Quantifying Coral Reef Composition of Recreational Diving Sites: A Structure from Motion Approach at Seascape Scale

Marco Palma,João Fernandes,Ubaldo Pantaleo,Chiara Magliozzi,Carlo Cerrano,Paul Leinster,Gianpaolo Coro,Monica Rivas Casado

doi:10.3390/rs11243027

Abstract

Recreational diving is known to have both direct and indirect impacts on coral habitats. Direct impacts include increasing sedimentation, breaks and diseases that lead to a decrease in the richness and abundances of hard corals. Indirect impacts include urban development, land management and sewage disposal. The ecological effects of scuba diving on the spatial composition metrics of reef benthic communities are less well studied, and they have not been investigated at seascape scale. In this study, we combine orthomosaics derived from Structure from Motion (SfM) photogrammetry and data-mining techniques to study the spatial composition of reef benthic communities of recreational diving sites at seascape scale (>25 m 2 ). The study focuses on the case study area of Ponta do Ouro Partial Marine Reserve (Mozambique). Results showed that scuba-diving resistant taxa (i.e., sponges and algae) were abundant at small (>850 m 2 ) and highly dived sites (>3000 dives yr − 1 ), characterized by low diversity and density, and big organisms with complex shapes. Fragile taxa (i.e., Acropora spp.) were abundant at low (365 dives yr − 1 ) and moderately dived sites (1000–3000 dives yr − 1 ) where the greater depth and wider coral reef surfaces attenuate the abrasive effect of waves and re-suspended sediments. Highest taxa diversity and density, and lowest abundance of resistant taxa were recorded at large (>2000 m 2 ) and rarely dived sites. This study highlights the potential applications for a photogrammetric approach to support monitoring programs at Ponta do Ouro Partial Marine Reserve (Mozambique), and provides some insight to understand the influence of scuba diving on benthic communities.

Highlights

Coral reefs across the world are facing multiple threats with anthropogenic activities playing a key role in the type and magnitude of the impacts
We combine orthomosaics derived from Structure from Motion (SfM) photogrammetry and data-mining techniques to study the spatial composition of reef benthic communities of recreational diving sites at seascape scale (>25 m2)
This study highlights the potential applications for a photogrammetric approach to support monitoring programs at Ponta do Ouro Partial Marine Reserve (Mozambique), and provides some insight to understand the influence of scuba diving on benthic communities

Summary

Introduction

Coral reefs across the world are facing multiple threats with anthropogenic activities playing a key role in the type and magnitude of the impacts. Several small scale studies on diving impacts have demonstrated that coral composition differs significantly between more and less frequented diving sites, with lower hard coral richness and abundance at diving sites with high levels of diving activity [2,3,16,17]. These effects show considerable variability in both ecological (i.e., taxonomic and functional coral groups) and physical (i.e., size, geomorphology, hydrodynamic, depth) characteristics of the reef [2,18]. In addition to the effects on small scale taxonomic structure, scuba diving can lead to seascape [21] reef physical deterioration and functional changes (i.e., reproductive cycles and trophic interactions) [2,22]

Results

Discussion

Conclusion