Physical Disturbance

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 (Fonseca, 1992).

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., 1997).

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.

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