Spatial dynamics of crustose coralline algae and turf algae as an indicator of reef recovery

In 2015, prolonged warming caused severe bleaching across reefs in the Main Hawaiian Islands resulting in widespread coral mortality. In addition to understanding changes in coral populations, it is also important to study algal dynamics on these reefs, especially with a focus on crustose coralline algae (CCA) and turf algae. CCA can serve as indicator for coral growth and recovery, thus playing a significant role in determining the future of bleached coral reefs. Meanwhile, turf algae quickly occupies available space, competes with corals and prevents coral settlement. To assess the state of these Hawaiian reefs, a large-scale mosaic imaging technique has been in use since 2014 to document the conditions of select sites before and after the bleaching event. These 33 m2 photomosaics cover three reefs in leeward Maui across a gradient of herbivore biomass and sediment input. Analysis of the mosaics shows marked differences in turf and CCA spatial dynamics and percent cover between sites. Sites more protected from anthropogenic influences had the greatest CCA growth, while the site most impacted by local stressors lost CCA abundance and had the highest increase in turf algae. This suggests local management and stressors around a reef may affect CCA and turf growth, and therefore influence coral recovery. The large-area mosaic imaging method may serve as a useful model for analyses of reef resilience on coral reefs.