Subtidal Sediments

Subtidal Sediments - John Miller - Explore Kent

Current Status

The seabed around Kent is generally shallow and currents can be very strong. The water is a mix of warm Lusitanian waters from the Channel and cold Boreal waters from the North Sea. High turbidity restricts plant life to shallow depths. Information on the extent of seabed habitats and associated communities around Kent or the UK is far from complete, and priorities for action will evolve as more data becomes available. A few significant rocky habitats exist around Kent, but the majority of Kent's seabed is sediment, comprising various mixes of pebbles, gravel, sand, and mud (see map below). The local diversity of flora and fauna varies according to the precise physical habitat characteristics, and the level of environmental stress.

Gravels occur mainly in ancient river courses, on elevated areas of seabed near rocky outcrops and in areas with strong currents. These can be sufficiently stable for attachment of various sessile species such as sponges and anemones. Sands accumulate where currents are moderate to strong, occurring as a thin cover over underlying rock, or forming deep banks and dynamic ridges. There are numerous sandbanks around Kent, including the Goodwin Sands, Margate Sands and Varne Bank, some of which rise up to 40m above the surrounding seabed. Mobile sand habitats support only a few highly robust, fast-growing species, while more stable sands support many burrowing species of invertebrates and fish, providing food for many birds and fish. Sandbanks that are not uncovered on low tides are important for key fish species such as sandeels and bass.

Sandbanks exposed at low water provide haul-out and pupping sites for seals. Whiteweed hydroid is harvested for the florist trade from the Thames estuary's muddy sandbanks. Sand and gravel habitats are important spawning areas for many fish species including plaice, sole, herring and many others of high commercial value, and successful spawning and recruitment often requires extensive areas of suitable habitat.

Blue mussel beds occur both subtidally and intertidally on a variety of sediment types and in a range of conditions. They are often ephemeral, but play an important role in functioning marine ecosystems, including in coastal sediment dynamics, as a food source for over-wintering waders, and enhancing habitat diversity in sediment-dominated environments. Intertidal mussel beds can occur on a variety of sediments forming dense aggregations supporting various age classes. In the subtidal, dense mussel beds can form on the upper faces of tide-swept sediment seabeds. (The BAP “blue mussel bed” habitat only covers ‘natural’ beds on a variety of sediment types, and excludes artificially created mussel beds, and mussel beds on rock and boulders.


Subtidal sediment habitats and communities are particularly affected by pollution, bottom trawling, dredging and aggregate extraction, and construction of coastal and offshore infrastructure, including offshore windfarms, cables, defence and other developments.

Blue mussel beds are under pressure from commercial fisheries, pollutant bioaccumulation, coastal developments, anchoring, removal for fishing bait and physical trampling. Exploitation of Kent’s mussel beds is managed by the Sea Fisheries Committee through use of byelaws.

Current Action


  1. To better quantify and understand Kent’s subtidal sediments and blue mussel beds.
  2. To identify and protect key sites of marine nature conservation value around Kent.
  3. To promote an integrated cross-sectoral, ecosystem approach to sustainable marine resource management in Kent.

Relevant Habitat Action Plans

Subtidal sands and gravels

Blue mussel beds

Relevant UK SAPs