Photogrammetric quantitative study of habitat and benthic communities of deep Cantabrian Sea hard grounds

To study the highly complex deep-sea habitats of the Cantabrian Sea and their macro-epibenthic communities a new towed underwater sled was designed to carry out quantitative visual transects based on photogrammetric analysis. The main objective of the study was undertaken to provide a first approach for gaining a better understanding of the correlation between hard substrates, depth and ecology in this region; thereby enabling researchers to determine the extent to which benthic communities depend on physical factors. sults were compared from two areas with different characteristics and methodological problems: one in the central Cantabrian Sea outer shelf (150 m depth), near the head of the Lastres Canyon, and another at the summit of the Le Danois Bank (555 m depth). Two image databases corresponding to two transects were analysed, with every photo being linked to a faunal list and a set of environmental variables. To assess the amount of variation in faunal densities related to the set of habitat environmental characteristics, a redundancy analysis (RDA) was used. The set of environmental variables comprised depth, temperature, salinity, substrate type and seafloor reflectivity. Using the hierarchical classification proposed by EUNIS, three habitats were identified from a Cantabrian Sea shelf visual transect: A4.12—Sponge communities on circalittoral rock (14.5% coverage), A5.35—Circalittoral sandy mud (56.8%) and A5.44—Circalittoral mixed sediments (28.7%). A typical community appeared on the rocky habitat, made up of yellow coral Dendrophyllia cornigera and the cup sponge Phakellia ventilabrum. On Le Danois Bank, three habitats were identified and the cnidarians (Caryophyllia smithii and Callogorgia verticillata) and the sponges (Asconema setubalense, Aplysilla sp., hexactinellids) characterized rocky habitats and patchy rock-sand habitats. tudy provided groundtruthing for the existing surficial seafloor features and very valuable information about the autoecology of sessile and vulnerable species, since the scale used was more appropriate for this kind of study than the macroscale of trawling, with several microhabitats being sampled in the same sample. Also, the use of this non-extractive methodology, which does not cause damage or alterations to benthic communities, is particularly necessary in vulnerable ecosystem studies and Marine Protected Areas monitoring.