Descriptor 7 focuses on permanently altered hydrographical conditions, which predominantly arise from coastal activities causing topographical changes (e.g. land claim, barrages, sea defences) and coastal and offshore infrastructures (e.g. ports, wind farms, oil rigs, pipelines, heat and brine outfalls).

© Fotolia, Author: Halberg
Permanent hydrographical changes can occur due to changes in the thermal or salinity regimes, changes in the tidal regime, sediment and freshwater transport, current or wave action and changes in turbidity. The degree of change and the period over which such change occurs varies considerably, depending on the type of modification. Assessment of the degree of change can be related to both the water column and the sea-floor, and consequently to their biological communities. These types of change are normally triggered by building activities, such as extensions or alterations to the coast, or the building of artificial islands and other infrastructural works in the marine environment (such as outfalls from power stations, bridges and causeways to islands, and offshore installations). This Descriptor addresses all such developments (existing and new infrastructures) including both large- and small-scale structures. The cumulative pressure of 'localized activities' will have to be considered to assess the significance of the aggregated effect of many small-scale changes. Specific importance is given to new planning activities that will have to fulfill Environmental Impacts Assessments (EIA). 

Changes, such as altered erosion patterns or residence time can modify local conditions in a way that negatively impact sensitive species and habitats and can therefore compromise the achievement of the biodiversity and eutrophication Descriptors (D1, D4, D5 and D6). Consequently the cumulative impact on the ecosystem from pressures resulting from the alteration of hydrographical conditions is therefore intimately linked to the assessments of these other Descriptors.
D7 Commission Decision (EU) 2017/848 of 17 May 2017
Permanent alteration of hydrographical conditions does not adversely affect marine ecosystems.
D7C1 – Secondary: Spatial extent and distribution of permanent alteration of hydrographical conditions (e.g. changes in wave action, currents, salinity, temperature) to the seabed and water column, associated in particular with physical loss ( 1) of the natural seabed.
D7C2 – Secondary: Spatial extent of each benthic habitat type adversely affected (physical and hydrographical characteristics and associated biological communities) due to permanent alteration of hydrographical conditions. Member States shall establish threshold values for the adverse effects of permanent alterations of hydrographical conditions, through regional or subregional cooperation.