Access Number 30980 

Study Title: Integrated Report On The Impact Of Marine Aggregate Dredging On 

Physical And Biological Resources Of The Seabed 

Report Title: Integrated Report On The Impact Of Marine Aggregate Dredging On 

Physical And Biological Resources Of The Seabed – Final Report 

Contract Number: 14-35-01-99-CT-30980 

Sponsoring OCS Region: Headquarters – International Activities and Marine Minerals 

Division 

Applicable Planning Area(s): OCS 

Fiscal Years of Project Funding: FY 1999 to 2002 

Completion Date of Report: October 2002 

Costs: FY 1999 - $229,713; FY 2000 – $109,266; FY 2001 - $48,532; 

FY 2002 - $10,970 

Project Manager: David R. Hitchcock, Ph.D. 

Affiliation (Project Manager): Coastline Surveys Ltd 

Address: Bridgend Farmhouse, Downton Road, Bridgend, Gloucestershire GL10 2AX, 

United Kingdom 

Principal Investigators: David R. Hitchcock, Ph.D., Richard C. Newell, Ph.D., D.Sc., 

Lindsay Seiderer, Ph.D. 

Key Words: Aggregates, Dredging, Marine Mining, Benthic Resources, Environmental 

Impact, Dredge Plume, Sand and Gravel, Screening 

Background: The project encompasses several key elements of interest to INTERMAR 

and the MMS's Marine Minerals Programme. Principally, the requirements for integrated 

information regarding the impact of benthic and surface sediment plumes, on physical 

and biological resources of the seabed, previously identified as a key requirement in 

several INTERMAR and State/Federal Task Force documents, are addressed by this 

project 

Objectives: Overspill and, more importantly, screened and rejected material will 

disperse downstream and cause an impact. It is important that this is quantified. There 

have been many predictions of dispersion by modelling techniques and many of these 

have had to be modified substantially in the light of our fieldwork programmes. 

Assessment of the actual impact on the seabed (physical and biological) will enable 

correlation with the predicted dispersion of material, actual dispersion of material 

(footprint) and impact of settled material (footprint of significant impact, rather than any 

impact no matter how small). 

• Does the use of ADCP techniques supported by traditional water sample 

characterisation still provide a best value approach to defining the gross morphology 
Access Number 30980 

of the dispersing plume and any sub-divisions attributable to different sources and 

processes? 

• Is there a detectable impact on the sedimentary provinces that may be caused by 

marine aggregate mining? 

• Can high-resolution sidescan sonar mosaic imagery provide broad scale 

mapping at sufficient resolution to identify any impacts due to mining operations 

either due to changing sedimentary or biological community? 

• Is there a detectable impact of marine aggregate mining on key features of 

benthic biological community structure including species diversity (S), population 

density (N), biomass (B) or body size (B/N)? 

• Is there a detectable impact on community structure as assessed by nonparametric multivariate techniques? 

• How far beyond the immediate limits of the dredged area do such impacts 

extend? 

• Can any impact on community structure beyond the boundaries of the dredged 

area be related to ‘far-field’ deposition of material in the outwash? 

• Are there differences in impact of anchor dredging where material is essentially 

exploited at one site, and trailer-dredging over a wider area? 

• What is the nature and rate of the recolonisation and recovery processes in a 

commercially dredged area? 

• Can any recommendations be made on the scale, frequency and number of 

samples required for cost-effective monitoring of the impact of sand and gravel 

mining on sea bed resources? 

Description: An integrated study of the impacts of dredging on the physical and 

biological resources of a non-screened dredge area on the South Coast of Britain has 

been completed. This site has been dredged since 1991 by anchored suction dredgers 

and occasionally (since late 1998) by trailing suction dredgers, removing a total of 

nearly 2 million tonnes over that period. The gravelly resource has been extracted from 

a very small target area of roughly 400m x 400m, within a larger licence. 

 Over 350km of high-resolution sidescan sonar mapping has confirmed the areas 

that have been dredged by observing the extent of small pits formed on the seabed. 171 

seabed samples have been obtained and the majority analysed for sedimentary and 

faunal analysis. The study area extended up to 10km either side of the dredge zone 

(one tidal excursion) in order to identify far-field effects. 
Access Number 30980 

Study Results: The results clearly show that the physical impact of dredging on the 

seabed (without screening) is limited to a zone within approximately 300m down tide of 

the dredge area. There is no visible evidence of suspended sediments falling to the 

seabed beyond this zone, which may be manifested as infilling of small pits by fine 

sediments, siltation within crevices or development of migratory sand ripples. However 

there is some statistical evidence that the surface sediment samples have a greater 

sand fraction within the excursion track of the plume than those samples either side. 

 The biological “footprint” of impact has been established. Species diversity, 

population density and biomass of benthic macrofauna of the study site is typical of that 

recorded in UK waters. Average benthic macrofauna biomass as a whole is equivalent 

to 4.06 grams Carbon per m2. 

 The studies show that dredging at anchor using a modern 2300 tonne suction 

dredger is associated with a reduction of species diversity of 66%, population density 

(87%) and biomass (80-90%) of benthic invertebrates. The deposits are loaded as an 

‘all-in’ cargo with no discharge of screened material at this site. In this case the 

suppression of invertebrate species variety, population density and biomass appears 

confined to the dredge sites themselves, with no evidence of impact outside the 

boundaries of the dredge pit. 

Some distance outside the dredge site, there is evidence of an enhancement of benthic 

diversity and biomass in an elongated ‘halo’, which extends for a distance of up to 3 km 

from the dredge site. Average benthic macrofauna biomass is equivalent to 17 grams 

Carbon per m2, some 4 times greater than the surrounding deposits. 

 Our monitoring aboard the dredge vessel determined some 17.36 tonnes ashfree dry weight of organic matter may be released per year in the outwash of dredgers 

operating within the restricted worked site of the much larger North Nab licence. This 

material is likely to be carried beyond the boundaries of the Licence Area along the axis 

of the tidal excursion: whether this is sufficient to account for the enhanced values of 

biomass 1-3km from the dredge site is unknown. 

 In contrast with the intensively anchor dredged site, the trailer dredged site has 

been exploited less intensively. Communities within this site are largely similar to those 

in the surrounding deposits. This suggests that the processes of recolonisation and 

recovery are sufficient to keep pace with the rate of removal of biomass when dredging. 

It must be noted that the key factor here may be intensity rather than method of dredging 

used. 

 Sites which have been left undredged for known times suggests that initial 

recolonisation by mobile components of the benthos can occur within weeks with some 

70-80% of the species variety returning. This process is often accompanied by a 

similarly rapid increase in population density, although not as frequently, but with both of 

these stages in the recolonisation sequence being substantially completed within 3-6 

months after cessation of dredging. Restoration of biomass is achieved by growth of the 

small individuals that recolonise the deposits. This stage is incomplete even after 18 

months compared with areas some distance away from the dredge site, and this finding 

is in keeping with anecdotal information available from the literature. 

 The results for trailer-dredged studies elsewhere indicate that species diversity 

may initially recover much quicker, as mentioned above. Population density is not 

dissimilar to anchor dredge sites, with biomass recovering to within 80% of the 

undredged sites within 3 months. 

 
Access Number 30980 

We conclude with the following general hypothesis based on this study and another 

partial study carried out in the Southern North Sea on a trailer dredge study site:- 

(1) The degree of suppression of the fauna in the dredge site itself is clearly 

dependent on the intensity of dredging. In high intensity dredging (North Nab) the 

suppression of population density, species variety and biomass can be as high as 60-

80%. In areas that are dredged less intensively by trailer techniques, the suppression is 

either less than at anchor dredge areas (North Nab), or undetectable (North Sea). 

(2) There is no evidence of an impact outside the immediate dredge sites. 

(3) Both sites show some evidence of an enhanced biomass and population density at 

some distance from the dredge site, possibly reflecting the deposition of organic 

components from fragmented invertebrates discharged in the outwash. 

(4) Recovery of population density and species variety can be very rapid indeed. This 

depends on the degree of disturbance to which the area is subjected under natural 

conditions. In shallow water wave disturbed areas such as the North Sea, colonising 

species are mobile and well adapted to rapid recolonisation. In more stable (equilibrium) 

communities such as occur on coarse rocks and cobbles, recolonisation is slower. 

(5) Recovery of biomass is achieved by growth of the recolonising individuals. In this 

case restoration of biomass generally requires at least several years. In some of the 

deeper water communities that we have recently analysed, individual species may be at 

least 20 years old. This implies that deep-water stable equilibrium communities may 

require a time of at least 20 years for recovery, compared with 2-3 years in shallow water 

coastal sands. 

(6) Anchor dredging has a significant impact on the species variety, population density 

and biomass of benthic macrofauna, although without screening is largely limited to 

within a hundred metres of the active dredged zone. Trailer dredging, on the other hand, 

appears to have a much lesser impact on species variety, population density and 

biomass, although this may be limited to the lower intensity of trailer dredging activities in 

the study areas. However, species recovery data suggests that recovery is quicker for 

trailer dredge areas, due to the reduced distance of ‘inwalk’ for colonising species (only 

the widths of trailer tracks), compared with the more substantial impact of an anchor 

dredged area. 

(7) On the available evidence collected herein, we would suggest that trailer dredging 

over a wide area at an intensity carefully matched to the potential times for species 

recovery (indicated by the response times to natural disturbances e.g. turbulent shallow 

water or less disturbed deeper waters) will be more sustainable than intensively dredging 

small areas of seabed. 

 Importantly, the detailed analyses of these and other data for this project have 

revealed the susceptibility of analysis methods to ‘noise’ within the datasets. This is 

caused by inter-sample variability due to significant under sampling of the diverse 

benthic macrofauna of sands and gravels by conventional methods. We have shown 

that single samples of macrofauna obtained from a ‘Hamon’ type grab contain sufficient 

taxa to use non-parametric multivariate analytical techniques to define community 

composition. Values for individual variables, such as species variety are, however, 

heavily dependent on the number of replicate samples taken. At least 3 replicate 

samples are required to obtain a satisfactory assessment of the species composition of 

the macrobenthos of sands and muds, but that 13 or more replicates are required for 

gravels. The repercussions of this in terms of scale, frequency, density of sampling sites 

and number of replicate samples and subsequent cost implications must be carefully 

considered when designing suitable monitoring protocols. 
Access Number 30980 

Study Products: 

Hitchcock, D.R., Newell, R.C., Seiderer, L.J. (2002c) “Integrated Report on the Impact 

of Marine Aggregate Dredging on Physical & Biological Resources of the Sea Bed” U.S. 

Department of the Interior, Minerals Management Service, International Activities & 

Marine Minerals Division (INTERMAR), Washington, D.C. OCS Report Number 2002-

054. 

Hitchcock, D.R. and Bell, S. (2003) “Physical Impacts of Marine Aggregate Dredging on 

Seabed Resources in Coastal Deposits”. Journal of Coastal Research. 12pp 

Hitchcock, D.R., Newell, R.C. and Seiderer, L.J. (2002a) “On the impacts of marine 

aggregate mining on the seabed: Part I Physical”. Proceedings of XXIst Information 

Transfer Meeting, Gulf Coast Region INTERMAR, New Orleans, USA. 9pp 

Hitchcock, D.R., Newell, R.C. and Seiderer, L.J. (2002b) “On the impacts of marine 

aggregate mining on the seabed: Part II Biological”. Proceedings of XXIst Information 

Transfer Meeting, Gulf Coast Region INTERMAR, New Orleans, USA. 9pp 

Newell, R.C., Hitchcock, D.R. and Seiderer, L.J. (1998) “Organic enrichment associated 

with outwash from marine aggregate dredging: a probable explanation for surface 

sheens and enhanced benthic production in the vicinity of dredging operations”. Marine 

Pollution Bulletin, 38(9), 809-818. 

Newell, R.C., Seiderer, L.J. and Hitchcock, D.R. (1998) “The impact of dredging works 

in coastal waters: a review of the sensitivity to disturbance and subsequent recovery of 

biological resources on the seabed”. Oceanography and Marine Biology Annual Review, 

36, 127-178. 

Newell, R.C., Seiderer, L.J., Simpson, N.M., and Robinson, J.E. (2002) “Impacts of 

marine aggregate dredging on benthic biological resources in coastal deposits”. Journal 

of Coastal Research (in press)