Background to saline lagoons in the UK
Distribution and conservation importance
Saline lagoons are defined as areas of shallow,
coastal saline water, wholly or partially separated
from the sea by sandbanks, shingle or, less frequently,
rocks (Brown et al 1997). Five main sub-
types have been identified in the UK, on the basis
of their physiography, as meeting the definition
of the habitat type, and these sub-types have been
used in identifying and designating statutory sites
including SACs (Brown et al 1997) and SSSIs
Isolated lagoons: these are separated completely
from the sea by a barrier of rock or sediment. Seawater
enters by limited ground water seepage or by over-topping
of the sea barrier. Salinity is variable but often
low. Isolated lagoons are often transient features
with a limited life-span due to natural processes
of infilling and coastal erosion. Isolated lagoons
may have less water exchange than percolation lagoons
and consequently a more impoverished biota.
Percolation lagoons: these are normally separated
from the sea by shingle banks. Seawater enters by
percolating through the shingle or occasionally
by over-topping the bank (e.g. in storms). The water
level shows some variation with tidal changes, and
salinity may vary. Since percolation lagoons are
normally formed by natural processes of sediment
transport, they are transient features, which may
be eroded and swept away over a period of years
or decades or may become infilled by movement of
the shingle bank.
Silled lagoons: water in silled lagoons is retained
at all states of the tide by a barrier of rock (the
>sill=). There is usually little tidal rise and
fall. Seawater input is regular and frequent and
although salinity may be seasonally variable, it
is usually high, except where the level of the sill
is near to high tide level. These lagoons are restricted
to the north and west of Scotland and may occur
as sedimentary basins or in bedrock (where they
are called >obs=). Muddy areas are dominated by filamentous
green algae, amongst which may be colonies or rare
charophytes, such as foxtail stonewort Lamprothamnium
papulosum. Beds of tasselweeds, Ruppia
spp. and, in the deeper, most stable lagoon, eelgrass
Zostera marina may be present.
Sluiced lagoons: sluiced lagoons develop where
the natural movement of water between the lagoon
and the sea is modified by human mechanical interference
such as the construction of culverts under a road
or valved sluices. Communities present in sluiced
lagoons vary according to the substrate type and
salinity but may resemble those of silled lagoons.
Lagoonal inlets: seawater enters lagoonal inlets
on each tide and salinity is usually high, particularly
at the seaward part of the inlet. Larger examples
of this sub-type may have a number of different
basins, separated by sills, and demonstrate a complete
gradient from full salinity through brackish to
fresh water. This salinity gradient significantly
increases the habitat and species diversity of the
sites in which it occurs.
The water in lagoons can vary in salinity from
brackish (due to dilution of seawater by fresh water)
to hypersaline (i.e. more salty than sea water as
a result of evaporation). Lagoons can contain a
variety of substrata but most commonly include soft
sediments. The plant and animal communities of lagoons
vary according to the physical characteristics and
salinity regime of the lagoon, and therefore there
are significant differences between sites. Although
a limited range of species may be present compared
with other marine habitats, many of these species
are specifically adapted to conditions of varying
salinity and some are found only in lagoonal habitats.
The vegetation may include beds of eelgrasses Zostera
spp., tasselweeds Ruppia spp, the alga Chaetomorpha,
pondweeds Potamogeton spp. or stoneworts
such as foxtail stonewort Lamprothamnium papulosum.
In more rocky lagoons, communities of fucoid wracks
Fucus spp., sugar kelp Laminaria saccharina
and red and attached green algae are also found.
The fauna is often characterised by mysid shrimps
and other small crustaceans, worms which burrow
into the sediment, prosobranch and gastropod molluscs
and some fish species. Species that are particularly
found in lagoons and consequently have restricted
distributions in the UK include starlet sea anemone
Nematostella vectensis, lagoon sandworm Armandia
cirrhosa, lagoon sand shrimp Gammarus insensibilis
and foxtail stonewort Lamprothamnium papulosum.
Lagoons may also provide important habitat for waterfowl,
marshland birds and seabirds.
Distribution and conservation
Lagoons are localised in Europe and on the Atlantic
coast have a restricted distribution. They are listed
as a priority Annex I habitat in the Habitats Directive.
The habitat type is complex, and a wide range of
physical types and origins can be included in the
broad definition. The habitat is also relatively
uncommon in the UK (and consequently is listed as
a priority habitat under the UK's Biodiversity Action
Plan (UK Biodiversity Group 1999)). However, in
the UK there is a range of geographical and ecological
variation in the habitat type, and some of the types
of lagoon found in the UK are rare elsewhere in
Europe. Therefore, a high proportion of the sites
identified as meeting the definition of the habitat
type have been selected as Special Areas of Conservation
(SACs). Only sites on natural substrata have been
selected as SACs. Sites that are entirely artificial
in origin, e.g. some docks, have been excluded from
the definition, even though, in some cases the communities
present may be similar to those of more natural
sites. Ten sites (incorporating over 60 individual
lagoon sites) have been identified as SACs for lagoons
in the UK (see linked figure
and the table below). As a priority habitat
there are likely to be numerous lagoon SACs in other
EU Member States but no information was available
at the time of writing on sites selected.
Summary of UK lagoon SACs and component individual
lagoons. Data for England based on Bamber 1997,
for Scotland on Thorpe 1998 and Thorpe et al 1998
site, type (and size/ha)
|North Norfolk coast
and Gibralter Point Lagoons
Holkham Salts Hole (0.5), Abraham=s Bosom (1.5),
Half Moon (0.1), New Moon (0.1), Seahorse
Marsh (3.4), Salthouse Broad (3.0), Little
Eye (0.2), W. Gramborough Hill (0.2), E. Gramborough
Hill (0.2). All percolation lagoons.
|Benacre to Easton
Denes Pool (2.1), N.E. Denes Pool (0.7), Easton
Broad (1.7). Isolated: Benacre Broad (8.0),
Covehithe Broad (0.8).
|Orfordness to Shingle
||Aldeburgh P8 (0.3),
Shingle Street #p1 (0.5) #p3 (0.3) #p4 (0.5)
#p5 (0.0003) #p6 (0.1) #p7 (0.25) and Shingle
street Cobb #0 (1.1). All percolation lagoons.
|Solent and Isle of
||Lagoon inlet: Shut
Sluiced lagoons: Fort Gilkicker Moat (4.3),
Normandy Farm (5.0), Eight Acre Pond (3.0),
North and South Salterns Lagoons (1.9), Oxey
(0.93), Pennington (1.54), Keyhaven (0.62)
Isolated lagoons: Farlington Marshes (0.02),
Bembridge Harbour Lagoon (3.0), Harbour Farm
#1 (3.9) and # 2 (3.7).
|Chesil and the Fleet
||Lagoonal inlet (with
characteristics of percolation type to a small
|Loch Roag Lagoons
||Tob Valasay (30.0),
Loch Shader (7.0). Both silled lagoons.
|Obain Loch Euphoirt
||Loch Obisary (300.0),
Oban nam Fiadh (41.0), Oban Sponish (15.0),
Bagh Uaine (2.0). All silled lagoons.
|Loch nam Madadh
||Sluiced: Loch an
Duin (43.0), Alioter lagoon (15.0), Loch an
Strumore (60.0), Bagh Ostram (2.0).
Silled: Bac-a-stoc lagoon (2.0), Loch Minish
lagoon (1.0), Oban nan Stearnan (10.0).
Lagoonal inlet: Loch Houram (37.0), Loch
na Ciste and Strom Ban (12.0), Leiravay Bay
|Loch of Stenness
||Lagoonal inlet (860.0).
||Lagoonal inlet (with
basins separated by sills) (61.0)
of candidate Special Areas of Conservation for marine