Evidence of nutrient enrichment by seabirds on coral reefs in Oahu, Hawaii

Background/Question/Methods

Seabirds forage offshore, then deposit marine-derived nutrients on their breeding islands in the form of guano. Nutrient-rich guano has been documented to subsidize recipient island- and freshwater communities through bottom-up enrichment, but cross-ecosystem flux into nearshore marine communities is debated. We visited four islets offshore of Oahu, Hawaii that varied approximately 100-fold in wedge-tailed shearwater (Puffinus pacificus) abundance to test the hypothesis that seabirds increase dissolved phosphate, nitrate, and algal δ¹⁵N on adjacent coral reefs. We collected replicate seawater samples and Halimeda spp. algal samples on the leeward side of each islet in January and February 2012. We filtered seawater and used a Lachat QuikChem 8000 Flow Injection Analyzer to measure dissolved nitrate and phosphate levels (μM). We oven-dried, homogenized and analyzed algal samples for δ¹⁵N using an elemental analyzer coupled to an isotope ratio mass spectrometer (IRMS).

Results/Conclusions

Dissolved phosphate was significantly higher at the most abundant seabird islet and decreased incrementally with decreasing seabird abundance. However, dissolved nitrate showed no significant difference across islets. Algal δ¹⁵N was significantly higher offshore of the most abundant seabird islet, but showed no differences across the three other islets. Our results suggest that seabirds are a potential source of nutrients for surrounding reefs. Seabird-derived phosphate may be a limiting nutrient in this system, while seabird-derived nitrogen may be swamped out by anthropogenic sources of nitrogen. Our isotope results support the notion that retention of seabird-derived nutrients may be a function of seabird abundance at breeding islets. Seabird-derived nutrients may have varying consequences for recipient coral reef ecosystem health depending on the level of anthropogenic nutrient inputs and fishing pressure in the system. Seabirds and coral reefs are two of the world’s most threatened marine communities, and understanding the impacts of seabird-enrichment on coral reef ecology can help managers identify potential synergies between seabird and coral reef conservation actions.