Sensitivity to human activities
Activities listed are those which influence, or are likely to influence this habitat
and which are assessed in the UK marine SAC project review. The sensitivity rank may
require amendment in the light of new information becoming available.
||Development: land claim
|Extensive areas of intertidal mudflats have been removed
through land-claim coupled in some areas with rising sea levels (Davidson et al.
1991; Burd 1992). Some estuaries have lost up to 80% of the available area, most of which
has been the land-claim of intertidal mudflats. The greatest impact of land reclamation is
due to depletion of the main prey rather than simply to area loss and each prey and
predator species will differ in their response (McLusky, Bryant & Elliott, 1992).
Although the area of intertidal mudflats in estuaries is smaller than the subtidal area,
it provides the dominant feeding area for the fish populations (Elliott & Taylor
1989). For example, land claim in the Forth Estuary has removed 24% of the natural fish
habitats but 40% of their food supply (McLusky, Bryant & Elliott 1992). The greatest
effect of land claim is on flatfish such as flounder and juvenile plaice.
||Waste: industrial effluent discharge
|Silt, which is often associated with industrial pollution,
may be deposited onto the mudflats thus raising their height and therefore increasing the
exposure time of infaunal communities at low tide.
|Heavy metal contamination/
|Waste: industrial effluent discharge
|Industrialised and urbanised estuaries and coastlines receive
effluent discharges which contain conservative contaminants i.e. those with a long
half-life, are likely to bioaccumulate (remain within the food chain) and thus have a
toxic effect (Clark 1997). Such contaminants include heavy metals, radionucleides and
synthetic organic compounds. The lethal and sub-lethal effects of these pollutants vary
according to the state and availability of the compound, its characteristics and the
organisms. Some effects may be lethal, by removing individuals and species and thus
leaving pollution -tolerant and opportunistic species. Other effects may be sub-lethal, in
affecting the functioning of organisms such as the reproduction, physiology, genetics and
health, which will ultimately reduce the fitness for survival (Nedwell 1997). Sheltered,
low-energy areas such as intertidal mudflats in enclosed bays or estuaries will be most
susceptible to these pollutants as dispersion is low and the finer substrata in these
areas will act as a sink (McLusky 1982; Somerfield, Gee & Warwick 1994; Ahn, Kang
& Coi, 1995; Nedwell 1997). The pollutants will enter the food chain and be
accumulated by predators, as shown by the seasonal loading of heavy metals in tissues of
wading birds in the Wash (Parslow 1973).
||Uses: boats/shipping (oil spills)
|Oil spills resulting from tanker accidents can cause
large-scale deterioration of communities in intertidal and shallow subtidal sediment
systems (Majeed 1987). Oil covering intertidal muds prevents oxygen transport to the
substratum and produces anoxia resulting in the death of infauna. In sheltered low-energy
areas such as intertidal mudflats pollutant dispersion will be low and the finer substrata
in these areas will act as a sink (McLusky 1982; Somerfield, Gee & Warwick 1994; Ahn,
Kang & Coi, 1995; Nedwell 1997). The pollutants will then enter the food chain and be
accumulated by predators.
|Changes in nutrient levels
||Waste: sewage discharge
|High organic inputs coupled with poor oxygenation leading to
conditions of slow degradation will produce anaerobic conditions in the sediments. In turn
this increases microbial activity and reduces the redox potential of the sediments
(Fenchel & Reidl 1970). Ultimately this increases the production of toxic chemicals
such as hydrogen sulphide and methane. The changed status to anaerobiosis will limit the
sediment macroinfauna to species which can form burrows or have other mechanisms to obtain
oxygen from overlying water. Moderate enrichment provides food to increase the abundance
and a mixing of organisms with different responses increases diversity (Elliott 1994).
With greater enrichment, the diversity declines and the community becomes increasingly
dominated by a few pollution-tolerant, opportunistic species such as the polychaete Manayunkia
aesturina. Such a symptom on intertidal mudflats is an increased coverage by
opportunistic green macroalgae such as Ulva sp. and Enteromorpha sp.
resulting in the formation of green tide mats. Anoxic conditions form below
the mats, reducing the diversity and abundance of infauna (Simpson 1997). In grossly
polluted environments, the anoxic sediment is defaunated and may be covered by
sulphur-reducing bacteria. Such a change will affect the palatability of any remaining
prey and thus impair functioning of marine areas.
|Removal of non-target species
||Collecting: bait digging
|The potential effects of bait digging are to reduce community
diversity and species richness, especially by commercial digging for worms and other
macrofauna on intertidal mudflats (Brown & Wilson 1997). This removal of target
species, leading to community and population changes at the ecological and genetical
levels, will affect predators.