An analysis of mesophotic macroalgal species richness and abundance in Puget Sound

Mesophotic ecosystems are important, light-dependent communities that act as an area of transition for shallow and deep-water organisms. Macroalgae are prominent members of these communities whose growth is influenced primarily by depth and light levels. Even though they are important community members and habitat-builders in these mesophotic ecosystems, macroalgae are highly understudied, especially in temperate environments. To examine these understudied communities, macroalgal coverage, richness, and light intensity at 3 different depth levels across 4 sites in Puget Sound, USA were compared using videos collected during SCUBA diving. All but three of the transects were within the surface irradiance limits classified as mesophotic zones. No clear trends between overall coverage and depth were identified with each site exhibiting unique patterns. These findings indicate that there may be interactions between site and depth or other factors influencing temperate mesophotic macroalgal ecosystems such as bathymetry and substrate composition