Characterization and sources of colored dissolved organic matter in a coral reef ecosystem subject to ultramafic erosion pressure (New Caledonia, Southwest Pacific).

International audienceThe eastern lagoon of New Caledonia (NC, Southwest Pacific), listed as a UNESCO World Heritage site, hosts the world's second longest double-barrier coral reef. This lagoon receives river inputs, oceanic water arrivals, and erosion pressure fromultramafic rocks, enriched in nickel (Ni) and cobalt (Co). The aimof this study was to characterize colored dissolved organic matter (CDOM), aswell as to determine its main sources and its possible relationships (through the use of Pearson correlation coefficients, r) with biogeochemical parameters, plankton communities and trace metals in the NC eastern lagoon. Water sampleswere collected inMarch 2016 along a series of river/lagoon/open-ocean transects. The absorption coefficient at 350 nm (a350) revealed the influence of river inputs on the CDOMdistribution. The high values of spectral slope (S275–295, N0.03m−1) and the lowvaluesof specific ultraviolet absorbance (SUVA254, b4 L mg-C−1 m−1) highlighted the photodegradation of CDOM in surface waters. The application of parallel factor analysis (PARAFAC) on excitation-emission matrices (EEMs) allowed the identification of four CDOM components: (1) one humic- and one tyrosine-like fluorophores. They had terrestrial origin, exported through rivers and undergoing photo- and bio-degradation in the lagoon. Thesetwo fluorophores were linked to manganese (Mn) in southern rivers (r = 0.46–0.50, n = 21, p b 0.05). (2) A tryptophan-like fluorophore, which exhibited higher levels offshore. It would be potentially released from the coral reef. (3) A second tyrosine-like (“tyrosine 2-like”) fluorophore. Linked to Prochlorococcus cyanobacteria (r= 0.39, n =47, p b 0.05), this fluorophore would have an oceanic origin and enter in the lagoon through its southern and northern extremities. It also displayed relationshipswith Ni and Co content (r=0.53–0.54, n=21, p b 0.05). This work underlines the diversity of CDOM sources in the NC eastern lagoon