Abstract

Animal forests promote marine habitats morphological complexity and functioning. The red gorgonian, Paramuricea clavata, is a key structuring species of the Mediterranean coralligenous habitat and an indicator species of climate effects on habitat functioning. P. clavata metrics such as population structure, morphology and biomass inform on the overall health of coralligenous habitats, but the estimation of these metrics is time and cost consuming, and often requires destructive sampling. As a consequence, the implementation of long-term and wide-area monitoring programmes is limited. This study proposes a novel and transferable Structure from Motion (SfM) based method for the estimation of gorgonian population structure (i.e., maximal height, density, abundance), morphometries (i.e., maximal width, fan surface) and biomass (i.e., coenenchymal Dry Weight, Ash Free Dried Weight). The method includes the estimation of a novel metric (3D canopy surface) describing the gorgonian forest as a mosaic of planes generated by fitting multiple 5 cm × 5 cm facets to a SfM generated point cloud. The performance of the method is assessed for two different cameras (GoPro Hero4 and Sony NEX7). Results showed that for highly dense populations (17 colonies/m2), the SfM-method had lower accuracies in estimating the gorgonians density for both cameras (60% to 89%) than for medium to low density populations (14 and 7 colonies/m2) (71% to 100%). Results for the validation of the method showed that the correlation between ground truth and SfM estimates for maximal height, maximal width and fan surface were between R2 = 0.63 and R2 = 0.9, and R2 = 0.99 for coenenchymal surface estimation. The methodological approach was used to estimate the biomass of the gorgonian population within the study area and across the coralligenous habitat between −25 to −40 m depth in the Portofino Marine Protected Area. For that purpose, the coenenchymal surface of sampled colonies was obtained and used for the calculations. Results showed biomass values of dry weight and ash free dry weight of 220 g and 32 g for the studied area and to 365 kg and 55 Kg for the coralligenous habitat in the Marine Protected Area. This study highlighted the feasibility of the methodology for the quantification of P. clavata metrics as well as the potential of the SfM-method to improve current predictions of the status of the coralligenous habitat in the Mediterranean sea and overall management of threatened ecosystems.

Highlights

  • The term animal forest refers to the underwater population of suspension feeders mainly represented by sponges, cnidarians and bivalves which are well known for enhancing the morphological complexity of the seascape and structuring the habitat of the communities, promoting their functionalities [1]

  • This study proposes a novel and transferable Structure from Motion (SfM) based method for the estimation of gorgonian population structure, morphometries and biomass

  • P. clavata is of special interest in coralligenous habitats because its canopy reduces the range of environmental variability and supports key associated biota [3,5,6]

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Summary

Introduction

The term animal forest refers to the underwater population of suspension feeders mainly represented by sponges, cnidarians and bivalves which are well known for enhancing the morphological complexity of the seascape and structuring the habitat of the communities, promoting their functionalities [1]. In the Mediterranean Sea, animal forests are well represented by several octocoral species such as the gorgonian Paramuricea clavata (Risso, 1826). This is a slow-growing, long-lived and low fecundity species which forms dense forests along outcrops, cliffs and biogenic substrates [2] from −15 m to −200 m depth [3,4]. P. clavata is of special interest in coralligenous habitats because its canopy reduces the range of environmental variability and supports key associated biota [3,5,6]. The annual linear growth rate for the species ranges between 2.7 cm and 3.0 cm, with larger rates observed for smaller colonies [7]. The drag effect of the fans promotes particle retention, this supporting both invertebrate and vertebrate and favoring the trophic energy transfer between benthos and plankton [9]

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