Dredging Activities, Spoil Disposal and Aggregate Extraction

Both dredging and spoil disposal will affect the sediment and hydrographic regimes. In addition, any activities upstream of subtidal sand banks will affect the hydrographic regime and/or the sediment balance. Dredging of subtidal mobile sandbanks will occur where they interfere with navigation and also dredging to deepen and widen channels for shipping will affect the low water mark and thus directly remove parts of the biotope complex. It can cause loss and damage by the indirect effects of increased scour and erosion on artificially steepened slopes.

Dredged material disposal over subtidal mobile sandbanks will occur where the latter are adjacent to dredged areas. However, in areas of strong tidal currents dispersion of dredge plumes may be high and thus the effects minimal. For example, in one of a few cases, dredging of a subtidal mobile sandbank produced low levels of suspended sediment and did not appear to smother the benthos (Poiner & Kennedy, 1984). Furthermore the substratum changed little during sampling.

Dredging and dredged material (spoil) disposal has the potential to:

  • increase the amount of suspended particles and thus influence turbidity, light penetration and primary production of the water column and substratum (Iannuzzi et al, 1996);
  • modify the sediment composition where the dredge spoil is of a different nature to the receiving area (SOAEFD, 1996);
  • smother the benthos, if it is unable to migrate through the material, and clog feeding or respiration apparatus, especially of suspension feeding invertebrates, hence there may be a change in the feeding guilds present;
  • create a disturbed benthic community and possibly reduce the number and diversity of benthic species and affect larval recruitment (Rosenberg, 1977); however, there may be an initial increase in species richness following dredging, possibly due to an increase in available resources or an increase in the number of available niches (Poiner & Kennedy, 1984; Stephensen et al, 1978; Jones & Candy, 1981; SOAEFD, 1996); and
  • produce an upstream addition of sediment-bound contaminants through sediment disturbance or liberation and hence affect water quality. Similarly, as many banks are at the centre of a gyre (see Chapter II), this may increase the delivery and accumulation of polluting materials. However, resuspension and liberation of sediment-bound contaminants may be a short-term phenomenon.

However, it is emphasised that subtidal sandbanks are the result of relatively high energy conditions. As such they will be naturally disturbed by large changes in the hydrographic conditions, e.g. by storms. The ability of the community to recover from sediment disturbance is high because of the predominant mobile nature of the component species (Rees, 1994; Kaiser & Spencer, 1996). Similarly the influx of material either by natural phenomena or man-induced conditions such as dredge spoil, will be accommodated by the community.

Communities in the dynamic and physically-controlled environments of subtidal mobile sand banks will be able to tolerate sediment movement and, after disturbance, will be re-established more rapidly than those in more stable environments. For example, as was shown by an experimental dredging operation carried out in a small sandy bay, with the aim of quantitatively assessing the effects of scallop dredging on the benthic fauna and the physical environment. The infaunal community, which consisted of bivalve molluscs and pericarid crustaceans, which were adapted morphologically and behaviourally to a dynamic environment, did not show any significant changes in abundance or biomass.

The most important effect shown by Eleftheriou and Robertson (1992) was on the epifaunal and large infaunal organisms whereby sessile forms such as polychaetes decreased and the burrowing urchin Echinocardium was reduced in the dredged area. Large numbers of molluscs, echinoderms and crustaceans were killed or damaged by the dredging operations. Very large numbers of the burrowing sandeel Ammodytes were also destroyed. Thus the effect of dredging may be limited to the selective removal some fragile and sedentary components of the infauna, and the destruction of the large epifaunal and infaunal organisms.

The speed of recovery of the benthic community, through an influx of colonising organisms, is dependent on the timing of the dredging in relation to the timing of reproduction and or migration. While this may only be on a few occasions during the year for the macrobenthic organisms, studies of meiofaunal recolonisation after sediment disturbance indicate that partial recovery occurs within a few tidal cycles (Sherman & Coull, 1980) following recruitment through hyperbenthic populations. Both passive suspension and active emergence to ensure rapid dispersion and recolonisation has been observed in harpacticoids (Palmer, 1988) and turbellarians (Giere, 1993).

In addition to dredging and disposal effects, subtidal mobile sandbanks may be in demand for aggregate extraction, depending on the relative mixture of coarse and medium sand and the size, depth and nature of the deposit (IECS, 1995). This may be of particular concern for commercial crustacean populations. Crab and lobster spawning areas occur where sand banks meet rocky areas and thus they are affected by changes to the sandbanks.

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