Abstract
We assess the occurrence and extent of cold-water coral (CWC) species Madrepora oculata and Dendrophyllia cornigera, as well as gorgonian red coral Corallium rubrum, in La Fonera canyon head (Northwestern Mediterranean Sea), as well as human impacts taking place in their habitats. Occurrence is assessed based on Remotely Operated Vehicle (ROV) video imaging. Terrain classification techniques are applied to high-resolution swath bathymetric data to obtain semi-automatic interpretative maps to identify the relationship between coral distribution patterns and canyon environments. A total of 21 ROV immersions were carried out in different canyon environments at depths ranging between 79 and 401 m. Large, healthy colonies of M. oculata occur on abrupt, protected, often overhanging, rocky sections of the canyon walls, especially in Illa Negra branch. D. cornigera is sparser and evenly distributed at depth, on relatively low sloping areas, in rocky but also partially sedimented areas. C. rubrum is most frequent between 100 and 160 m on highly sloping rocky areas. The probable extent of CWC habitats is quantified by applying a maximum entropy model to predict habitat suitability: 0.36 km2 yield M. oculata occurrence probabilities over 70%. Similar predictive models have been produced for D. cornigera and C. rubrum. All ROV transects document either the presence of litter on the seafloor or pervasive trawling marks. Nets and longlines are imaged entangled on coral colonies. Coral rubble is observed at the foot of impacted colonies. Some colonies are partially covered by sediment that could be the result of the resuspension generated by bottom trawling on neighbouring fishing grounds, which has been demonstrated to be responsible of daily increases in sediment fluxes within the canyon. The characteristics of the CWC community in La Fonera canyon are indicative that it withstands high environmental stress of both natural and human origin.
Highlights
Reef-forming cold-water corals (CWC) provide ecological niches and substrate that harbour a large variety of other species acting both as refuges for prey as well as nursery areas [1]
We present a systematic Remotely Operated Vehicle (ROV) exploration of different canyon environments within La Fonera canyon head aiming at detecting CWC communities and quantifying their extent by applying predictive habitat mapping techniques based on high-resolution swath bathymetry data
[51] concluded that canyon walls seem to be relatively less impacted than the canyon floor in terms of number and density of litter items, our observations indicate that within the inspected depth range of 79 to 401 m, litter is specially abundant on the canyon walls and crests, whereas is relatively less abundant along the floor
Summary
Reef-forming cold-water corals (CWC) provide ecological niches and substrate that harbour a large variety of other species acting both as refuges for prey as well as nursery areas [1]. The term “cold-water corals” encompasses stony corals (Scleractinia), true soft corals (Octocorallia, including red corals), black corals (Antipatharia), and calcifying lace corals (Stylasteridae); being scleractinian white corals Lophelia pertusa and Madrepora oculata the predominant reef-forming species [9], often accompanied by Dendrophyllia cornigera [2]. These sessile suspension feeders prefer hard substrates within high energetic hydrodynamic environments that prevent deposition of sediment and where nutrient-rich waters stimulate food supply [9, 10] in a temperature range of 4–13°C and a salinity range of 32–38.8‰ [1]. Access to advanced instrumentation to explore deep-water environments such as manned and Remotely Operated Vehicles (ROVs) is progressively revealing the scale and abundance of CWC colonies, the rough topography of coral-prone areas such as submarine canyons makes difficult an accurate positioning and endangers the operation of underwater vehicles
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