The benthic substrate of the Greater North Sea is predominantly characterized by soft sediments (from muds to gravel beds; Figure 14). The spatial distribution of habitats follows gradients with respect to depth and latitude. The relatively shallow southern North Sea (0–50 m) is subject to greater natural disturbance due to wave and tidal effects giving rise to relatively coarse seabed sediments, for example the Dogger Bank and parts of the Southern Bight. Further north, beyond the Dogger Bank, mud content increases, as observed in the Fladen Ground at depths of 100–150 m and in the Norwegian trench at depths of 400 m. Coarse gravel and rock habitat types are predominately located in the English Channel.
The combination of depth, nearbed hydrodynamic conditions, and sediment types give rise to a large diversity of benthic communities within the ecoregion. Nearbed hydrodynamic stress is an important determinant of the species composition of benthic communities. High nearbed tidal forcing results in the transport of sands and fine gravel which act as natural stressors on the benthos, favouring benthic organisms that have specific traits adapted to such conditions (e.g. relatively short‑lived and fast‑growing species), whereas more hydrodynamically stable conditions with low levels of nearbed stress favour a greater diversity of life strategies. More locally, especially in the southern part of the North Sea, seabed geomorphological variations create localized habitats which can favour disturbance-resistant and long-lived species.
The impacts on seabed habitats and associated benthic community by bottom trawling in the ecoregion have been assessed by combining data on benthic species-specific longevity, local depth, sediment data, and bottom‑trawling intensity to generate a map of potential benthic impacts (Figure 15).
Fishing-induced physical disturbance is estimated to have resulted in an overall decrease of invertebrate benthic biomass of approximately 20% in the ecoregion when compared to an unfished state. This impact is patchy and may be as high as 90% in the most heavily fished area.
Except for patches of Sabellaria spinulosa (Ross worm) and Modiolus modiolus (horse mussel) reefs and scattered glacial erratic boulder fields, the North Sea contains limited biogenic and geogenic reefs. However, until the 1920s, dense oyster beds (Ostrea edulis) were widespread in parts of the southern central North Sea, creating diverse and productive benthic habitat. This biogenic habitat has, however, largely disappeared following the mechanization of fishing fleets. In this respect, the North Sea remains one of the most impacted shelf sea regions in the world. Other human-mediated disturbances on the seabed consist mainly of sand and gravel extraction and the growing introduction of offshore renewable energy structures such as wind farms, leading to potential smothering and habitat loss. Artificial hard substrates, such as hydrocarbon production platforms, wind turbines, and ship wrecks provide new different habitat types that can locally increase biodiversity. Sea grasses (Zosteraceae) used to be common off the coasts of the southern North Sea; however, their extent is now more limited due to the loss of shallow intertidal and delta areas.
Figure 15: Impact of physical disturbance (abrasion by bottom trawling) on the benthic invertebrate community biomass. The highest impact is found in areas with high sensitivity and high abrasion. Low impact means low abrasion, low sensitivity or both.
