Bay of Biscay: plankton and temperature, the food of cetaceans

The Bay of Biscay, a nourishing ocean for cetaceans

The Bay of Biscay is among the richest areas in Europe for cetaceans: common dolphins, striped dolphins, common bottlenose dolphins, pilot whales, fin whales, sperm whales and beaked whales are regularly sighted there. This abundance owes nothing to chance: it rests on the biological productivity of its waters, itself governed by two parameters that can be followed from space, the sea surface temperature and the chlorophyll concentration, a marker of phytoplankton.

Chlorophyll: the base of the food chain

Chlorophyll-a measured by satellite indicates the density of phytoplankton, the microalgae that, through photosynthesis, form the first link in the marine food chain. Phytoplankton feeds the zooplankton (including krill and copepods), which in turn feeds the small pelagic fish (sardines, anchovies, sprats) that are the prey of dolphins and large rorquals. In the Bay of Biscay, chlorophyll undergoes a powerful spring bloom (March to May) over the continental shelf, then a secondary autumn bloom, two peaks that set the rhythm of the bay’s ecological year.

Surface temperature and thermal fronts

The surface temperature moves from about 11 °C in late winter to 21-22 °C at the height of summer. Beyond the mean, it is above all the thermal fronts, those lines of contrast between water masses, that matter to cetaceans: the mixing there concentrates nutrients, hence plankton, hence prey. The continental slope of the bay, where the seabed drops abruptly from 200 to 4,000 metres, generates upwellings that are particularly favourable to deep-diving sperm whales and beaked whales.

Copernicus and NASA satellites

The map draws on open satellite imagery: the surface temperature comes from the GHRSST MUR analysis (1 km resolution, cloud-free), and the chlorophyll from the Sentinel-3 OLCI optical sensors (European Copernicus programme) and MODIS-Aqua (NASA). Since optical data are blocked by cloud, some days show areas without measurement: scroll through the days to find the clearest images.

Going further

The oceanography of the bay sheds light on the lives of the cetaceans that frequent it: thermal preferences of the species, annual migrations, sighting hotspots, diets, ocean currents and impacts of climate change.