Marine Habitats

Marine habitats can be divided into separate areas characterised by the relationship to sea levels.

Coastal saltmarshes comprise the upper, vegetated portions of intertidal mudflats, lying approximately between mean high water neap tides and mean high water spring tides. For the purposes of the biodiversity action plan, however, the lower limit of saltmarsh is defined as the lower limit of pioneer saltmarsh vegetation (but excluding seagrass Zostera beds) and the upper limit as one metre above the level of highest astronomical tides to take in transitional zones. This means that such habitats do get inundated by the highest tides, but not by every tide.

Status in Essex - The Essex estuaries support c.2,878ha of saltmarsh extending from the River Stour through to the north Thames. This represents about 6% of the national total. The evidence shows considerable, long-term loss of saltmarsh in all estuaries and coastal areas. The Stour estuary has suffered greatest loss at nearly 60% and the Colne least at 12%. Overall the net area lost (for the areas we have figures for) between 1973 and 1998 is 25%. This represents a loss of nearly 1000ha at an average rate of approximately 40ha per year. Many saltmarshes are being 'squeezed' between an eroding seaward edge and fixed flood defence walls.

Intertidal Mudflats at lower levels are sedimentary intertidal habitats created by deposition in low energy coastal environments, particularly estuaries and other sheltered areas. Their sediment consists mostly of silts and clays with a high organic content. Towards the mouths of estuaries where salinity and wave energy are higher the proportion of sand increases. Mudflats are intimately linked by physical processes to, and may be dependent on, other coastal habitats such as soft cliffs and saltmarshes. They commonly appear in the natural sequence of habitats between subtidal channels and vegetated saltmarshes. In large estuaries they may be several kilometres wide and commonly form the largest part of the intertidal area of estuaries. However, in many places they have been much reduced by land claim.

Mudflats, like other intertidal areas, dissipate wave energy, thus reducing the risk of eroding saltmarshes, damaging coastal defences and flooding low-lying land. The mud surface also plays an important role in nutrient chemistry. In areas receiving pollution, organic sediments sequester contaminants and may contain high concentrations of heavy metals.

Mudflats are characterised by high biological productivity and abundance of organisms, but low diversity with few rare species. The mudflat biota reflects the prevailing physical conditions.

Status in Essex - With its long coastline of winding estuaries Essex has an important mudflat resource, supporting large flocks of overwintering shore birds, making these areas internationally important. Beds of the common mussel Mytilus edulis occur and give rise to an economically important industry.

Saline Lagoons comprise a third habitat type, essentially bodies, natural or artificial, of saline water partially separated from the adjacent sea. They retain a proportion of their seawater at low tide and may develop as brackish, full saline or hyper-saline water bodies.  Lagoons can contain a variety of substrata, often soft sediments which in turn may support tasselweeds and stoneworts as well as filamentous green and brown algae. In addition lagoons contain invertebrates rarely found elsewhere, often in huge numbers. They also provide important habitat for waterfowl, marshland birds and seabirds. The flora and invertebrate fauna present can be divided into three main components: those that are essentially freshwater in origin, those that are marine/brackish species and those that are more specialist lagoonal species. The presence of certain indigenous and specialist plants and animals make this habitat important to the UK's overall biodiversity.

There are several different types of lagoons, ranging from those separated from the adjacent sea by a barrier of sand or shingle ('typical lagoons'), to those arising as ponded waters in depressions on soft sedimentary shores, to those separated by a rocky sill or artificial construction such as a sea wall. Sea water exchange in lagoons occurs through a natural or man-modified channel or by percolation through, or overtopping of,  the barrier. The salinity of the systems is determined by various levels of freshwater input from ground or surface waters. The degree of separation and the nature of the material separating the lagoon from the sea are the basis for distinguishing several different physiographic types of lagoon.

Status in Essex - drainage channels behind seawalls associated with garzing marshes can be saline, and pools on saltmarsh provide another type of saline lagoon.