Impacts of Aggregate Dredging

Removal of habitat
1. Areas near dredging site may be of conservation significance or important nursery and feeding grounds for commercially valuable species e.g. Queen Scallop (Aequipecten opercularis) and edible crab (Cancer pagurus).
2. Full restoration of fauna and sediments can take 2-3 yrs from cessation of dredging - short term dredge areas. But in the UK most licensesfor 15 years
  1. Changes in sediment composition, associated communities change.
    1. Organisms can be associated with certain sediment types/particle size (Warwick & Uncles 1980, Severn Estuary)
      1. Different faunal composition of silty sand to coarser sand
    2. Increase in the proportion of fine sands on increase in gravel
  2. Long term effects not known but areas around UK still not recovered after 7 years
    1. Recovery often only assessed on parameters of biomass, abundance and diversity
      1. But restructuring of biomass dominants and age structures could take longer than other community attributes, perhaps 15-20 yrs (Newell et al, 2004)
        Newell et al, 2004.Studied impacts of dredged sites, surrounding areas and axis of transport of discharged materials
        Impacts on bathymetry and sediment composition
        Significant differences in sediments in heavily and lightly dredged sites
        Benthic communities significantly different between dredged and undredged sites
        Major suppression of species richness, population density, biomass and average body size
        Decreased 'equilibrium' species e.g. bivalves & echinoderms
        More 'opportunistic species well adapted to disturbed sediments
        An impoverished community dominated by polychaetes
        A 'footprint' of the impacts of dredging and screening in surrounding areas
        Majority of sediments settle within 500m but evidence of 2km
        'Footprint' can extend 2-3km along axis of net sediment transport
        Distance depends on net sediment transport at the seabed
3. Immediate loss of fauna in extraction area
Impacts on Marine Life
1. Direct - removal of sediment
  1. Severe physical disturbance of seabed. Physical effects seen for up to 7 years after dredging ceased (at a moderate energy site)
    1. Weathered dredge tracks - marks the seafloor
2. Indirect - Sediment resuspension
  1. Indirect impacts may be more important than direct impacts. Can result in reduced species richness, low densities & reduced biomass
  2. Plumes of suspended material from action of dredge head itself - underwater
  3. 1) Outwash of material from spillways from vessel generate much more. 2) Rejection of unwanted sediment
    1. These two termed "surface plumes"
      1. Extent depends on particle size, volume, speed of discharge and local hydrodynamics
        Sand particles usually within few 100m, finer particles transported much greater distances from point of discharge
      2. Impact depends on habitat type
        Low levels of natural disturbance - more significant. Encrusting epifaunal taxa affected by abrasive action of suspended sediment
        Levels of natural disturbance high with high levels of suspended sediment due to waves and tidal action - less impact
Suggested Mitigation Measures
1. Reduction of overboard screening; material accumulates at dredge site, means more dredging and more discharge required in the exploited area over time
2. Adjustment of time for exploitation; dredging only when tidal stream will carry sediments away
3. Establishment of refuge areas to enhance recolonisation; non-dredged areas left between areas of dredged seabed
4. Measures need to be cost effective to really be implemented
21% of the sand and gravel used in England and Wales is extracted from the marine environment
1. Extracted using huge ships - in UK usually trailer dredging, sometimes static suction hopper dredging
  1. Useful materials are washed and screened, rest thrown back overboard

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Impacts of Aggregate Dredging

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	Creado por Madison Burt hace más de 10 años