The Ecosystem Consequences of Sediment Disturbance


Title: The Ecosystem Consequences of Sediment Disturbance
Publication Date:
March 31, 2005
Document Number: AE 1224
Pages: 23

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Centre for Environment Fisheries and Aquaculture Science (CEFAS) (2005). The Ecosystem Consequences of Sediment Disturbance. Report by Centre for Environment Fisheries and Aquaculture Science (CEFAS) and UK Department for Environment Food and Rural Affairs (DEFRA). pp 23.

The seabed of the UK coastal waters and shelf is subject to a variety of pressures from human activities. These include: fishing by bottom trawlers; the dredging and disposal of dredge spoil from estuaries and harbours, to maintain navigation, which may contain contaminants; and, the extraction of sand and gravel for use in construction. Such activities may lead to the disruption of benthic communities (micro-, meioand macro-fauna) by physical disturbance or removal of habitat; the introduction of contaminants; and changes in the cyling of carbon and nutrients within the seabed. These perturbations in turn may result in an undesirable disturbance to the overall health of the ecosystem (in terms of biodiversity, resilience and impacts on higher trophic levels).


The overall aim of the project was to establish the impact of sediment disturbance due to human activities on: the biogeochemical functioning of the seabed ecosystem (i.e. the storage and cycling of carbon and nutrients, and organic degradation); the bio-availability of sediment-bound contaminants; and, the redistribution of toxic algae. This was placed in the context of natural disturbance due to regular tidal motions and episodic storm events. We sought to address each of these issues within the project and provide a conceptual model that could be developed as a tool for use in environmental impact assessment, and to provide a means for conveying information on the consequences of disturbance to regulators, policy makers and the general public, and to contribute to the formulation of guidance through international fora such as OSPAR ( This was based on the DPSIR approach to assessing ecosystem ‘health’ (Driver, Pressure, State, Impact & Response). The project was multidisciplinary with chemists, biologists, sedimentologists, physical oceanographers and information scientists working together. A combination of field observations, laboratory experiments, modelling and GIS (Geographic Information Systems) techniques were used.


Five main scientific objectives were identified at the inception of the project, with a further two added later, and these were managed as separate ‘work strands’:


  1. To develop a conceptual model of the consequences of sediment disturbance to support impact assessments and the development of activity-related ecological targets;
  2. To assess the relative magnitude of sediment disturbance due to natural (storms and tides) and human (dredging, fishing and construction) activities, at both local and regional scales;
  3. To assess the effect of sediment disturbance on the biogeochemical functioning of coastal marine sediments;
  4. To assess the effect of sediment disturbance on contaminant (PAHs, TBT, Cu and other metals) bio-availability and toxicity;
  5. To assess the effect of sediment disturbance on the occurrence and distribution of toxic algal cysts and the conditions under which the toxic algae would thrive.
  6. To support a project funded under the EU 6th Framework programme: COBO - towards a Coastal Ocean Benthic Observatory. This commenced in March 2004, consisting of a consortium of 12 partners from 6 countries.
  7. To undertake a review of existing information and conduct additional research into environmental conditions on and around the Souter Point disposal ground off the NE coast of England.


The results of the project have been reported in a number of contract reports, peer-reviewed publications, conference presentations and other outputs. One of the main findings was that about 70% of the seabed in the southern North Sea is physically impacted by human activities (aggregate extraction, dredge disposal, trawling). Natural disturbance and fishing impacts are of most importance on a regional scale whilst aggregate extraction and disposal are spatially restricted (<0.1%) but intense. The spatial distribution of the trawling fleets is patchier than originally thought with significant inter- and intra- year variations in impact intensity which can be linked to management actions (i.e. closure of the Cod Box in 2001). Natural disturbance appears to mediate the sediment type and mobility, redox state and hence associated rate processes, to a significanr extent, based on a combination of field measurements and modelling techniques. The continuum from low natural disturbance (diffusional) to high natural disturbance introduces an increasing degree of natural variability against which human impacts are difficult to detect. In terms of field observations, the use of a SPI camera (Sediment Profile Imaging) proved to be a relatively cost-effective and rapid method for assessing the impact of dredge spoil disposal on the benthic fauna and general sediment characteristics. Some of the sediment process measurements are costly and time-consuming, which may impose limits on their use. The EU COBO programme will provide a range of novel in-situ technologies which will complement the more conventional methods, for future applications. The work on algal cysts illustrated the patchy distribution of particular species. The assumption that the Thames embayment would contain significant quantities was ill-founded. The experimental work allowed the conditions for excystment to be explored, generating a modelling module for inclusion in the GIS framework and providing a predictive capability of the conditions under which a toxic bloom might occur.


A novel approach to impact assessment has been developed which combines GIS-based disturbance information and pressure-impact models within a DPSIR framework. It can be applied on varying spatial scales and to a range of human activities, and has potential for helping to select reference areas for establishing EcoQOs (Ecosystem Quality Objectives). The impact assessment approach is quick to apply, given the availability of appropriate data and impact models (empirical or conceptual). Consequently, it can be used as a screening tool to identify limitations in scientific knowledge or help decide if a full modelling exercise is required. In this way, existing impact models can be applied within a GIS environment to give a preliminary assessment of potential effects, in terms of: sensitivity to changing disturbance distributions (management scenarios); investigations of critical model criteria (environmental conditions); or by combining information layers to provide risk assessments. In the present study the approach was applied to three case studies: the impact of trawling on sediment biogeochemistry; the potential impact of disturbance of toxic dinoflagellate cyst viability; and, the behaviour of TBT (tributyl tin, used in anti-fouling paint) at the Tyne disposal site.


The results will be fed into a number of on-going Defra projects, including the development of indicators of ecosystem health and promoting the ecosystem approach to environmental management. It will inform our input to a number of international collaborations and advice fora, including OSPAR, ICES and the EU Marbef Network of Excellence (marine biodiversiy, The work on the Tyne disposal site will be used to develop improved assessment techniques and monitoring strategies at this and other sites.


A number of areas would benefit from future research. This should include geochemical processes in coarser sediments, which were not targeted in this project but which represent the bulk of sediments on the UK shelf and may be key in addressing carbon cycling. The mediation/response links between the faunal community and biogeochemical changes proved difficult to establish but undoubtedly are of significance, as is the rate of post-impact recovery. The current models of contaminant behaviour following disturbance are insufficiently robust and rely on assumptions due to lack of site specific information. This needs to be recognised with the implied need for appropriate field and experimental data, to cover the range of sites, contaminants and biota likely to be encountered. The work on algal cysts would benefit from additional experiments on a wider range of organisms, to provide basic information on the conditions for excystment and encystment, cyst viability and probability of toxic events, from oceanographic observations combined with improved modelling descriptions. Such research would greatly strengthen the evidence base upon which management decisions depend.


Acknowledgement: This article was identified by the Crown Estate Wave and Tidal Knowledge Network.


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