Dredging and disposal: Organic matter and nutrients

The release of organic rich sediments during dredging or disposal can result in the localised removal of oxygen from the surrounding water. Depending on the location and timing of the dredge this may lead to the suffocation of marine animals and plants within the localised area or may deter migratory fish or mammals from passing through. However it is important to stress that the removal of oxygen from the water is only temporary, as tidal exchange would quickly replenish the oxygen supply. Therefore, in most cases where dredging and disposal is taking place in open coastal waters, estuaries, bays and inlets this localised removal of oxygen has little, if any, effect on marine life (Bray, Bates & Land 1997). However, despite the temporary nature of the effect, if oxygen depletion were to occur during important life stages of sensitive species, such as the peak spring migration of salmon and sea trout smolt (young) through estuary and bay habitats, the effects could be adverse. The Environment Agency has the general duty to maintain and protect freshwater fisheries, including salmon and sea trout, with jurisdiction out to 6 miles from freshwater baselines.

The resuspension of sediments during dredging and disposal may also result in an increase in the levels of organic matter and nutrients available to marine organisms. This can result in two main effects:

  • In certain cases, such as environments adapted to low nutrient conditions or sensitive to the effects of eutrophication which can simply be described as nutrient enrichment leading to the formation of algal blooms. These blooms can reduce the surrounding water quality by causing the removal of oxygen as the blooms break down or occasionally by the release of toxins which may disturb marine wildlife. The potential formation of algal blooms in coastal and estuarine areas is generally limited by high turbidity levels and tidal flushing (ABP Research R701 1997), however blooms are known to occur in certain marine SACs, particularly during spring and summer months.
  • In other cases, increased organic material, nutrients and algal growth may provide more food for zooplankton and higher organisms, with possible knock-on effects on the productivity of the marine ecosystem. For example, there is evidence of increased productivity of benthic communities surrounding a disposal site in Liverpool Bay that receives considerable amounts of dredged silts. The beneficial effects are reported to be a result of organic enrichment from the dredged material and due to the stabilisation of sediments through the incorporation of fine organic matter (Murray 1994b). Increased suspended sediments as a result of dredging operations in the Walney Channel, Morecambe Bay may have resulted in increased numbers of filter-feeding brittlestar and fanworm (George et al 1996). However, if the communities that are present in the vicinity of disposal sites rely on low nutrient levels then any nutrient enrichment is unlikely to be beneficial.

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