A Comparison of Macrobenthic Organisms in Shallow-Water and Mesophotic Coral Reef Ecosystems of the Northwestern Hawaiian Islands

Mesophotic Reef Ecosystems (MCEs) are predicted to be an important refuge for shallow-water species in response to climate change. This has led to an increasing amount of research on MCEs in recent years. The Papahānaumokuākea Marine National Monument (PMNM) surrounding the Northwestern Hawaiian Islands (NWHI) represents one of the largest marine protected areas in the world. There has been an increased emphasis on characterizing the biodiversity of the NWHI since the establishment of the PMNM in 2006. The fauna of this remote region has previously been surveyed extensively in shallow-water depths (<30 m) using snorkeling and conventional SCUBA diving, and numerous times in deep-water (>200 m) through the use of trawling, manned submersibles, and remotely operated vehicles. However, little is known about the MCEs found in the intermediate depth ranges. Since 2009, annual research expeditions have surveyed the biodiversity of MCEs within the PMNM using mixed-gas technical diving. Coral Point Count software was used to analyze photoquadrat images taken at shallow-water reefs and MCEs along transects near Midway Atoll, Lisianski Island, Pearl and Hermes Atoll, and French Frigate Shoals. A multivariate analysis of these communities showed a spatial segregation between shallow-water and mesophotic reefs. There was a significantly higher percentage of coral cover at shallow water reefs than mesophotic reefs (p=<0.0024). There was also a significantly higher cover of green, red, and brown macroalgae at MCEs than shallow-water reefs (p<0.01, p<0.0023, p<0.0004). It is important to understand the composition of these ecosystems in order to predict their role in the changing ocean.