Honig, S. E., and B. Mahoney (2016), Evidence of seabird guano enrichment on a coral reef in Oahu, Hawaii, Marine Biology, 1–7, doi:10.1007/s00227-015-2808-4.
Seabirds are often found nesting in large numbers on islands near coral reefs. They forage for food out in the open ocean and come back to nest and breed in island colonies. Oahu, the capital-city island of Hawaii with roughly 1 million residents is surrounded by several small islets that are mostly uninhabited by humans. It is on these relatively secluded islets that the majority of the seabird population of Oahu makes their home (Fig. 1).
Coral reefs and seabirds are both threatened marine communities. Their coexistence in close proximity to one another may help the coral reef community thrive by adding concentrated oceanic nutrients to the system. Seabirds find food far out at sea and the nutrients they consume become concentrated in their guano (a.k.a. poop). When the guano washes back into the ocean from their island nesting grounds, these nutrients may become available to the marine environment of the nearby coral reefs. In this way, seabirds connect offshore and coastal marine environments and turn the food chain into something of a loop, connecting the top directly back to the bottom.
Working in some of the isolated islets off Oahu, Honig and Mahoney designed an experiment to determine whether seabird-derived nutrients really do end up in coral reef waters.
A natural experiment
During the rainy season, when seabird guano on islands could most easily wash into the ocean, Honig and Mahoney studied four islets and their neighboring reefs (see map, Fig. 2). Moku Nui, the southeastern-most island has a very large seabird population while fewer seabirds live on the other three islets. All four islets are uninhabited by humans and have coral reefs nearby. Mokoli’I (in the northwest) is arguably the most influenced by human activities since it is nearest any freshwater runoff, but all of the islets are relatively isolated from human activity.
The island with the large seabird population had more dissolved phosphate in the waters nearby than any of the other islands. Phosphate is an essential nutrient that plants need to grow; more phosphate in the water means more resources are available to plants in the coral reef ecosystem. Just because they’re available, though, doesn’t mean that the extra nutrients are being used. To be sure, the researchers also sampled the macroalgae Halimeda (Fig. 3) and found that the seabird-derived nutrient signal also showed up there. Fish and invertebrates feed upon these macroalgae, so a boost in production at the plant level can affect higher and higher trophic levels. Not only are nutrient concentrations higher near the seabird colonies, but their guano could be fueling production at all levels of the coral reef ecosystem.
The link between seabirds and nutrient enrichment becomes complicated in human-influenced areas, like Oahu, where nutrient enrichment can also occur as a result of unnatural causes like runoff from sewage and fertilizers. Too much nutrient loading can often have harmful effects on highly diverse coral reef ecosystems by favoring fast-growing algal species, which tend to support low-diversity ecosystems.
Sewage and fertilizer runoff exist on Oahu and macroalgae growing near those sources show signals that indicate it utilized those unnatural nutrient sources, so how do we know the excess nutrients in the waters off Moku Nui came from the birds? First of all, the islets studied here are relatively far from any direct sources of anthropogenic nutrient input. More importantly, of the four islets studied, only the one with a large seabird population had higher nutrient concentration in the water and the algae. Since all the islets lie in similar proximity to human activities, we can safely assume that the excess nutrients came from the seabirds.
A complicated story for conservation
Seabirds are endangered and conservation efforts are underway to help them recover. But corals are also endangered. Interactions are between the two groups are complicated, and still not well understood. It is possible that increasing the number of seabirds on the island could lead to over-loading of nutrients on reefs that are already under stress due to their proximity to human activity. It’s also possible that these excess nutrients could support a larger fish population that would feed on the excess algae. Conservation efforts, therefore, should make sure to take the whole system into consideration. For example, combing efforts to rebuild seabird populations with stricter fishing regulations may bring the ecosystem into balance. Seabirds and coral reefs coexist naturally in many locations and, with careful planning, both communities should be able to thrive in the Hawaiian Islands.