Mesmerizing Macroalgae in Mesophotic Coral Ecosystems in the Hawaiian Archipelago

Mesophotic Coral Ecosystems (MCEs) in the Hawaiian Archipelago are low-light and presumably oligotrophic environments, yet contain a high diversity and abundance of macroalgae. What processes are driving the high abundance and diversity of these algae? Measurements of macroalgal tissue nutrients, stable isotopes, and irradiance profiles in the water column were conducted across a gradient of anthropogenic impact in the Hawaiian Archipelago to determine how bottom-up processes may be influencing macroalgal abundance. Invasive macroalgae were abundant offshore of densely populated areas and contained elevated tissue nutrients, suggesting that eutrophication may be impacting some MCEs via hypothesized submarine groundwater discharge. Several genera of endemic native and invasive macroalgae co-occurred in dense beds to 90 m depths. These data suggest that some macroalgae have evolved to thrive in these low-light habitats and have created islands of biodiversity that may be under threat from invasive species influenced by anthropogenic nutrients.