Quantifying sponge communities from shallow to mesophotic depths using orthorectified imagery

Mesophotic coral reefs are estimated to represent up to 80% of the total areal coverage for coral reefs worldwide. Quantifying mesophotic coral reef community structure and function, at multiple spatial and temporal scales, and the ability to monitor these attributes repeatedly and accurately, is an ecological priority. Recent discussions on the relative merits of remotely operated vehicles and autonomous underwater vehicles (ROVs and AUVs, respectively) versus the use of technical divers using quadrats or photoquadrats to obtain quantitative imagery undervalues the distinct and important complimentary roles that both approaches bring to the study of mesophotic coral reefs. However, all platforms must adhere to fundamental photogrammetry principles to accomplish the goal of accurate and repeatable surveys of coral reefs. Here we show that quantifying the projected surface area of sponge populations for a tropical coral reef on Puerto Rico using ROV imagery, not originally collected for ecological characterizations, requires specific screening guidelines to ensure the images are orthogonal and processed to obtain orthorectified images for the quantification of benthic communities. This is required to minimize multiple sources of error that could confound quantitative estimates of percent cover, biomass, and/or abundance of benthic taxa on shallow and mesophotic coral reefs using plot, or plotless, designs.