Offshore Overfalls MPA

Status: Marine Conservation Zone (MCZ)

Offshore Overfalls MCZ is a joint site with Natural England, located in the eastern English Channel, approximately 18 km south-east of the Isle of Wight. The seabed is predominantly coarse sediment with areas of sand, mixed sediments and exposed bedrock.

Site

Located in the eastern English Channel, approximately 18 km south-east of the Isle of Wight, Offshore Overfalls MCZ is a joint site with Natural England. The seabed is predominantly coarse sediment with areas of sand, mixed sediments and exposed bedrock.

The site protects 593 km² of seabed, including the English Channel outburst flood geomorphological features that are Quaternary fluvio-glacial erosion features. The variety of habitats found support a diverse range of species, including sponges, hydroids, bryozoans on the cobbles and boulders and crabs, sea stars and sea urchins. Burrowing worms live within the sediment alongside burrowing anemones and bivalves such as scallops.

More detailed site information can be found in the Summary section below.

 

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Site Timeline

The diagram below is a summary of the key milestones involved in the selection and designation of Offshore Overfalls MCZ. More detail can be found in the Relevant Documentation section.

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Relevant Documentation

The documents referred to below and any other historical documents relating to Offshore Overfalls MCZ were produced during the selection and designation process and therefore may be out of date. This Site Information Centre is the most up-to-date source of information for this MPA, and will reflect any additional information gathered since these documents were produced. Further information about the Marine Conservation Zone site selection process and historic MCZ advice is available on JNCC's MCZ webpages.

• Offshore Overfalls MCZ Designation Order – the official description of the site designation under the Marine and Coastal Access Act 2009. The Designation Order includes boundary co-ordinates, conservation objectives and a list of the designated features. More information on the designation, including a designation map and factsheet, is available on Defra's website.
• JNCC's pre- and post-consultation scientific advice for features proposed for designation in 2016.

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Summary

Last updated: October 2018

Information for this site summary was adapted from JNCC’s scientific advice to Defra on the designation of Offshore Overfalls MCZ and incorporates any further information gathered since this document was produced. Please refer to this document in the Relevant Documentation section for further details and information sources.

 

Site overview

The Offshore Overfalls MCZ is located in the Eastern English Channel approximately 18 km south-east of the Isle of Wight and just north of the Offshore Brighton MCZ. It includes a mixture of sediment types which create a dynamic seabed environment and host a diverse ecosystem. The site depth ranges from 20 m to 70 m, the deeper areas coinciding with a valley system running through the site from the south to the north-east. This valley is part of the English Channel Outburst Flood Features (Quaternary fluvio-glacial erosion features) which are protected within the site for their importance to the study of geomorphology.

The English Channel Outburst Flood Features were formed during the Pleistocene Epoch more than 200,000 years ago when sea levels were much lower than they are today and the English Channel was not yet covered by seawater. A large glacial meltwater (freshwater) lake, in part of the area now occupied by the southern North Sea, burst its retaining bank to the south. This created a vast discharge of sediment and water that carved out large-scale longitudinal valleys, the outburst flood features, which were later submerged by seawater in the English Channel when sea levels rose again after the Ice Age. These erosional features have scientific value as they help us to understand the Ice Age history of the area, including sea-level change, and how massive-scale flood features develop, which may provide important information about tsunami events. Within the Offshore Overfalls MCZ these erosional features are evident as deeper valleys within the site which run along the south east corner.

Offshore Overfalls MCZ was originally recommended by the Balanced Seas Regional MCZ Project in 2011. The site was prioritised for additional evidence collection in a verification survey undertaken by JNCC and Cefas in 2012 to improve confidence in the presence and extent of broad-scale habitats and habitat features of conservation importance (FOCI).

The 2012 survey confirmed the presence of the broad-scale habitats (BSH) designated within this MCZ (subtidal coarse sediment, subtidal mixed sediments and subtidal sand sediment), along with moderate energy circalittoral rock which was recorded sporadically across the MCZ.

The BSH subtidal sand is located in two isolated patches that fringe the northern margin of the Northern Palaeovalley and associated with marine bedforms that form a collection of sediment ripples and waves. These marine bedforms are predominantly comprised of sandy sediment although some coarse or mixed sediments may be present in the troughs of the sediment waves. The BSH subtidal mixed sediments is confined to the northeast of the MCZ. Evidence from the 2012 survey shows bedrock structures visible at the seabed in this region, covered with a thin veneer of mixed sediments. Bedrock structures are sporadic along the south east of the site and in an area to the north-west known as the ‘Overfalls’ which is the namesake of Offshore Overfalls MCZ. Subtidal coarse sediment is predominant within the MCZ, covering almost three-quarters of the site. It is located on the flanks and terraces of the Northern Palaeovalley and within the valley floor.

Offshore Overfalls MCZ was included in the proposed network because of its contribution to ENG criteria to broad-scale habitats (BSH), and its added ecological importance. The area is incredibly diverse with 278 infauna species and 45 epifauna species identified from the 2012 survey. This data was reviewed by Envision Mapping Ltd for JNCC in 2016 and statistically analysed to reveal the potential biotopes present within the site. The majority of stations (52 out of 59) were assigned to the biotope SS.SCS.CCS.MedLumVen (Mediomastus fragilis, Lumbrineris spp. and venerid bivalves in circalittoral coarse sand or gravel). This biotope fits with the broad scale habitats designated within the site and was characterised by comparatively high numbers of the bristle worm Notomastus latericeus, along with the pea urchin (Echinocyamus pusillus).The full report of this analysis can be found in the Monitoring and Evidence section.

The infauna biological communities appear to be dominated by a diverse range of burrowing worms (polychaetes). Bivalves such as the Queen scallop (Aequipecten opercularis) occur in smaller numbers along with the long-clawed porcelain crab (Pisidia longicornis) and the common brittlestar (Ophiothrix fragilis). The epifauna, living on top of the sediment, are dominated by hydroids and bryozonas and also include a range of sponges (Porifera), sea anemones (Actinaria) and sea stars such as the common starfish (Asterias rubens) and the common sun star (Crossaster papposus). Various species of fish are also present including thornback ray (Raja clavata), red gurnard (Chelidonichthys cuculus), small-spotted catshark (Scyliorhinus canicula), and bib (Trisopterus luscus). Further detail on the evidence for this MCZ can be found in the Monitoring and Evidence section.

Site location:  Co-ordinates for this MCZ can be found in the Designation Order listed in the Relevant Documentation section.

Site area:  593 km², which is similar to the size of the New Forest National Park (566 km²).

Site depth range:  20 m to 70 m. The seabed depth drops significantly in the middle of the site where it overlaps with the northern Paleovalley.

Charting Progress 2 biogeographic region: Eastern Channel.

Site boundary description: The north-west corner and limit of the northern boundary of the site are concurrent with the original boundaries proposed by the Balanced Seas Regional Group. The boundaries around the larger offshore part of the MCZ were set to include an area of higher biodiversity and are determined by geographical co-ordinates alone.

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Monitoring and Evidence

Last updated: April 2025

The full overview of the various data used to support site identification, along with information on confidence in feature presence and extent are available in the Scientific advice on possible offshore Marine Conservation Zones considered for consultation in 2015 and JNCC's post-consultation scientific advice for Tranche Two offshore Marine Conservation Zones proposed for designation in 2016. JNCC will be adding relevant survey data for this MCZ to its MPA Mapper in due course.

Some of the data for this MCZ have been collected through JNCC-funded or collaborative surveys and some through other means. Data gathered provide direct evidence confirming the presence of the protected features within the site.

 

Survey and data gathering

• Offshore Overfalls MCZ and Offshore Brighton MCZ Survey (2020) – JNCC collaborated with Cefas on a data gathering and monitoring survey undertaken in January 2019. Video, still images, multibeam bathymetry data, grab samples and DNA samples were collected.
• Offshore Overfalls MCZ MB0120 Survey (2012) – JNCC collaborated with Cefas on an MCZ site verification survey to Offshore Overfalls MCZ, funded through the MB0120 Defra data collection. Video, images, acoustic data and grab samples were collected across the site.

 

Data analysis reports

• Offshore Overfalls Marine Conservation Zone (MCZ) Monitoring Report 2019 (2024) – Monitoring report for the 2019 JNCC and Cefas survey of Offshore Overfalls MCZ in which imagery and grab sample data were used to describe the epifaunal assemblages and (within the context of data limitations) the infaunal assemblages that characterise the designated features of the site. An improved habitat map was produced, combining 2019 acoustic data and derived epifaunal assemblage groups, which provides greater resolution and confidence in the extent and distribution of the designated features across the site. 
• Community analysis of Offshore Overfalls MCZ data (2016) – JNCC contracted Envision Mapping Limited to complete a community analysis of offshore MCZ grab and video data to establish biotopes present. The following biotopes were assigned after multivariate analysis of the 2012 survey data. The Marine Habitat Classification for Britain and Ireland (version 15.03, JNCC 2015) has been provided in brackets:
  Circalittoral coarse sediment (SS.SCS.CCS)
  Mediomastus fragilis, Lumbrineries spp. and venerid bivalves in circalittoral coarse sand or gravel (SS.SCS.CCS.MedLumVen)
• EUSeaMap – Provides supporting information on the presence and extent of subtidal coarse sediment, subtidal sand and subtidal mixed sediments from a predictive seabed habitat map of European waters.
• Offshore Overfalls rMCZ Post-survey Site Report 2015 (Published in 2017) – Report detailing the findings of a dedicated seabed survey at Offshore Overfalls MCZ in 2012, providing evidence of the presence and extent of broad-scale habitats and habitat FOCI outlined in the original site assessment document. 
• British Geological Survey Particle Size Analysis (PSA) Data Points – Particle Size Analysis of historical British Geological Survey data was collated and used to identify habitat type within the MCZ. PSA was used to provide habitat type in Modified Folk classification. This has been converted by JNCC to the EUNIS habitat using JNCC's 'Correlation Table showing Relationships between Marine Habitat Classifications (2004 and 2007 versions) and Habitats Listed for Protection'.
• MB0102 Task 2A Erosional Fluvio-glacial Features (2009) – Habitat map providing a shapefile of the English Channel Outburst Flood Feature (Quaternary fluvio-glacial flood features) taken from Gupta et al. 2007.
• Marine ALSF Regional Environmental Characterisation Surveys South Coast Synthesis (Surveys conducted 2007) – The South Coast Synthesis Study combines the Eastern English Channel REC habitat map with the South Coast REC habitat map and synthesises the gaps to create coverage across the English Channel. The study proposed some alternative habitat types that are not part of the EUNIS habitats classification system and JNCC translated these into the closest official EUNIS habitat types.

 

Additional relevant literature

References for further supporting scientific literature consulted during the identification of this site can be found in the annexes of our advice. Please be aware that although these sources contain information in relation to this MPA, they do not necessarily represent the views of JNCC.

• Gupta, S., Collier, J.S., Palmer-Felgate, A. and Potter, G. (2007) Catastrophic flooding origin of shelf valley systems in English Channel. Nature, 448, 342–345.

 

Knowledge gaps

As part of the UK Marine Monitoring and Assessment Strategy (UKMMAS), JNCC led the development of a UK Marine Biodiversity Monitoring Strategy, working with partners across the UK monitoring community. The Strategy spans UK territorial and offshore waters, focusing on biodiversity in the wider environment and within Marine Protected Areas. Its aim is to implement efficient, integrated monitoring of marine biodiversity to provide the evidence needed for all the UK's policy drivers.

The evidence collected during MPA monitoring surveys is used in combination with other available evidence to:

• Enable assessment of condition of the features within sites;
• Contribute to the assessment of the degree to which management measures are effective in achieving the conservation objectives for the protected features;
• Support the identification of priorities for future protection and/or management; and,
• Enable Government to fulfil its national and international assessment and reporting commitments in relation to MPAs and help identify where further action may be required.

More detail on offshore MPA monitoring can be found on the Offshore MPA monitoring webpage. A list of monitoring surveys and relevant reports can be found on the MPA monitoring survey reports webpage.

If you are aware of any additional information not referred in the Relevant Documentation section listed on the main page or the annexes of the MCZ advice documents, please contact us.

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Conservation Advice

Last updated: October 2017

Conservation objectives

Conservation objectives set out the desired state for the protected feature(s) of an MPA. The conservation objectives for the protected features of an MPA are useful if you are:

• Planning measures to conserve the site and its protected features;
• Monitoring the condition of the protected features; and/or
• Developing, proposing or assessing an activity, plan or project that may affect the protected features of the site

The conservation objectives for the protected features of the MCZ are:

Subject to natural change, the broad-scale habitats Subtidal coarse sediment, Subtidal sand and Subtidal mixed sediments features are to remain in or be brought into favourable condition, such that its:

• Extent is stable or increasing; and
• Structures and functions, its quality, and the composition of its characteristic biological communities are such as to ensure that it is in a condition which is healthy and not deteriorating.

Subject to natural change, the English Channel Outburst Flood Features (Quaternary fluvio-glacial erosion features) geomorphological interest feature is to remain in or be brought into favourable condition, such that its:

• Extent, component elements and integrity are maintained;
• Structure and functioning are unimpaired; and
• Surface remains sufficiently unobscured for the purposes of determining whether the conditions in the points above are satisfied.

More information regarding the conservation objectives for the protected features of the Offshore Overfalls MCZ is available in the site Designation Order, In addition to the conservation objectives above, General Management Approaches (GMAs) have been set by JNCC for each feature, which provide a view as to whether a feature needs to be maintained in or be brought into favourable condition (i.e. recover), based on our knowledge about its condition. For more information on the General Management Approach for MCZs see Defra’s MCZ Designation Explanatory Note.

 

Advice on operations

Section 127 of the Marine & Coastal Access Act (2009) states that JNCC may provide advice and guidance regarding matters capable of damaging or otherwise affecting the protected features of an MCZ.

JNCC has published the following advice on activities which are capable of damaging or otherwise affecting protected features in MCZs:

• MCZ Fisheries Advice (joint advice with Natural England);
• MCZ Licensed activities Advice (joint advice with Natural England); &
• JNCC's pre-consultation and the post-consultation scientific advice for Tranche Two MCZs.

For the most up-to-date information about the biological communities present within the site and their spatial distribution, please see the Monitoring and Evidence section. Sensitivity information for the protected features within the site can be found in a Technical Report commissioned by Defra to support the MCZ designation process.

The information contained within the Monitoring and Evidence section, the above Technical Report and the advice listed above on activities which are capable of damaging or otherwise affecting the protected features in MCZs are useful if you are:

• Carrying out any activity that may impact the protected features of the site and need to find out how to operate within the law;
• An authority providing advice on specific proposals; and/or
• An authority responsible for putting management measures in place.

Our scientific understanding of the ecology of the protected features of the site and how activities can affect them may change over time. Similarly the activities taking place within the site may also change over time. JNCC’s conservation advice will be kept under review and will be periodically updated to reflect this. Further information on JNCC’s conservation advice work is available on our 'Conserving MPAs' webpage.

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