The Influence of Seabird Guano on Pacific Coral Reef Ecosystems

The biosphere is composed of a mosaic of ecosystems that are connected to one another through biological and physical processes. Animals that migrate over large spatial scales can play a significant role in vectoring nutrients and energy from one ecosystem to another. For example, seabirds forage in pelagic ecosystems on fish, krill, and squid and then migrate thousands of kilometers to breeding islands where they deposit large concentrations of nutrient-rich feces, or guano. Over sixty years ago, G.E. Hutchinson hypothesized that nutrients from seabird guano could increase primary productivity in recipient marine ecosystems. The nutrients in seabird guano have been widely documented to dramatically alter recipient terrestrial and freshwater ecosystems, but the influence of guano on nearshore marine ecosystems remains elusive. In the following dissertation, I assess the impact of seabird guano on one of the most biodiverse marine ecosystems on earth; coral reefs. First, I examine the influence of seabird guano subsidies on coral reef benthic community composition and fish biomass across one of the largest marine protected areas on earth, the Pacific Remote Islands Marine National Monument. I demonstrate that coral reefs next to islands with high densities of seabird guano have more herbivorous fishes and significantly different benthic communities. Next, I use stable isotope analysis and benthic percent cover data to evaluate the nitrogen-footprint from seabirds on coral reef macroalgae adjacent to Rose Atoll, American Samoa. I demonstrate that algae adjacent to Rose Island, which is home to a large seabird colony, is enriched in delta15N. However, algal ecological responses to seabirds are taxon-specific: fleshy algal cover increases with increasing proximity to Rose Island, whereas calcareous algal cover decreases with increasing proximity to Rose Island. Finally, I assess relative differences in seawater nutrients and algal nitrogen isotope values in the complex, anthropogenically-modified coral reefs of Oahu, Hawaii to see how seabird community subsidies may alter coral reefs in pristine vs. impacted Pacific islands. Even in the midst of anthropogenic fishing and nutrient enrichment, I demonstrate that islets with large seabird colonies have greater dissolved phosphate values and higher algal delta15N compared to islets with small seabird colonies. Together, these three data chapters are the first test of Hutchinson’s local enrichment hypothesis in coral reef ecosystems, and they provide strong evidence that seabirds significantly influence coral reef ecology and biogeochemistry via cross-ecosystem nutrient subsidies. Coral reefs and seabirds are two of the most threatened marine communities on the planet, but they are currently managed in isolation. The results of this research indicate that seabirds should no longer be ignored as a significant source of nutrients on coral reefs. Instead, seabird and coral reef conservation should be integrated in order to maximize efficient and potentially synergistic management of these two globally-imperiled marine communities