Seagrasses are generally not physically robust. Their root systems are
typically located within the top 20 cm of the sediment and so can be dislodged easily by a
range of activities, including trampling, anchoring, digging, dredging and powerboat wash
Physical disturbance can reduce the stability of Zostera beds.
The removal of plants typically results in increased patchiness. This may destabilize the
bed and increase the likelihood of additional losses. Disturbance can lead to reduced
sedimentation rates or increased removal of sediments. Sediment removal can also increase
turbidity, which may affect the success of Zostera reestablishment (Holt et al.,
Trampling may be caused by recreational activities such as walking,
horse-riding and off-road driving. Some watersports (eg. swimming, windsurfing) may result
in damage to subtidal beds. Trampling damage may also be caused by environmental
mitigation work. Thom (1993) reported that Z. marina beds in Washington State were
damaged by trampling when mitigation work was being carried out in response to crab
mortalities. Trampling damage resulting from oil clean-up attempts has also been reported.
After the Sea Empress oil spill, near Milford Haven in Wales, damage to Zostera
appeared to be limited to those plants living on areas of shore traversed by clean-up
vehicles (SEEEC, 1996).
Zostera beds are particularly vulnerable to physical disturbance of
the sediment caused by activities such as anchoring, hand-gathering of cockles,
bait-digging, dredging or suction dredging. The wash from powerboats and jet skis can also
cause physical disturbance to the sediment. Rhizomes are damaged or broken-up and seeds
are removed or buried too deeply for successful germination. The frequency and season of
such activities are important in determining the level of impact.
Cockle collection can be particularly damaging, as cockle beds and Zostera
beds are frequently associated. Perkins (1988) discussed cockle harvesting by suction
dredges in the Solway Firth. Harvesting of cockles by hand is a traditional practice here,
but with the introduction of mechanical dredgers, the fishing effort rose dramatically
between 1987 and 1992. In undredged areas, the substratum was characteristically hummocky
and covered with abundant Zostera. In dredged areas, the substratum surface was
smoothed and no Zostera were present. Perkins observed that the removal of Zostera
was accompanied by a loss of silt from the substratum and suggested that this fishery
could cause widespread damage or even completely eradicate Zostera from the bay.
Due to concerns over the sustainability of this fishing activity, the impacts on cockle
and Zostera stocks, and the effects on overwintering wildfowl, this fishery was
closed to all forms of mechanical harvesting in 1994 (Solway Firth Partnership, 1996).
In Strangford Lough, a practice known as sand ploughing
where farmers drive onto the mudflats and pull ploughs through the sand to remove rust, is
a notifiable operation under the ASSI regulations, due to the damage it causes to
intertidal Zostera beds and invertebrate communities.
However, physical disturbance can have positive consequences in certain
circumstances. Rae (1979) found that small-scale disturbance encouraged new growth of
intertidal Zostera in the Moray Firth. She suggested that this could be due to the
opportunistic colonization of newly-disturbed sediment when seeds or viable rhizome
fragments were deposited in newly created hollows on the shore or when viable but deeply-
buried seeds were brought closer to the surface where they could germinate successfully.