Habitats des fonds marins des côtes de France métropolitaine

• The importance of understanding our seafloor became apparent only a few years ago. This knowledge, initially pursued by academics seeking to describe marine ecosystems on dedicated sites, has become essential today for the sound development of increasingly numerous marine activities. Policies and management actions, now grouped under the term "Marine spatial planning" must be based on the most reliable environmental information if we want to implement sustainable development.

• The seabed, as much as the waters themselves, is subject to multiple disturbances, direct from in situ actions or indirect through continental practices (such as river inputs) or climate change (such as storm frequency or sea level rise). Creating seafloor habitat maps typically involves survey campaigns to acquire biotic data (relating to living organisms) and abiotic data (characteristics of the physical environment) which are then interpreted to produce thematic maps representing habitat units as polygons, similar to land-use maps on terrestrial sectors.

To harmonize the interpretation of these maps across Europe, significant efforts have been made over the past ten years to establish a common typology for all. This delicate exercise, due to the diversity of facies and the complexity of dependent biology, resulted in the marine component of the hierarchical EUNIS system (European Nature Information System), which contains approximately 1,500 classes and today covers the Atlantic and Mediterranean.

EUNIS first describes physical characteristics acting at a regional level (biogeography, temperature, salinity), then local (substrate, light or energy at the bottom) before introducing biological communities and then species at the finest level (level 5). By comparison, for the terrestrial component biogeography is represented by climatic zones.

• To best assign the most detailed possible EUNIS code to a seafloor habitat, in situ data that best reflects the biocenosis (or biological community) is necessary. These are acquired through observation techniques (divers, underwater video) or sampling (grab or dredge). It is also necessary to have "parent" data describing upper levels. Indeed, a biological community or species can be harbored by several types of abiotic parameter combinations, as is the case with many endofaunal communities (living within sediment). Environmental data are obtained partly through observation but also through remote sensing, acoustic and optical methods on the shore or in shallow waters. Increasingly, physical modeling, through its ability to create climatologies of physical forcings (currents, wave action, light), provides a large part of this information.

• Over the past four decades, numerous benthic habitat maps have been produced along French coasts, ranging from highly detailed maps (1/10,000 scale) over small areas to maps covering, for example, part of the English Channel at 1/500,000. The entire French Atlantic portfolio, initially drafted in "author typologies," has recently been translated into the EUNIS typology, with the advantage of simplifying its use and enabling comparisons and syntheses.

The idea of generating a simplified map arose from the need for planning and management documents that are homogeneous and comparable from country to country, while directing future efforts toward more detailed mapping. A representation at EUNIS level 3 quickly emerged as an achievable goal by maximizing existing data sources, with significant compilation, conversion and formatting effort using geomatics tools. The global map thus serves as an intermediate between older regional maps and more recent local—but scattered—maps.

To create this map it is necessary to gather minimum knowledge of: 1. bathymetry and seafloor forms (topography), 2. substrate nature (facies and grain size), 3. available light quantity at the bottom, 4. bottom disturbance induced by wave action and currents.

An immediate user question will be: "what is the map scale?" This scale—often also called resolution due to the grid-based nature of data—was defined as 1/300,000, given the original scale of the base layers. During updates, it will improve as more detailed models or datasets appear.

Recall that the global map does not claim to replace the local maps mentioned above but rather fills gaps between them. Finally, while the map's "global" designation stems from level 3 EUNIS classes integrating complex reality, there is no limit to the map's spatial detail, which depends only on the fineness of the base layers.

The map is a faithful transcription of the EUNIS system. First, it is necessary to define the classes present in a given marine basin. For example, for zones of the French continental shelf (English Channel and Atlantic), 7 substrate classes, 4 depth levels and 3 bottom-energy classes (only significant on rocks) were adopted to conform to the EUNIS typology.

• Creating the map first requires building the three layers presented in the figure below. This phase requires considerable effort because source data are scattered among many producers and of very diverse nature. The most complex case is substrate nature, where maps present different seafloor classification nomenclatures. These layers are classified into EUNIS categories. For example, "biological zones" are the levels characterizing seafloor, namely intertidal, infralittoral, circalittoral, bathyal, abyssal, defined according to physical criteria. The infralittoral corresponds to the well-lit zone (photic), obtained through satellite data on light extinction in the water column. The bathyal level, for its part, begins at the shelf edge, defined by a slope criterion calculated on bathymetry. Once the layers are georeferenced, they are simply combined through simple processing in a GIS.

The map of all France at 1/300,000 clearly highlights several distinctive features of the continental shelf: predominance of coarse sediments in the English Channel (macrotidal regime), large rocky plateaus of the Atlantic infralittoral and circalittoral subject to varied hydrodynamic regimes, the Grande Vasière clearly visible in green in southern Brittany, steep shores in Provence-Alpes-Côte d'Azur and Corsica, Gulf of Lion shelf.

The large Mediterranean depths are almost uniformly covered in mud except sometimes the detrital cones of canyons made of coarser sediments. Note that in the Mediterranean, it was deemed necessary to add Posidonia seagrass beds from detailed maps (in blue) because these beds are recognized as shaping the bottom to the point of being considered as a substrate in its own right.

• The map of the Natura 2000 zone at the Penmarc'h Rocks site is also represented with level 3 EUNIS symbology, without inclusion of biological data. It demonstrates that the global map concept applies under the same conditions to a local site. Indeed the data used here are: a) a very fine-mesh bathymetric digital terrain model down to 15 m depth, then coarser (1/20,000 scale), b) substrate nature obtained through acoustic lateral sonar surveys (1/20,000 scale), c) light and disturbance data at the bottom at approximately 250 m resolution. The final combination on a 5 m mesh naturally results in very coarse resampling of the least-resolved type (c) data, of which users of this map (provided as 1/20,000) must be warned. The immense rocky platform is striking, with its exposure in the infralittoral (purple and dark pink) allowing inference of large algae presence. To the southwest, circalittoral rocks, located between 60 and 100 m depth, are poorly exposed to surface wave action. At the center, the Grande Vasière begins, extending toward the southeast.

These maps have been used in several ways in a regional European context, particularly in the Interreg MeshAtlantic project, where a global map of Atlantic space was produced. In the marine protected areas (MPAs) of the four partner countries, Tempera (2013) extracted mapping statistics for each type of biological zone.

For France, the MPAs considered are marine Natura 2000 sites as well as the Iroise Marine Natural Park, at the tip of Brittany. Using this map shows that the infralittoral and coastal circalittoral are very well represented in our MPAs, but conversely the offshore circalittoral levels (extending to the continental shelf edge) and the bathyal (slope leading to great depths) are insufficiently represented.

Distribution statistics of marine biological zones within French MPAs

• Regarding substrate nature, statistics stand at 39% rock and 12% sediment. French MPAs thus achieve good coverage of rocky zones—with particularly rich fauna and flora—but encompass too low a proportion of soft sediment bottoms. Indeed, it is generally considered that below 20%, habitat protection risks being insufficient. The MPA agency (Odion 2013) performed similar work within the Interreg MAIA project. • Distribution statistics of marine substrate types within French MPAs • Author: French Research Institute for the Exploitation of the Sea (Ifremer).