Full-spur mapping: refining coral reef carbonate production estimates using underwater photogrammetry

Traditional methods for estimating calcium carbonate production on coral reefs rely on in-water line transects that sample only a fraction of the reefscape, inherently introducing spatial biases and under-representing the heterogeneity of coral assemblages. Here, we diagnose how these spatial assumptions influence carbonate production estimates by comparing line transects with full-spur, plan-view mapping derived from structure-from-motion photogrammetry. Three entire reef spurs (ca 850 m² total) within a highly degraded Caribbean reefscape were mapped to quantify coral cover, species composition and gross calcium carbonate production. Full-spur mapping revealed lower but more representative mean coral cover and production than conventional transects, indicating systematic overestimation driven by transect placement and reefscape-structure effects associated with partial spatial sampling (i.e. the spatial configuration of edges, flanks and gaps that can be under-represented by conventional and low-density transect sampling designs). While transect-derived averages masked heterogeneity into near-homogeneous, higher-mean matrices, full-spur mapping resolved strongly patchy production, with small, low-cover patches concentrating a disproportionate share of output amid extensive low-production areas. By coupling spatial structure with process at the scale of natural reef units, this framework exposes hidden biases in long-standing monitoring practices and strengthens the foundations for predicting geomorphic persistence in an era of accelerating reef decline.