Publication Abstract

On the variability of gravel habitats in the central and eastern English Channel: information to support policy decisions relating to the management and monitoring of offshore aggregate extraction sites

On the variability of gravel habitats in the central and eastern English Channel: information to support policy decisions relating to the management and monitoring of offshore aggregate extraction sites

R. Coggan*, C. Barrio-Froján*, M. Diesing* and J. Barry*. ME1102 Project report for Derfa: ME1102 Aggregates Study Report

The work reported here is one part of a larger project involved with studies of the seabed in the central English Channel and addressed our need for improved evidence about the distribution and nature of seabed habitats so that we can better manage the area for conservation and sustainable use of resources. This part of the project addressed the need to know more about the nature of seabed communities associated with expansive gravel habitats in the eastern and central Channel, to inform Government policy on the way human activities are managed in these areas. The Government has recently licensed several new aggregate extraction sites in the eastern Channel, and the way in which these are managed needs to take account of the degree to which gravel habitats and fauna in the east are similar to, or different from, those in the central Channel region.

This study sampled a longitudinal transect of stations across the central Channel region, complementing samples from the eastern Channel collected in prior studies. All sampling stations lay in UK waters between the 12 nautical mile and the UK/France median lines, and between Longitude 3oW and 1oE. Complementary sampling techniques, using seabed imagery and grab sampling, provided data on the epifaunal and infaunal elements of the benthic communities respectively, as well as samples and information relating to the physical nature of the substrate at each sampling station. Further information and data on topography, hydrography and sediment transport pathways were derived from published material and hydrodynamic modelling.

All of the stations sampled by seabed imagery had sediment at the seabed surface, but in some areas it comprised only a thin layer (<0.5 m thick) over solid rock. Detailed analysis of video and photo images enabled expert judgement to be used to assign each site to a recognised biotope class, and these varied along the transect. When augmented by similar analyses from a prior study in the eastern Channel, the central part of the larger study area was found to contain notably different biotopes than the western or eastern extremes, generally having larger particles (pebbles and cobbles) and attached fauna (e.g. sponges) indicating a more stable seabed environment.

The analysis of infaunal communities was based on sediment and faunal data from 312 grab samples collected by this and other projects across the gravel substrates in the central and eastern Channel. The data set included records of 653 taxa, and excluded colonial organisms (such as sponges and hydroids). Data were subject to statistical analysis using univariate and multivariate techniques. These showed that infaunal communities were not uniform across the study area, neither was there a simple longitudinal gradient from east to west (or vice versa). Instead, the communities towards the western and eastern ends of the transect were more similar to each other than to those from the central area (south of the Isle of Wight). Analysis of particle size data from a selected transect of stations revealed a similar pattern in the sediments, having a greater proportion of sand towards the western and eastern ends but more gravel in the centre. The faunal analysis also showed a statistically significant difference between communities in the 0o to 0.5o East sector, which contains the majority of the licensed aggregate extraction sites, and the adjacent sector, from 0o to 0.5o West.

Consideration of hydrodynamic models and sediment transport showed this divergence from the central area appeared to have a simple explanation, in that the net effect of tidal currents across the region is to transport particulate material (e.g. sand) away from the centre, either to the east or the west. Such areas of divergence are known as ‘bedload parting zones’, and one is know to exist at approximately 1o West in the central Channel. The companion to this, a ‘bedload convergence zone’, exists just east of Dungeness (beyond 1o East), and particulate matter will begin to settle out of suspension as this zone is approached. Hence there is a physical mechanism that explains the observed broad scale distribution of seabed sediment types mapped by the British Geological Survey, and the corresponding pattern of variability in the faunal communities.

This study shows for the first time that gravel communities are not uniform throughout the central and eastern Channel and provides a causal mechanism that explains why the observed variability exists. It indicates that management options should view the region as a number of smaller management units rather than one large unit. The evidence suggests that the immediate environs of the newly licensed aggregate extraction sites should be treated as a single management unit. Management decisions should also be informed by the broader picture, considering the magnitude of impacts likely to arise from aggregate extraction against the background of the broad scale regional mechanism that has been acting for millennia to shape the sediments, habitats and communities that currently exist in this part of the channel.

Reference

R. Coggan*, C. Barrio-Froján*, M. Diesing* and J. Barry* (2012) On the variability of gravel habitats in the central and eastern English Channel: information to support policy decisions relating to the management and monitoring of offshore aggregate extraction sites. ME1102 Project report for Derfa: ME1102 Aggregates Study Report 300309_full.doc

R. Coggan*, C. Barrio-Froján*, M. Diesing* and J. Barry*

ME1102 Project report for Derfa: ME1102 Aggregates Study Report 300309_full.doc