Benthic habitats

Benthic marine habitats include all biological communities associated with the sea floor, from the top of the intertidal zone and inner reaches of estuaries down to the deep sea. UK waters encompass the transition zone between northeastern, cold-water communities and southwestern, temperate-water communities found along western Europe. For this reason our waters are particularly important at a European scale for their exceptional variety of benthic habitats and thus high overall biodiversity. However, many of these habitats are affected by human activities.

We assessed whether human activities were putting pressure on the different habitats, using thresholds derived from the EU Habitats Directive and work by OSPAR. These assessments were based on the area of habitat considered to have been impacted by each pressure. Since habitat maps derived from survey data cover just 10% of the UK continental shelf, we had to rely on models for much of this assessment. Where the data were incomplete or unavailable, we based our assessments solely on expert judgement (in such cases, this is reflected in a low confidence ranking in the assessment).

Figure 3.1 Distribution of six broad habitat types found throughout UK waters. Subtidal and deep-sea habitat types are derived from modelling; intertidal habitat types are derived from survey data. The inset box shows an example of intertidal habitats in the Cardigan Bay area in larger scale. The white space in the map indicates where there are insufficient data to model the habitats.

We made separate assessments for six broad habitat categories: intertidal rock; intertidal sediments; subtidal rock; shallow subtidal sediments; shelf subtidal sediments, and deepsea habitats. In each case, we judged the current status of the habitat relative to an expert view about former natural conditions (i.e. in the absence of human pressures), the trends over the past ten years, and the prospects for the next two decades.

Red sea fingers

© Keith Hiscock

Intertidal rock

Intertidal rocky habitats, including rocky and boulder shores and sea cliffs, occur in all UK seas. Although these habitats are generally in good condition the harvesting of edible shellfish and the occurrence of non-native species are adversely affecting some local rocky shore communities. In addition, species composition of intertidal rocky communities in the Western Channel and Celtic Sea region is already impacted by warmer waters due to climate change.

Intertidal sediments

Along the south-eastern and north-western coasts of England and parts of Wales, intertidal sediments form extensive beaches, sandbanks, saltmarshes and muddy shorelines. In Scotland and Northern Ireland, such stretches of intertidal sediments are often interspersed with rocky promontories and headlands. Human pressures have adversely affected moderate to large areas of these habitats, notably mudflats and saltmarshes, in most of the UK seas apart from those around northern and western Scotland. Historical land claim and the construction of coastal defences and other structures have caused widespread habitat loss, particularly in England. Such structures also affect these habitats by changing current patterns and sediment distribution. In the Southern North Sea and Eastern Channel, the presence of non-native species such as common cordgrass (Spartina anglica) has led to widespread changes to saltmarshes and mudflats. Water quality can affect these habitats and although water quality has improved overall, there are still some small inshore areas where hazardous substances and nutrient enrichment are a problem. Beach litter levels are high in most regions but impacts remain largely unknown.

Subtidal rock

Expanses of subtidal rock are less common in UK waters than sediments (see Figure 3.1). The largest known areas occur in Scottish waters, particularly to the west of the Hebrides and around Shetland, although some extensive areas also occur off Devon and Cornwall. Elsewhere this habitat occurs mainly as a narrow band adjacent to rocky shores. There are also offshore biogenic reefs built by marine species including horse mussels (Modiolus modiolus, found mainly to the north), and ross worms (Sabellaria spinulosa), which are more common in the south and east. Overall, only limited areas of subtidal rocky habitats appear to be directly impacted by human activity. Some have, however, been permanently damaged or removed by mobile fishing gears such as bottom trawls. Bottom trawling has had a particular impact on fragile habitats such as biogenic reefs. Locally (such as near some large ports around England and Wales), subtidal rocky habitat has also been lost because of construction, coastal infrastructure or disposal of dredged materials.

Shallow and shelf subtidal sediments

Subtidal sediments – sand, gravel, muds and mixed sediments – cover almost all the continental shelf around the UK as well as coastal habitats such as sea lochs and lagoons. Shallow subtidal sediments, which can be regularly disturbed by surface waves are especially widespread in the Irish Sea, the Eastern Channel and the Southern North Sea, where they occur out to considerable distances offshore; they also occur in coastal lagoons, particularly in southern England and western Scotland. Shelf subtidal sediments are only rarely disturbed by surface waves because of their greater water depth, and can therefore support more stable communities. They occur throughout offshore areas of most regions, but also much closer to coasts where the water deepens rapidly, such as around most of Scotland, Northern Ireland and Cornwall. They are also found on Rockall Bank, west of Scotland.

Large areas of subtidal sediments in most regions have been adversely affected by mobile fishing gears such as bottom trawls and dredges, with less severe impacts on the Scottish Continental Shelf and the Eastern Channel.

Locally, extraction of aggregates has damaged the seabed in the Eastern Channel and Southern North Sea. Some estuaries and subtidal coastal habitats along the south coast of England and in the Irish Sea continue to experience nutrient enrichment and pollution. Non-native species are spreading in the subtidal coastal areas in most regions.

Deep-sea habitats

Deep-sea habitats occur below 200 m, beyond the edge of the continental shelf. Within UK waters they mainly occur to the north and west of Scotland and west of Rockall, although there are also small areas in the extreme south-west Celtic Sea. Most of these are sediment habitats, with rocky habitats and reefs largely confined to seamounts and similar structures. Similar to other subtidal habitats, deep-sea habitats are vulnerable to the impacts of some types of mobile fishing gears. Although this represents the main pressure on these habitats, their current status varies by region, with large areas of habitat impacted in the Scottish Continental Shelf Region, and limited areas known to be impacted in the Atlantic North-West Approaches.

Future work

We have based most of this assessment on benthic habitats on expert judgement, considering the relationship between habitats and pressures and drawing upon limited evidence from monitoring studies and research. While the present approach represents a significant advance in methodology since Charting Progress there are still many uncertainties. These would be greatly reduced and the approach enhanced if more robust evidence were available on the distribution and intensity of pressures, and the distribution and condition of a wider range of habitats. We do not know how some of the pressures identified as currently impacting habitats (e.g. bottom trawling and aggregate dredging on subtidal sediments) will change in the future in their intensity and likely distribution. Some specific small-scale habitats at significant risk from particular pressures which were not captured in the assessment process have been identified separately.

Finally, the threshold values against which benthic habitats were judged in this chapter need to be reviewed to ensure that they are set at an appropriate level. The targets that will be established for Good Environmental Status under the EU Marine Strategy Framework Directive will help to address the main causes of human impacts on marine habitats.

Examples of the diversity among benthic marine habitats in UK waters:

Intertidal rock with brown algae (Fucus sp.)
Intertidal mudflats
Soft corals and other colonial invertebrates on subtidal rock
Shallow subtidal sediments with lugworm casts
Brittlestars on shelf subtidal sediment
Deep-sea coral reef