilitate nesting by a range of important species. Where positioned across the wind these will also reduce wind fetch.

Three types of lagoon can be created:

Shallow and artificial lagoons– water exchange is usually partly natural and the lagoon may support both specialist lagoon species and important bird assemblages.
Shallow scrapes or floods – tideless systems with artificially controlled input. They may support important assemblages of breeding waders, and wintering waders and wildfowl, but are unlikely to support specialist lagoon species.
Deep and artificial lagoons – these may be separated from the sea by a permeable barrier, e.g. shingle, or sluices or weirs. They may be colonised by specialist lagoon species and can support important little tern colonies where suitable islands exist.

Establishment techniques

Colonisation of new lagoons is by passive dispersal, i.e. eggs or larvae drifting on currents and tides. The time taken for target species to colonise lagoons will depend to some extent on the method of exchange with the sea; lagoons with direct exchange, such as over-topping, rather than percolation, can be colonised by some species very quickly.

Some material could be introduced to assist with colonisation, particularly tasselweeds (Ruppia spp), which can support a number of faunal species. Note a licence may be required to capture and transport biological material as many saline lagoon species have special protection - consult Natural England locally. Channel inlets and sporadic inflows of large volumes of seawater may be important for colonisation.

Long-term management

It is necessary to maintain the exchange of sea water: to balance salinities, to manage water level particularly during the bird nesting season, and to ensure that emergent plants such as the common reed do not encroach the habitat. A planned programme of management and monitoring is required. This need not be onerous, but is critical given that some of the specialist species have very precise requirements. The needs of birds, such as the avocet, and specialist lagoon species may not be compatible, so objectives need to be considered at the outset.

Relevant case study

Cliffe pools case study

The information set out within this advisory sheet in no way constitutes legal or regulatory advice and is based on circumstances and facts as they existed at the time Nature After Minerals compiled this document. Should there be a change in circumstances or facts, then this may adversely affect any recommendations, opinions or findings contained within this document

Design

Habitat design

Establishment techniques

Long-term management

Further reading

Relevant case study