High-resolution mapping and terrain classification of Halimeda bioherms in the northern Great Barrier Reef

Halimeda is a prolific marine calcifying green alga that forms vast green meadows at mesophotic water depths (20–50 m) on tropical continental shelves. Over time, their carbonate skeletal remains accumulate and build enigmatic interconnected seafloor structures known as bioherms. This study presents the first sub-metre resolution digital elevation model for three sites in the northern Great Barrier Reef, significantly advancing our understanding of their geomorphology since Halimeda bioherms were described in the 1970s. These high-resolution data reveal complex bioherm features, including fine-scale characteristics not previously observed. Geomorphometric analysis was used to quantify surface characteristics within, and between the three sites, and produce the first benthic terrain classification identifying benthic structures. The analysis reveals extensive positive-relief crests encircling hollows, higher prevalence of pinnacles within the hollows, and intricate slope asymmetries suggestive of sediment transport processes. Large internal depressions (150 to 700 m diameter) exhibit weak east-west orientation and increasing irregularity and elongation with size, potentially reflecting the influence of water depth and hydrodynamic conditions on bioherm accumulation over time. Six different bioherm-specific benthic structures were identified including plane, peak or pinnacle, crest, slope, gentle slope, and hollow with a consistent northward shallowing trend across the three sites (15 0 –13 0 S latitude). Their relative area coverage mimics the mound-like bioherm structure, while terrain metrics underscore their structural complexity. These findings reveal that Halimeda bioherms are more morphologically complex than previously described and offer new insight into their formation processes. The terrain classification provides a valuable foundation for future benthic habitat mapping, paleo-environmental analysis and management of these important ecosystems. • First sub-metre multibeam bathymetry of Halimeda bioherms in the northern GBR • Bioherms comprise crest-hollow-pinnacle architectures with fine-scale heterogeneity. • Slope asymmetries and E -W depressions suggest hydrodynamic influence on growth. • South-to-north shallowing trend reflects antecedent surfaces and flooding history. • Six benthic terrain classes as new framework for habitat mapping and management.