Seabirds, which feed exclusively at sea and breed on islets, play an important ecological role in transferring nutrients between terrestrial and marine ecosystems. While the influence of guano on terrestrial ecosystems is widely recognized, very few studies have examined a possible effect on adjacent marine ecosystems. A preliminary study by our team demonstrated a significant enrichment of nitrogen (nitrates + nitrites) in the surrounding waters near islets hosting large colonies of seabirds as well as an assimilation of this nitrogen by a branchy coral but also by other components of the coral ecosystem such as macroalgae (assimilation estimated thanks to the high values of tissue d15N in relation to a reference site). However, several questions remain, such as the effect of this guano on coral health.

The reefs far from Entrecasteaux are home to large colonies of seabirds, are isolated, and allow the effect of natural enrichment in nitrogen and phosphate from guano to be tested without any anthropogenic pressure. The project is structured into 3 distinct objectives to assess the role of nutrients from guano on coral ecosystem functioning. Sampling of water, plankton, corals and macroalgae around these remote islands will be carried out at 10, 25, 50, 50, 100 and 400m from the islet where important seabird colonies breed, as well as at 50 and 400m on reference sites, without any bird droppings. The characterization of seabird communities at island level and their quantification will also be carried out during the campaigns, as well as the harvesting of guano from several species by a terrestrial ecologist who will land on the islet.

The specific objectives of the project are:

(1) Evaluate how nutrients from guano (N and P) are released into the coral ecosystem in the vicinity of breeding islets (Entrecasteaux reefs), focusing on the role of the tide, through nutrient sampling (ammonium, nitrates, phosphates) at different time steps, and salinity measurements (link with the freshwater lens under the islets). Dissolution/degradation experiments on guano will also be carried out to assess the potential contributions of dissolved inorganic nitrogen, phosphates and dissolved organic compounds.

(2) Test the use of d15N from the carbonate matrix of massive corals to make an history of the influence of guano on coral communities (analysis of d15N in a core of Porites sp.). Two coral cores will therefore be collected to determine whether this isotopic information can be recorded not only in the living tissues of corals but also in the organic matrix of their skeletons, and thus reconstruct the variation in guano inputs over time.

(3) Diagnose the impact of the assimilation of nutrients from guano on coral physiology: photosynthetic efficiency measurements in situ using a fluorimeter diving-PAM and controlled incubation of Pocillopora damicornis corals on the boat in small flow chambers (photosynthesis measurements).

Studying the impact of nitrogen and phosphate inputs on coral health is particularly important in a context of global change where bleaching episodes are becoming more frequent, particularly in New Caledonia where a massive bleaching wave took place in February 2016. To date, work on the effects of nutrient enrichment on the resistance of corals to climate change has shown divergent results. Finally, this campaign will provide information on the as yet unknown role of seabirds in the functioning of coral reefs in a context where both bird populations and coral reefs are threatened worldwide.

This project involves 6 UMRs as well as a European partner (Max Planck Institute, Germany).