Recovery of benthic communities following dredging activities

During all dredging operations, the removal of material from the seabed also removes the animals living on and in the sediments (benthic animals). With the exception of some deep burrowing animals or mobile surface animals that may survive a dredging event through avoidance, dredging may initially result in the complete removal of animals from the excavation site.

Where the channel or berth has been subjected to continual maintenance dredging over many years, it is unlikely that well-developed benthic communities will occur in or around the area. It is therefore unlikely that their loss as a result of regular maintenance dredging will significantly effect the marine ecology of SACs. However, certain marine species and communities are more sensitive to disturbance from dredging than others. For example, dredging where maerl beds (calcified seaweed) or Sabellaria reefs (reef forming marine worms) are present may result in the irreversible damage of these sensitive, slow growing species. These are important habitats, generally associated with the Annex I habitat subtidal sandbanks, found in only a few UK marine SACs (Birkett et al 1998). It is, however, unlikely that such sensitive marine communities would develop in close proximity to the disturbed habitat of a regularly maintained navigation channel.

The recovery of disturbed habitats following dredging ultimately depends upon the nature of the new sediment at the dredge site, sources and types of re-colonising animals, and the extent of the disturbance (ICES 1992). In soft sediment environments recovery of animal communities generally occurs relatively quickly and a more rapid recovery of communities has been observed in areas exposed to periodic disturbances, such as maintained channels i .

A review of dredging works in coastal areas world-wide showed that the rates of recovery of benthic communities following dredging in various habitats varied greatly (Nedwell & Elliot 1998; Newell, Seiderer & Hitchcock 1998), which is indicated as follows:

Recovery rates were most rapid in highly disturbed sediments in estuaries that are dominated by opportunistic species. In general, recovery times increase in stable gravel and sand habitats dominated by long-lived components with complex biological interactions controlling community structure.

These findings are supported by studies of the Georgia Estuary system, USA, which suggest that maintenance dredging has only a short term effect on the animal communities of the silt and clay sediments. Although almost complete removal of organisms occurs during dredging, recovery begins within 1 month and within 2 months the communities were reported to be similar to pre-dredge conditions (Stickney & Perlmutter 1975).

Other studies suggest that dredging impacts are relatively short term in areas of high sediment mobility (Hall, Basford & Robertson 1991). For example, the complete recovery of benthic animals in a channel in the estuarine Dutch Wadden Sea occurred within 1 year of the removal of sediments from this highly mobile sand environment (Van der Veer et al 1985).

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