German Federal Agency for Nature Conservation (BfN)



Habitat destruction

Both extraction methods cause massive disturbance to the marine environment.

Local impacts include up to 80% loss of benthic communities and destruction of unique habitats. Depending on local geological and hydrological conditions and the type of ecological community at a site prior to sediment removal, it may take benthic flora and fauna communities years or even decades to recover their former colonisation structure and species composition. If changes are too extensive, species previously found at a site will be unable to recolonise at all.

The probability of dredging pits and furrows refilling and the length of time this takes vary widely according to water depth, sediment type, exposure and dredging method. Dredging traces are particularly long-lasting in gravel/sand deposits, where hydrodynamics, sediment dynamics and sediment availability do not permit replenishment or even regeneration of the sea floor. Dredging or subsequent natural replenishment can also lead to changes in sediment type compared with the prior state.

Changes in particle size can hinder or even prevent the return of a site’s former benthic organisms by recolonisation from nearby. Local demersal currents can also modify the furrows and pits in the sea floor.

The large pits left by a cutter suction dredger can develop low oxygen zones (‘dead’ zones) that will not be recolonised by benthic organisms. 

Impairment of the food web

Sand eels are particularly hard hit by the effects of sand extraction. They prefer well-oxygenated coarse sand sediment, which they burrow into and lay their eggs in. Sand eels are a major food source for birds, grey seals, harbour porpoises and fish. Hence if sand eels and many other benthic organisms such as mussels, snails and worms lose their habitat, then dredging impairs the entire food web. The consequences can thus spread to other groups of animals.

Disturbance through underwater noise

Marine mammals such as grey seals and harbour porpoises are exposed to severe underwater noise from dredging vessels and dredges. This noise exposure has direct adverse effects on the animals (see also
underwater noise – impacts). A site off the Isle of Sylt where sand is dredged for coastal replenishment, for example, is close to a designated protection area for harbour porpoises, which use the area as a nursery.

Long-distance impact of turbidity plumes

Turbidity plumes are a far larger-scale problem. They arise when dredging itself stirs up sediment and when sand and gravel is washed or screened on board ship. Superfluous fine sediment is then released back into the sea where it is caught up and drifts along in currents. When this fine material ultimately drops to the sea floor, it can fall on other ecological communities remote from the excavation site, such as valuable, biologically diverse reefs. Reef fauna such as some mussels and sea squirts that filter microscopic particles out of the water can be impaired in their filtering ability and die.

Turbidity plumes can also directly impact fish, as the plumes significantly impair the viability of fish eggs.