Evidence of seabird guano enrichment on a coral reef in Oahu, Hawaii

Seabirds and coral reefs are two of the most threatened marine communities on earth, and they co-occur on many tropical islands and subtropical islands and atolls. Seabirds concentrate marine-derived nutrients on breeding islands in the form of feces (guano), and these nutrients dramatically alter terrestrial ecosystem ecology. Recent work in the remote Pacific indicates seabird-derived nutrients may also subsidize nearshore coral reefs, but the consequences of guano on complex, anthropogenically modified coral reefs are unknown. The impact of seabird guano on nearshore coral reefs around Moku Nui, an islet with a large seabird colony in Oahu, Hawaii, was investigated in comparison with coral reefs around three islets with lower seabird abundance. Reefs in close proximity to Moku Nui (where thousands of wedge-tailed shearwaters, Puffinus pacificus, breed) had greater concentrations of dissolved phosphate in seawater and greater delta N-15 in adjacent subtidal macroalgae relative to reefs next to smaller breeding colonies. However, dissolved nitrate was not different among islets. These results indicate that seabirds may be a source of nutrients for the waters surrounding Moku Nui that are already inundated with local and global stressors.