The Ecological Role of Patchy Cold-Water Coral Habitats: Does Coral Density Influence Local Biodiversity in Submarine Canyons of the Bay of Biscay?

Poster presentation at ATLAS 3rd General Assembly.

In the Bay of Biscay, Cold Water Coral (CWC) habitats have been mapped in 24 out of a hundred known submarine canyons. The footprint of ROV or towed-camera transects is small compared to the size of these canyons but still, known CWC habitats totalize a linear of 46 km. The distribution of these habitats, dominated by reef-building scleractinians, antipatharians, alcyonaceans and pennatulids, is however patchy and their median size is small, ranging from 6 to 65 m. While the preservation of these Vulnerable Marine Ecosystems has become essential, all CWC habitats will not be benefit from conservation strategies due to their widespread and patchy distribution. Priorities must thus be defined. Among the criteria for such prioritization, we explore here the ecological role that CWCs provide as habitat engineer.

The abundance, the diversity and the taxonomic composition of megafaunal assemblages associated with each CWC habitat have first been compared. Scleractinian reefs, antipatharians and alcyonaceans on hard substrates as well as pennatulids on soft substrates host different megafaunal assemblages and each should be considered as separate management units. Within each of these three units, we further tested the influence of coral cover or coral density on the abundance and the diversity of the associated fauna. In scleractinian reefs, the abundance and diversity of the megafauna linearly increases with coral cover, providing evidence for the ecological role of coral reefs. Coral cover ranging from 80 to 100% also is common, representing a third of all images where scleractinian reefs have been recorded. Coral cover may thus offer a good proxy for prioritization of management strategies. In habitats dominated by antipatharians and alcyonaceans on hard substrates or dominated by pennatulids on soft substrates, higher abundance and diversity of the megafauna is only found associated with the highest densities of corals (i. e. over 16 colonies per image). The occurrence of such high densities however is quite rare, representing 2% to 3% of images where those habitats have been recorded. Other criteria than the ecological role of non-scleractinian corals as habitat providers for the megafauna may have to guide conservation prioritization.