Intertidal Sand and Mudflats

Subtidal Mobile Sandbanks

The conceptual model concentrated on the sets of relationships required to structure sedimentary habitats (see linked figures A and B). The information suitable for defining and understanding those relationships can be obtained from several sources, hence the conceptual diagram can thus be extended to indicate which information can be derived from existing databases, by measurement and by other, wide-scale survey techniques such as photography (see linked figure).

It is emphasised that much of the information necessary for determining the changes described below can be obtained rapidly by broad-scale surveys (P in Figure 8.0). These include aerial photography, skilled-eye assessment and Phase 1 surveys of Intertidal Sand and Mudflats, still and video photography and side-scan sonar techniques (such as RoxAnn) for Subtidal Mobile Sandbanks.

The use of the term database (D in Figure) implies that information is from other sources (published and unpublished) and as such it does not necessarily require to derived for a site. For example, the hydrographic information indicating the current regime over Subtidal Mobile Sandbanks can be derived from Admiralty Charts and the tidal regime influencing Intertidal Sand and Mudflats can be obtained from the published Tide-Tables (Office of HM Hydrographer). Similarly, biological information on the reproductive cycles and biology of the dominant faunal species can be derived from the literature, e.g. Rasmussen (1973), Wolff (1973).

The most cost-effective means of assessing change in the biotope complexes will be through an annual cataloguing and monitoring of activities at a qualitative and subjective level. This will indicate areas of human-induced change as well as natural but large-scale modifications. This will also indicate the areas and features or components of the system to be monitored in more detail. If there are no perceived major changes to the system, then a quinquenniel survey should be carried out as a phase 1 (habitat mapping) survey. This should focus on elements at risk of change and lead to a fully quantitative community analysis where change is observed, i.e. outside normal variability.

The interaction between the physical and biological attributes produces several related features which can be used for defining the Favourable Condition of the habitats and Targets for their management. The spatial extent, tidal regime and elevation of the flats dictates the size of the primary consumer populations, which in turn support the fish and birds. The tidal elevation is required to remain within certain limits, as the major component of the biological community, in terms of abundance and biomass, is located within the mid-tidal zone. A decrease in tidal height will force this zone towards the low water mark and thus perhaps expose it to the greater current speeds found close to the low water channels, whereas an increase in tidal height will increase drying periods and thus desiccation of the organisms. The nature of the substratum, both as sediment type and slope, is the result of the hydrographical regime, including the tidal, wind-induced and residual water movements, which will influence the water retention properties, the permeability and porosity, retention and degradation of organic matter and thus the sediment oxygen regime and redox characteristics.

With such soft substratum areas, the main assessment will be of change in sediment type and its relationship to water movements and, in certain cases, freshwater/land runoff. It is relatively easy to categorise which is regarded as sand or mud but there is still variation. For example, a slight variation in one component of sediment particle distribution can result in a substantial change in sediment type and features, e.g. silt increase affecting porosity.

This sedimentary regime reflects whether the zone is a high or low energy flat. This dictates whether the community is dominated by detritus and deposit feeders which will be in large abundance in low energy sheltered mudflats, or lower abundances of mobile small crustaceans or sparse suspension feeding bivalves found in higher energy intertidal sandflats. Whereas the former is utilised by waterfowl and some fishes, sandflats are used more as flatfish nursery areas. Because of this, an assessment of community functioning, as production supporting vertebrate consumers, is of paramount importance.

Within sand and mudflat areas, the important feature of community structure relates more to the abundance and biomass of a restricted set of species than to the presence of rare species. The latter are considered to be of minor importance and unlikely to occur given the homogeneity of the sand and mudflats both physically and biologically and the limited availability of niches.

The community in turn through biomodification (biostabilisation, biodeposition and bioturbation) will influence the sediment structure. This is particularly important for large Lanice, spionid and Zostera beds (biostabilisation), Macoma and Cerastoderma beds (bioturbation) and mussel beds (biodeposition). Microphytobenthos populations, such as diatoms, euglenoids and flagellates, will also increase the stabilisation of the flats.

Table - Summary of Favourable Conditions in Intertidal Sand and Mudflats and Targets for their Maintenance.

These habitats are amongst the simplest to monitor quantitatively although their spatial extent can make monitoring time-consuming. Remote sensing and skilled-eye surveys should be used to indicate the gross features, with repeat surveys carried out during a similar time period each year. Features to be assessed should include: nature of sediment, extent and shape of area and extent of run-off channels, algal, seagrass and mussel coverage.

An annual catalogue of human activities in the area will indicate the likelihood of change either directly or within the hydrographic or sediment cell. The likelihood of large-scale changes should be addressed by a quantitative survey of the sediment type, the shore elevation, the macrobenthos (and algae/seagrass where appropriate), the waterfowl population as winter counts, and the juvenile fish populations using the flats. Where their is no perceived changes as the result of human activities, a quinquenniel assessment of key features is considered suitable. However, this may not be feasible for extensive habitats, such as large estuaries, where an ongoing programme of small scale surveys may be more appropriate.

At present, there exists no quantitative, numerical values for the above attributes against which the compliance monitoring can be carried out (see below). Once these, as monitoring standards, have been developed then the monitoring can test for compliance with such values.

The sensitivity of the Intertidal Sand and Mudflats to natural and anthropogenic changes is discussed in Chapters V and VI. Each of the aspects identified thus will constrain the attributes; for example, as a summary:

• nutrient stimulation of the area, as hypernutrification leading to symptoms of eutrophication, will increase the coverage by opportunistic green macroalgae, such as Enteromorpha. In turn, this may prevent bird feeding on the flats and create anoxic conditions in the sediment below the mats.
• organic enrichment of the mud and sandflats by sewage and other organic discharges will impair functioning through the development of opportunistic populations and anoxic sediment which affect the palatability of prey. Such changes will also occur as a result of industrial discharges.
• land-claim will remove intertidal flats and thus remove productive areas (McLusky et al, 1993), a feature not easily reversed. The change in substratum type and the tidal elevation are the greatest threats to the integrity of the mud and sandflat sites and their carrying capacity in supporting waterfowl and fishes (nursery and adult).
• similarly, a loss of mud and sandflats can occur through an increase in the low water area following channel modification, or upper area through colonisation by pioneer saltmarsh flora. This will lead to a change of habitat and therefore use.
• as the flats are the result of a balance between sediment accretion and erosion, any input of sediment through, for example, hydrographic disruption or dredged-material relocation, will have an effect whose effect depends in size on the nature of the receiving material. For example, an increase of 5% silt and clay in muds will have little effect but the same in sands would have a large effect by preventing percolation and oxygen exchange.
• in addition to any local loss of area, the combined effects of global sea-level rise, isostatic rebound in the southern part of Great Britain and the constraining of the upper tidal region though seawalls will produce the tidal squeeze, again removing productive wetland areas.

Management of the Subtidal Mobile Sandbanks is required to ensure that the Favourable Conditions required for the protection of their integrity should be maintained. The area of sandbank is the result of: the underlying geology, the hydrographic regime incorporating wave, tidal and residual current patterns, and the supply and removal of sediment. While the first of these cannot be influenced anthropogenically other than by mineral extraction, which may produce subsidence, the latter two categories require to be protected by preventing any interference or disruption to the water patterns. Given that most colonisation of the habitats is from external sources, there is unlikely to be a critical size for the sandbanks although this is not known.

The depth of water above the sandbank requires maintenance within natural limits to prevent shallowing leading to exposure on low spring tides, or deepening which would alter the sandbank characteristics, perhaps creating conditions similar to those of the surrounding seabed. In addition, where the bank is dependent on some light penetration, for any algal component of the beds, then any increase in depth will affect that light regime. The hydrographic regime will also affect the water characteristics in terms of salinity, temperature and dissolved oxygen.

The majority of these sandbanks are marine and any reduction in salinity will disturb the marine communities whilst changes to depth and water exchange will affect the temperature and oxygen balance. In addition the upstream addition of polluting materials or the on-site increase in dredging or dredged material disposal will affect water quality. Similarly, where the banks are at the centre of a gyre, this may increase the delivery and accumulation of polluting materials.

Many sandbanks are the result of conical headland gyres being created either side of headlands by residual currents moving either clockwise or counter-clockwise. Such gyres require to be protected in order to maintain the sandbanks, for example those found either side of Flamborough Head. This requires control of coastal developments, which may affect changes in the hydrodynamic system, through the terrestrial and offshore planning process.

Table - Summary of Favourable Conditions in Subtidal Mobile Sandbanks and Targets for their Maintenance.

The sand banks can be modified by extreme natural events such as storms as well as anthropogenic interference with the substratum and hydrographic regime. Where there is no perceived changes due to human activities, the sites should routinely be assessed qualitatively on a five-year basis. However, if there is cause to suspect change has occurred or will occur as the result of human activities such as potential aggregate extraction or modification of the hydrographic regime, then shorter-interval quantitative monitoring should occur. In addition, a record should be maintained of climatic conditions given that large-scale freshwater inputs and severe storms will have a large effect on the areas and thus these data will be required for any prediction and interpretation of biological changes.

The sensitivity of the Subtidal Mobile Sandbanks to natural and anthropogenic changes is discussed elsewhere. Each of the aspects identified thus will constrain the attributes; for example, as a summary:

• such areas may be in demand for aggregate extraction, depending on the relative mixture of coarse and medium sand material, and this will affect the size, depth and nature of the substratum material.
• interference to the hydrographic patterns by dredging, construction of rigs, jetties and harbours will affect the deposition and erosion processes and the delivery of colonising organisms and organic matter. These activities may also reduce the heterogeneity of the bed and thus the number of niches available for colonisation.
• disruption of the light regime through an increase in turbidity as the result of increased land run-off or bed-disruption by dredging and spoil disposal, will affect any algal and angiosperm flora.
• commercial fishing by beam trawling will disrupt the integrity of the bed and damage the larger benthic fauna, especially the epibenthos. The margins of subtidal sandbanks adjoining rocky areas are likely to support edible crab and lobster populations and thus will be the site of commercial potting and creel fishing.

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