Descriptor 7: Hydrographical Conditions
“Permanent alteration of hydrographical conditions does not adversely affect marine ecosystems”
What are hydrographical conditions
Hydrographical conditions are characterized by the physical parameters of seawater: temperature, salinity, depth, currents, waves, turbulence, turbidity (related to the load of suspended particulate matter). They play a crucial role in the dynamics of marine ecosystems and can be altered by human activities, especially in coastal areas.
Why should we pay attention to hydrographical conditions?
The physical parameters of seawater are essential because they structure the water masses and determine the various habitats that provide the environmental conditions required for marine life. These conditions influence the production and the growth of plankton and fish species. The dispersion and dwelling of larvae for many benthic and pelagic species depend on hydrographical factors. They also play an important role for the exchanges between the sea and the atmosphere and between the various layers of water. For instance, the weakening of the surface mixing process can induce a poor oxygenation of the seawater and sometimes lead to anoxic situations and a sudden mortality of some species.
Hydrographical factors also control some features of the seafloor (for instance muddy seabeds require weak currents). Changes in the physical parameters of seawater can then have an impact on the spawning, breeding and feeding areas of marine organisms.
What are the main pressures on hydrographical conditions?
Hydrographical conditions of a specific area are often highly variable and shaped by large-scale forcings such as tide, general oceanic circulation and climate. They may however be affected by human induced pressures, especially in coastal seas.
These disruptions which generally have an impact on a local scale result from:
- infrastructure construction on the coast and offshore, i.e. embankments
- offshore platforms and marine renewable energy installations
- channel creation
- navigation channel dredging
- maritime traffic (in channels, shallow waters)
- sediment remobilization by fishing equipment (trawls, dredges)
- sand extraction, offshore mining
- changes in freshwater riverine inputs as a consequence of damming and irrigation
- changes in solid matter riverine inputs
- release of large quantities of warm (power plant cooling) or salty water (from desalination facilities)
What can be done?
The dynamics of the sea are very complex. The status of the ocean at a given moment results from mechanisms ranging from decadal (such as the North Atlantic Oscillation) to shorter time scales (such as wind or river forcings). Therefore large data sets are required to observe and detect changes in the environmental status. At present data are provided by in situ measurements and satellite observation (which yields only surface information). Synoptic situations can be built at different scales with the help of 3D models which are developed by “operational oceanography” programmes.
An example of an indicator is given by sea surface temperature (SST) maps that can be computed and compared in order to monitor changes