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Cefas Study in U.K. Informs Monitoring Process for Aggregate Dredging

Study area showing locations of aggregate dredging interest (blue - Humber dredging region, green - Anglian dredging region, yellow - Thames dredging region, orange -
Goodwin Sands dredging license, purple - eastern English Channel dredging region, red - South Coast dredging region, dark pink - Bristol Channel/Severn Estuary dredging
region, brown - North-West dredging region). (b) Sample locations and extent of submaps used to show faunal cluster groups and diversity. The underlying bathymetry is
from the 2015 updated version of Defra’s Digital Elevation Model (DEM)54. Maps created in RStudio (version 1.0.143, https://www.rstudio.com/).

Study area showing locations of aggregate dredging interest (blue - Humber dredging region, green - Anglian dredging region, yellow - Thames dredging region, orange - Goodwin Sands dredging license, purple - eastern English Channel dredging region, red - South Coast dredging region, dark pink - Bristol Channel/Severn Estuary dredging region, brown - North-West dredging region). (b) Sample locations and extent of submaps used to show faunal cluster groups and diversity. The underlying bathymetry is from the 2015 updated version of Defra’s Digital Elevation Model (DEM)54. Maps created in RStudio (version 1.0.143, https://www.rstudio.com/).


By Anna Townshend
In October, the Centre for Environment, Fisheries and Aquaculture Science (Cefas), an executive agency of the U.K. Department for Environment Food & Rural Affairs (Defra), published a study, which brings together 50 years of seabed grab survey data from the U.K. The study aimed to better understand the data through collation and standardization, informing better approaches to monitoring the environmental effects of aggregate dredging.
The paper provides details of the Regional Seabed Monitoring Plan (RSMP), an approach based on the new dataset, for monitoring the long-term impacts of aggregate dredging on the seabed.
“In the past, benthic monitoring tended to focus on the impacts of ongoing dredging (e.g., showing reductions in the number of species within the direct and indirect impact zones),” said Keith Cooper, Cefas senior marine ecologist and architect of the study. Those impacts have been widely studied and have become increasingly predictable, he said. “For reasons of sustainability, it is now right that the focus shifts to what’s likely to happen to the site after dredging (its recovery potential),” Cooper said.
Cefas provides government officials in the U.K. and internationally with impartial expert advice and evidence relating to marine science. The study was funded by Defra, the Marine Management Organization (regulators), The Crown Estate (seabed owner), the British Marine Aggregate Producers Association (BMAPA) and the Welsh Government.
The U.K. marine aggregates industry adopted the RSMP in the main English dredging regions (Humber, Anglian, Thames, East Channel and South Coast) in 2013. In 2015, the Welsh government and the Natural Resources Wales adopted the RSMP in its waters (Bristol Channel/Severn Estuary and North-West dredging regions).
“At present, the RSMP applies only to the marine aggregate dredging industry, although other sectors could also benefit from the approach,” Cooper said.
The current study brings together more than 33,000 samples from 777 surveys, including data from both industry and government, and produces a standardized dataset for benthic macrofauna and sediments. According to the study, “the resulting dataset is used to identify spatial and temporal patterns in faunal distribution around the U.K., and the role of sediment composition and other explanatory variables in determining such patterns.”
The U.K. marine aggregate dredging industry produces sand and gravel from licensed extraction areas located around the coast of England and Wales, with material used for construction, fill and coastal defense. According to the study, aggregate dredging can create some “localized environmental impacts (e.g. changes in seabed topography, alterations to sediment composition and loss of benthic fauna), although these vary considerably.” The RSMP works to ensure that the seabed after dredging is left in a condition that will allow for recolonization at the site and improve overall sustainability.
The Mahalanobis Distance Test
To implement the RSMP, it was necessary to produce a baseline assessment of the seabed; identify the range of sediment composition found with each baseline faunal assemblages; and develop a method for assessing the likely ecological significance of changes in sediment composition. These objectives were achieved with the data from industry and government resources. The assessment method for sediment composition is called the Mahalanobis distance test, which is a method for identifying outliers in a multidimensional dataset.
“We’re using it to assess whether the sediment composition of monitoring samples (the percentages of coarse gravel, medium gravel, fine gravel, coarse sand, medium sand, fine sand, silt/clay) falls within the normal range, in which case it should be possible for a full faunal recovery after dredging,” Cooper said. 
According to the study, “Notable differences in the composition of sediments were observed between samples from different faunal assemblage groups. Within these groups, some small differences in sediment composition were evident between samples from the different physical cluster regions, thus supporting the assessment of sediment change based on samples from the same faunal and physical cluster region.”
The Mahalanobis distance test was also successful in identifying known recovery problems at an existing monitoring site in the eastern English Channel dredging region, and at a former aggregate extraction site in the southern North Sea. According to the study, the sediment composition of the test samples, in comparison with that of the wider cluster groups, correctly identified the sediment fractions known to be responsible for the problem. In both cases, a lack of coarse sand was identified as the most likely cause for slow recovery. The study suggests that it is likely “that a reduction in the proportion of gravel would not necessarily lead to a decline in taxon richness, so long as sediment remained within the limits defined by the Mahalanobis distance test,” the study said. The work will continue to study that relationship and continue to add to the dataset.
“This is the first time so much benthic data has been brought together. The insight we get from the dataset (in terms of natural variability) can now be used to inform better, more informed management decisions,” Cooper said.
Each licensed area for the study and secondary impact zone is covered by an array of grab sampling stations. The nature of the seabed animal communities (i.e., assemblage type) at each station is identified at the baseline, usually at the Environmental Impact Assessment (EIA) stage. Cooper said sampling stations are then revisited during subsequent RSMP monitoring rounds (once every five years) to check whether sediment composition remains favorable for the return of the original assemblage type after dredging. “The assessment of favorability is based on whether sediments remain within the normal range seen for the particular group in the wider region,” Cooper said.
“The study represents a significant step forward in our understanding of how sediment change is likely to affect benthic faunal assemblages. Whilst it has long been recognized that faunal assemblages differ in their sensitivity to changes in sediment composition, and that a faunal recovery is often predicated on a physical recovery, it has hitherto not been possible to quantify such relationships. Both the close relationship between sediments and the benthos, reported here and in the literature, and the consistent temporal patterns in faunal assemblage distribution support the development of the RSMP approach,” the study said.
Mark Russell, executive director of BMAPA said: “The Regional Seabed Monitoring Program approach has allowed individual sample stations to apply across multiple license areas, therefore reducing duplication of sampling effort and increasing the robustness and consistency of the baseline data that is being acquired for environmental monitoring. By adopting a more integrated approach in the delivery of regulatory compliance, significant savings in time, effort and resources are expected to be realized by not only industry operators, but also regulators and statutory advisors throughout the life of all the licensed marine aggregate operations that are covered by the RSMP program.”
Cost Savings and Monitoring Efficiency
The cost savings could be significant. A 2014 study by BMAPA (“Marine-Aggregate Regional Seabed Monitoring Plans (RSMP): Cost/Benefit Statement on Behalf of the Marine Aggregate Sector,” estimated that the costs of monitoring may be reduced by 50 percent.
Cost savings result from collaboration. Within each dredging region, operators will collaborate, using one rather than multiple vessels for monitoring activities. Overall, the RSMP requires far fewer faunal samples, which have processing costs 200 to 500 pounds per sample, instead of focusing on samples for sediment particle size composition. It will also result in a more streamlined and focused reporting and review process, Cooper said. Savings and efficiency will further be aided by an online application to perform analysis of the data. This online application is in development now.

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