IRD scientists have recently revealed the little-known impact of bird droppings on the marine environment and corals.
Droppings from the sea birds that nest on the coasts do not just fertilise terrestrial ecosystems. For the first time, a study has established their impact on marine environments. “We have shown that the nitrogen contained in guano enriches local seawater,” explains biologist Anne Lorrain, specialist in trophic ecology and isotopic tracing. “It is even assimilated by corals!”
To establish this, IRD scientists worked around the Entrecasteaux and Chesterfield archipelagos, situated respectively 230 and 550 km from new Caledonia. Very isolated, uninhabited and spared the anthropic pollution that might alter the results of the study, these reefs provide the ideal conditions to see whether guano is incorporated into the marine food chain. They are in fact a particularly oligotrophic environment, and accommodate vast colonies of sea birds. “These tiny islands concentrate tens of thousands of seabirds, belonging to a great variety of species, which go there to reproduce,” explains island fauna specialist Éric Vidal. “They spend a few months to over a year there, and live the rest of the time on the high seas in the tropical Pacific.” Their permanent presence leads to the accumulation of huge quantities of excrement on the islands.
Concretely, research uses isotopic markers for the nitrogen contained in guano, to trace its entry into marine food chains. Analysis of samples clearly shows the presence of this element in the waters of the lagoons, but also in the tissues of corals adjacent to the islands. “We have yet to shed light on how the elements of guano end up in seawater,” the researcher admits. “It may leech out of the accumulated stocks on the islands, percolate via resurgences of fresh water at sea, come from direct deposit of droppings as birds fly over the lagoon in search of food, or a mixture of all of these.”
The mechanisms by which the corals assimilate the nitrogen from the bird droppings are also still to be clarified. Two hypotheses exist, or coexist. “This nutrient may be taken in by plankton, which is in turn absorbed by the corals. The nitrogen could also be captured in dissolved form by zooxanthellae, the microalgae living in symbiosis with the corals, and then made available to their hosts,” indicates Fanny Houlbrèque, specialist in coral physiology.
Enrichment with nitrogen from guano also benefits corals situated near to large bird colonies. We have yet to find out how this contribution affects coral health. It could help to improve the coral’s resistance to bleaching, which affects many coral reefs, or it could have a negative impact. Future research will attempt to verify this.