This doctoral dissertation improvement project will study the effects of seabird-derived nutrients on coral reef ecosystems across multiple atolls in the Northwest Hawaiian Islands. Specifically, the study will test the hypothesis that sea bird colonies augment nutrient import to coral reefs, leading to increased coral reef algal cover and abundance of herbivorous fishes. Three approaches will be used: a survey comparing coral reef communities with and without seabird colonies, an analysis of energy pathways from producers to consumers in single reefs, and a transplant experiment between coral reefs with and without seabird colonies.

Coral reef ecosystems and seabird colonies are two of the world?s most endangered marine ecosystems. Understanding the potential indirect impact of seabirds on coral reefs is important for the design of coral reef preservation efforts. This study will promote new synergies in international conservation efforts.

Project Report

Seabirds and coral reefs are two of the most threatened marine communities on the planet, and they co-occur on tropical islands. Seabirds deposit nutrient-rich feces on islands in the form of guano, and this guano accumulates in dense concentrations over time. We set out to measure the impact of nutrient-rich guano on the ecology and biogeochemsitry of adjacent coral reefs which are known to be very sensitive to human-derived nutrients like sewage and agricultural fertilizer. We collaborated with the National Oceanic Atmospheric Administration's Coral Reef Ecosystem Division to analyze coral reef data from the Northwestern Hawaiian Islands and Pacific Remote Island Areas. We used multivariate statistics to test if seabird guano production density was a signficant variable (in addition to many other variables) for explaining benthic percent cover and fish biomass data. We found that guano was indeed significant, and we are currently writing our results into a manuscript that will be submitted for publication. We also analyzed macroalgae from a subset of islands in the Pacific Remote Island Area region to test if guano increased the ratio of heavy to light nitrogen (δ15N) in Halimeda sp. We are currently analzying the resulting data, but preliminary results show that equatorial upwelling is a major factor explaining the δ15N content in this region. To test whether guano had an impact in complex systems at small-scales, we collected seawater nutrient data (dissolved phosphate, nitrate) and algal δ15N next to four offshore islets in Oahu, Hawaii where there were differing numbers of seabirds. We found that phosphate increased incrementally with increasing seabird abundance, and the island with the highest number of seabirds also showed the highest value of algal δ15N. While there are many factors at play, these results support the hypothesis that guano enriches adjacent reefs. We are currently submitting these results for publication in high impact journals, and we will continue to work with the federal agencies and non-governmental organizations (NOAA, Island Conservation, TNC) who manage both seabirds and coral reefs to integrate the management of these threatened communities. We have advanced the understanding of nutrient cycling on coral reefs by using landscape-level analyses to incorporate seabirds as a factor, and the results from these analyses have broad implications for the management and conservation of seabirds and coral reefs.

National Science Foundation (NSF)
Division of Environmental Biology (DEB)
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Henry L. Gholz
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University of California Santa Cruz
Santa Cruz
United States
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