Estimating coral reef carbonate budgets using Structure-from-Motion photogrammetry Coral Reefs

Carbonate budget assessments quantify rates of calcium carbonate production and erosion from habitat-altering marine taxa and can be used to evaluate the potential for reef growth and the persistence of coral reef frameworks. Tracking the key ecosystem processes that control carbonate budgets is increasingly critical as climate change threatens to shift reefs toward net erosional states. However, the complex and time-intensive nature of traditional in-water carbonate budget surveys have limited their application in logistically challenging locations. Here, we introduce a new field-efficient methodology for collecting benthic carbonate production, endolithic bioerosion, and urchin erosion data for census-based carbonate budget assessments using Structure-from-Motion (SfM) photogrammetry. To evaluate the efficacy of our approach, we compared SfM-derived metrics with in-water data collected following the ReefBudget methodology at sites in the main Hawaiian Islands, Mariana Archipelago, and American Samoa. Our image-based SfM approach generated gross carbonate production, urchin erosion, and net carbonate production rates that were comparable to in-water estimates while still capturing site-level variability in production states. We observed greater deviation between methods in several constituent metrics, as SfM estimates of rugosity and endolithic bioerosion were higher and coral cover and urchin densities lower than in-water data; yet, these differences did not ultimately result in major systematic methodological biases in net production across our sites. Although SfM techniques can require significant upfront costs and annotation time, they also offer substantial advantages over in-water surveys. These advantages extend across field efficiency, measurement precision for key parameters, scalability, and potential to leverage models for multiple applications.