scientific chapter |
Spalding HL, Amado-Filho GM, Bahia RG, Ballantine DL, Fredericq S, Leichter JJ, Nelson WA, Slattery M, Tsuda RT
Abstract Macroalgae in mesophotic coral ecosystems are generally understudied compared to corals and fishes yet may be more abundant than coral-dominated reefs given their lower depth limits (> 200 m) and ability to grow over soft and hard bottom habitats. These assemblages are abundant and diverse globally, with changing species composition with increasing depth. Ubiquitous macroalgal assemblages include the red algal rhodolith beds and nongeniculate and Peyssonneliales assemblages; green algal Halimeda beds, meadows, and bioherms and Caulerpa spp. beds; and brown algal Lobophora spp. or Distromium spp. beds, Sargassum spp., and kelps. The use of molecular techniques is elucidating macroalgal diversity and rates of endemism, and molecular data and phylogenetic analyses often show strong cryptic diversity or pseudodiversity when compared with morphoanatomical analyses. Mesophotic macroalgae are important as habitat and may serve as seedbanks or refugia for ecosystem resilience following environmental stress. Invasive algal blooms may be deleterious, particularly with the removal of native herbivores or increasing nutrients. Geomorphologically, calcified species such as rhodoliths and Halimeda spp. are significant global producers of calcium carbonate. Abiotic factors influencing the abundance and distribution of mesophotic macroalgae include temperature, water clarity, nutrients, and currents. In general, threats include rhodolith mining, oil spills, sedimentation, ocean acidification, invasive species, bottom trawling, and eutrophication. The impacts of global warming at mesophotic depths are unknown. Future studies should focus on collections for molecular analyses to evaluate population-level dynamics and connectivity between shallow and mesophotic depths and in situ manipulations to determine competitive interactions and ecophysiological processes in these low-light environments.