Physical loss and disturbance of the seabed

© Rob Bouwman.Trawler with two beam trawls. Beam trawls are relatively heavy gear that can influence the benthic fauna and flora.

Seabed habitats provide valuable ecosystem services to societies and industry in terms of food, raw materials, energy and space (e.g. fisheries). To obtain these benefits, we undertake several activities that disturb the seabed and its habitats by changing geological, physical and chemical conditions or directly affect the benthic biology. Human activities can cause physical loss or disturbance to the seabed, and the MSFD requires the assessment of the extent of such damage, as well as of the benthic habitat’s extent and condition and the adverse effects from physical disturbance produced on them.

Physical loss of habitat is an extreme pressure on marine ecosystem. Habitat is lost if its substrate, morphology or topography is permanently altered. The main activities causing such damage are offshore installations, port anchorage, dredging and dumping, windfarms, sand or gravel extractions and all kinds of constructions in or over the seabed.

Physical disturbance is caused by several human activities that affect the seabed either directly or indirectly. The main drivers are demersal fishing, shipping in shallow waters, finfish aquaculture, port anchorage, dredging and offshore installations. Sedimentation caused by dredging operations, demersal fishing gears, sediment disposal and construction projects spread sediments to the water column and cause direct sedimentation disturbance, which attenuates away from the activity zone. Abrasion of the seabed by benthic trawling or mussel and scallop dredging damages abiotic and biotic structures, whereas indirect abrasion is caused around built structures by altered current regimes or in shallow waters by wake induced turbulence from ships.

In relation to 'Sea-floor integrity', the Marine Strategy Framework Directive considers that 'good environmental status' is achieved when ‘Sea-floor integrity is at a level that ensures that the structure and functions of the ecosystems are safeguarded and benthic ecosystems, in particular, are not adversely affected.'

General outcomes from the regional assessments

OSPAR indicator shows the distribution and intensity of pressure from bottom-contact fishing activity and the associated disturbance to the seafloor at the OSPAR regional scale. The assessment covers the period 2010–2015. It shows that up to 86% of the grid cells assessed in the Greater North Sea and Celtic Seas show evidence of some physical disturbance of the seafloor from bottom contacting fishing gears, of which 58% of areas show higher levels of disturbance. Areas assessed in the Celtic Seas and the English Channel have higher levels of disturbance than other regions. There are no clear trends across habitats or regions.

Loss and disturbance to the seabed in the Baltic Sea is caused by multiple human activities such as extraction of seabed sand and gravel, modification of the seabed for installations, maintenance of open waterways by dredging, and bottom trawling. Based on the data available for the assessment period (2011-2016) and current knowledge, less than 1 % of the Baltic Sea seabed is potentially lost due to human activities while roughly 40 % of the seabed area is potentially disturbed.

Outcomes from the MSFD assessments

In 2018, Member States had to update the Good Environmental Status (GES) assessments performed under Marine Strategy Framework Directive Article 8. The present dashboard displays the overall status reported by countries for the features, where the results show which is the percentage of assessments where GES has been achieved, not achieved or is unknown or not assessed.