The long-term survival of Zostera beds requires an equilibrium between the processes of sediment accretion and erosion. Water clarity is also very important, as this affects the amount of light available for photosynthesis and so determines the depth to which the plants can grow. Many forms of coastal development can strongly influence the local hydrographic regime, causing profound changes in rates of sedimentation and erosion, and increasing the quantities of suspended sediment in the water column. The most common activities of this kind include:

• Construction of docks, piers, coastal defences and marinas
• Pipeline laying
• Channel dredging
• Land reclamation
• Seabed or water extraction

Increased sediment erosion has been strongly implicated in the loss of seagrass (Posidonia sp.) beds in the Mediterranean (Boudouresque & Meinesz, 1982) and Posidonia and Heterozostera beds in Australia (Shepherd et al., 1989). In both the Mediterranean and Australia, it has been shown that such seagrass losses can be self-perpetuating. Sediments that are no longer stabilized by seagrasses erode more quickly and the turbidity resulting from the increased sediment load in the water can lead to further degradation of beds (Shepherd, et al., 1989).

Increased sediment accretion has caused losses of seagrass beds in several parts of Australia (Shepherd et al., 1989). Reduction in light penetration is a major contributory factor in these declines but it has been suggested that other factors such as changes in the redox potential of surface sediments may have additional impacts (Thayer et al., 1975). Sediment accretion around the cofferdam for the Second Severn Crossing appears to have caused a decrease in the extent of the Zostera bed in the Severn Estuary pSAC (M. Hill, pers. comm.).

Butcher (1941a) suggested that the decline of Zostera beds in the Solent prior to the 1950s, may have been related to dock construction and channel dredging. Tubbs (1995) suggested that in the estuaries of southern and eastern England, where Zostera was formerly widespread and abundant, an adverse silt budget arising from a steepening shore profile, associated with a rise in sea level, may have limited recolonization.

Giesen et al. (1990a, b) suggested that in the Wadden Sea, increased turbidity caused by deposit extraction and dredging activities (and exacerbated by eutrophication) was a major factor in the decline of Zostera in the 1970s and 1980s and that fluctuations in salinity and temperature were of minor importance.

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