Photogrammetry as a promising tool to unveil marine caves’ benthic assemblages

Abstract

Traditionally, monitoring approaches to survey marine caves have been constrained by equipment limitations and strict safety protocols. Nowadays, the rise of new approaches opens new possibilities to describe these peculiar ecosystems. The current study aimed to explore the potential of Structure from Motion (SfM) photogrammetry to assess the abundance and spatial distribution of the sessile benthic assemblages inside a semi-submerged marine cave. Additionally, since impacts of recent date mussel Lithophaga lithophaga illegal fishing were recorded, a special emphasis was paid to its distribution and densities. The results of SfM were compared with a more “traditional approach”, by simulating photo-quadrats deployments over the produced orthomosaics. A total of 22 sessile taxa were identified, with Porifera representing the dominant taxa within the cave, and L. lithophaga presenting a density of 88.3 holes/m2. SfM and photo-quadrats obtained comparable results regarding species richness, percentage cover of identified taxa and most of the seascape metrics, while, in terms of taxa density estimations, photo-quadrats highly overestimated their values. SfM resulted in a suitable non-invasive technique to record marine cave assemblages. Seascape indexes proved to be a comprehensive way to describe the spatial pattern of distribution of benthic organisms, establishing a useful baseline to assess future community shifts.

First Survey of the Sponge Community of a Semi-Submerged Marine Cave along the Adriatic Apulian Coast

“…These data may be useful for monitoring purposes, as previous observations in other marine caves over a 50-year monitoring period have revealed a shift from three-dimensional to two-dimensional growth forms, which are more resilient to thermal anomalies [47,[50][51][52]. Related to this, it is also important to note the presence of a miniaturized growth form of A. aerophoba, already reported in the Ligurian, Ionian, and Northern Adriatic seas [52,53]. It is likely that the development of this variant over the last few years has been linked to climatic anomalies that have affected the Mediterranean Sea [54].…”

Sponge Communities of Submarine Caves and Tunnels on the Fernando de Noronha Archipelago, Northeast Brazil

“…This is the first study describing sponge communities in submarine caves and tunnels in Brazil. The study of submarine cavities in Brazil is still incipient, and many additional steps are needed for a better knowledge of marine caves and tunnels along the Brazilian coast and oceanic islands: (1) to provide quantitative data and detailed mappings using photoquadrats, photogrammetry, transects, echo-sounders, light sensors, and compasses [80,81]; (2) to investigate the dark zones of Sapata Cave and Ilha do Meio Cave, which remain largely unexplored; (3) to carry out more collections of sponges in the three cavities studied here, because many species appeared only in photographs and were not collected yet; (4) to publish taxonomic descriptions of the species that remain undescribed;…”