Seabed substrate of the Bay of Biscay: sand, mud, rock and what it changes for cetaceans

The presence of cetaceans in the Bay of Biscay is readily explained by water temperature and by the abundance of plankton. That is true, but incomplete. Beneath those moving variables lies a fixed one, which organises the rest: the nature of the seabed. Sand, mud, rock and coarse sediment do not carry the same prey, and that map does not change from one season to the next.

What the map shows

The data come from EMODnet Geology, which harmonises national surveys at European scale, here essentially those of the SHOM and of Ifremer. The classification used is Folk‘s, reduced to five classes, the European standard for describing a surface sediment by its proportions of gravel, sand and mud.

The map can be handled directly: scroll wheel or buttons to zoom, drag to move about, arrow keys as well. Without JavaScript it stays displayed as an overview.

The reading is clear. Sand covers nearly half the continental shelf. The muds are concentrated in two opposite settings: the coastal mudflats at the mouths of the Loire and the Gironde, where the rivers deposit their fine particles, and the abyssal plain, where only the lightest particles manage to settle out. Rock outcrops on the continental slope and off the Breton and Cantabrian coasts. Coarse sediment, gravels and pebbles, occupies the areas swept by the strongest currents.

Why this concerns cetaceans

A cetacean does not choose a sediment, it follows its prey. But its prey do choose a sediment.

The sandeel, a needle-shaped fish on which many predators depend, buries itself only in clean, well-sorted sands free of mud. Its distribution is therefore dictated by grain size, and with it that of the seabirds and marine mammals that exploit it. The mudflats, by contrast, harbour Norway lobster, sole and solenette, a quite different assemblage.

The harbour porpoise, a coastal, bottom-feeding species, is closely tied to the soft substrates of the shelf. The common dolphin, more mobile, exploits the water column over the shelf as well as over the slope. Cuvier’s beaked whale and the pygmy sperm whale, for their part, frequent only the deep water beyond the shelf break, where the map shifts to deep mud.

This structure throws light on what our other pages show: the modelled distribution of cetaceans and the sightings recorded in the Bay trace contrasts of which the geology of the seabed is one of the causes, alongside the thermal and bathymetric preferences particular to each species.

The shelf break, a dividing line

The most important feature of the map is not a colour but a boundary: the passage from the continental shelf to the slope, at around 200 metres depth. On the map it corresponds to the rocky band and to the abrupt appearance of the deep muds.

This break concentrates biological activity, because the currents that strike the slope force upwellings of nutrient-rich deep water there. It also separates two cetacean communities: the shelf species on one side, the deep divers on the other. The Capbreton Canyon, a submarine canyon that cuts into the shelf to within a few hundred metres of the Landes coast, is a spectacular local exception: it brings slope conditions within immediate reach of the shore, which explains why offshore species are sighted there, and stranded there, far more often than elsewhere on the French coast.

A geology map, and its three limits

Its scope needs to be stated plainly. This is a geology map, not a cetacean map. It says neither where the animals are, nor how many there are, nor whether they feed there. It describes the support of the food web.

Three limits are worth knowing. The five-class Folk classification is a summary: two seabeds classed as “sand” may differ in their fine grain size and harbour neither the same fauna nor the same prey. Next, the scale of rendering is 1:250,000, which rules out any local reading; the original surveys, finer in places, were generalised for the European harmonisation. Finally, surface sediment is not fixed: storms, tidal currents and bottom trawling rework it, and some surveys in the set are several decades old.

That last point is not incidental. Bottom trawling physically alters the sediment and the fauna that lives in it, which is among the pressures described in our page on fishing and ship strikes in the Bay.

Frequently asked questions

What is the Folk classification?

A nomenclature for marine sediments based on the relative proportions of gravel, sand and mud, proposed by the geologist Robert Folk in 1954. It exists in 16, 7 or 5 classes; the five-class version, the most robust for comparing surveys of different origins, is the one used here. Our glossary covers the technical terms used on the site.

Does the nature of the seabed determine the presence of cetaceans?

Not directly. It determines the benthic fauna and part of the fish, and therefore the food on offer, which in turn bears on the distribution of predators. It is a chain of causes, not a mechanical link: temperature, depth, currents and season count just as much.

Why so much mud offshore?

Because mud, made of very fine particles, settles only where the water is calm. These are the two ends of the system: the sheltered coastal mudflats, fed by the rivers, and the abyssal plain, beyond the reach of currents and swell. Between the two, the swept shelf keeps the sands and the gravels.

Where do these data come from, and can they be reused?

From EMODnet Geology, which distributes them under a Creative Commons Attribution 4.0 licence, so they are reusable with citation, including commercially. The underlying French surveys are those of the SHOM and of Ifremer. The Office français de la biodiversité publishes a record of its own for the SHOM dataset covering the Bay of Biscay, but with no mention of a licence, which led us to prefer the European source.

Sources

• EMODnet Geology, seabed_substrate_250k layer, WFS service, Bay of Biscay extent, 4,260 polygons. Licence Creative Commons Attribution 4.0. Portal: emodnet.ec.europa.eu.
• Office français de la biodiversité, record “Sédimentologie : nature des fonds superficiels du Golfe de Gascogne (SHOM)“, phy_atl_nature_fond_shom_dcsmm_pol_wgs84 layer, scale 1:100,000, distributed as a view service.
• Folk R. L., 1954. “The distinction between grain size and mineral composition in sedimentary-rock nomenclature”. The Journal of Geology 62: 344-359.
• Surface areas by class computed by cetace.info in a cylindrical equal-area projection, before geometric simplification.