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Descriptor 6: Sea-floor Integrity

“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”

What is sea-floor integrity?

The sea-floor integrity reflects the characteristics (physical, chemical and biological) of the sea bottom. These characteristics delineate the structure and functioning of marine ecosystems, especially for species and communities living on the sea floor (benthic ecosystems).

To characterize the sea-floor it is common to distinguish various types of seabed according to:

1. Depth

   
   Spider crab on black coral (deep cold coral) – IFREMER

   
   Coralligenous habitat in the Mediterranean - IFREMER

   
   Warty venus in shallow Atlantic waters - IFREMER

   The 200-metre depth contour separates the coastal and continental shelf areas from the deep sea. Coastal and shelf zones are under the direct influence of coastal activities and riverine inputs. In shallow waters the sea bottom receives also a part of the solar light (needed for photosynthesis), which does not penetrate deeper than a few hundred metres. The deep sea starts at the top of the continental slope and drops down to the abyssal plain (several thousand metres deep).
2. Substrate Type
   

   Substrate types are defined based upon their physical properties. There are a large variety of types but they can be grouped into the following broad classes:

   • Soft substrates, e.g. sand, mud, gravel and mixed sediments
   • Hard substrates, e.g. bedrock and boulders
   • Biogenic substrates, e.g. mussel beds and cold-water coral reefs
3. Species Composition
   

   The group of organisms (bacteria, invertebrates, algae, fish, etc.) living in a given space on the sea-floor forms an ecological unit that can be defined by several parameters (abundance, frequency, spatial patterns, etc.). As a general pattern, communities with good environmental status are assumed to be those with a few abundant species and many rare ones. Such a community has a high resilience potential to change (i.e. a high capability of adapting to changes).

Why should we pay attention to the sea-floor integrity?

In the ocean, the sea-floor is a key compartment for marine life, since it is a high biomass productivity area, especially in shallow waters. A great diversity of sea-floor types can be encountered depending on the substrate, the depth and the local environmental conditions. They form different kinds of habitats for fixed or mobile marine species that live on, inside and above the sea bottom.

Human activities may impact the structure (for instance the species composition of the benthic ecosystem) by damaging large or fragile species or modifying their functioning and favoring opportunistic or scavenging species that may profit from disturbance of the bottom and availability of dead organisms. A particular attention has to be paid to some remarkable habitats that, in spite of their reduced spatial extent, play an important role in marine ecosystems dynamics and biodiversity (e.g. biogenic reefs, cold corals, maerl beds).

Maintaining sea-floor integrity is therefore necessary to preserve marine biodiversity and living resources.

What are the main pressures on the sea-floor?

Human activities induce different kinds of pressures that can affect the sea-floor. The main pressures that directly impact the state of the sea bottom are:

1. Coastal infrastructures (ports, defences against erosion, etc.) and offshore installations (oil and gas platforms, wind farms, etc.);
2. Offshore mining and sand extraction;
3. Release of dredged sludge;
4. Moorings;
5. Some fishing practices (trawling, dredging, etc.);
6. Aquaculture (unused fish feed, fish faeces, etc.);
7. Introduction of non-indigenous species (trough ballast water for instance);
8. Pollution (chemical pollution, litter);
9. Changes in riverine inputs (organic enrichment of particulate matter, etc.);
10. Sediment remobilization by fishing equipment (trawls, dredges);
11. Changes in freshwater riverine inputs as a consequence of damming and irrigation;
12. Changes in solid matter riverine inputs; and
13. Release of large quantities of warm (power plant cooling) or salty water (from desalination facilities)
    
    

What can be done?

Monitoring the state of the seafloor

Due to the large variety of seafloor types, it is necessary to define indicators and standardized methods that can give a good image of the status of benthic ecosystems (i.e. living on the sea floor) and of their alteration by pressures from human activities. These indicators can be based on the presence of particularly sensitive or tolerant species or habitats. They can also be indices calculated from several parameters such as the species diversity, the number of species and the proportion of different types of species in benthic invertebrates samples.

Posidania meadows in the Mediterranean- IFREMER

Another example is given for the sea grass Posidonia oceanica which is an emblematic habitat in the Mediterranean Sea. The health of the Posidonia meadows can be assessed by measuring the leaf biomass (per surface unit) of the sea grass.

The monitoring of the pressures applied to the sea-floor, for example the proportion of the seabed significantly affected by human activities for the different substrate types, is also necessary to the assessment of the ecological status of the sea-floor. To this end, data of the distribution of human activities and mapping tools must be collated. The distribution of habitats can be mapped from geo-referenced data gathered by acoustic sonar and optical camera surveys. Repeated surveys give therefore information about the changes in habitat extent. However, this is only realistic for some particular habitats distributed over small/discrete areas.