Using photogrammetry to investigate coral reef erosion by clionid sea sponges

Abstract

Cliona is a genus of endolithic sponge that is steadily gaining dominance in areas such as the Caribbean Sea due to anthropogenically-caused ecosystem changes. These sponges erode and remove reef volume, reducing the carbonate budgets, structural complexity and hence ecological function of coral reefs. However, the behaviour of many Cliona species is still greatly understudied. Their erosion and lateral colonization rates have not been extensively studied in-situ, partly due to the practical difficulties in quantifying small-scale erosion over an extended time period.

This study develops and introduces a more practical and comprehensive method to conduct erosion/colonization rate studies of endolithic sponges, using Structure-from-Motion (SfM) photogrammetry and virtual 3D-model analysis.

This method was trialled on 5 Cliona caribbaea colonies in the Mexican Caribbean, south of the Puerto Morelos National Reef Park. The method required lesser fieldwork time and was capable of acquiring a much more comprehensive dataset to be analysed. In this study, Cliona caribbaea was found to erode almost exclusively at its outer perimeters, on average contributing to 965cm3/yr of volume loss within the visible sponge area, but remarkably further eroding an additional 1595cm3/yr of substrate outside the visible sponge area. Substrate rugosity was also found to be a key factor influencing lateral colonization rates and erosion depths between and within individual sponges.

This paper provides detailed workflows and video tutorials which research labs and citizen science initiatives worldwide can easily use to contribute to the limited knowledge and data on endolithic sponges and their erosional impacts on coral reefs.