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

Abstract

The 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 from ultramafic rocks, enriched in nickel (Ni) and cobalt (Co). The aim of this study was to characterize colored dissolved organic matter (CDOM), as well 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 samples were collected in March 2016 along a series of river/lagoon/open-ocean transects. The absorption coefficient at 350nm (a) revealed the influence of river inputs on the CDOM distribution. The high values of spectral slope (S, >0.03m) and the low values of specific ultraviolet absorbance (SUVA, <4Lmg-Cm) 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. These two fluorophores were linked to manganese (Mn) in southern rivers (r=0.46-0.50, n=21, p<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<0.05), this fluorophore would have an oceanic origin and enter in the lagoon through its southern and northern extremities. It also displayed relationships with Ni and Co content (r=0.53-0.54, n=21, p<0.05). This work underlines the diversity of CDOM sources in the NC eastern lagoon.

Bio-Optical Characterization and Ocean Colour Inversion in the Eastern Lagoon of New Caledonia, South Tropical Pacific

“…The coastal lagoons of New Caledonia are typically dominated by oligo-mesotrophic waters throughout the year (~0.3 mg·m −3 of Chla), with strong influence of the adjacent ocean. However, during the rainy season, and especially after intensive rainfall events, that wash out the eroded soil, organic and inorganic sediments, as well as nutrients that enhance phytoplankton growth, are carried into the lagoon increasing the water turbidity and altering the trophic state [9,10,42]. of the TREMOLO project [9], with three in situ campaigns up to date: the CALIOPE 1 (October 2011), CALIOPE 2 (March 2014), and CALIOPE 3 (March 2016, [10]).…”

“…The lagoon is connected to the adjacent Pacific Ocean, through strait passages that allow the intrusion of oligotrophic oceanic waters forced by tides, trade winds, and circulation of the external Vauban Current [50]. On the continental side, there are many small and medium size rivers that contribute to the system, as well as several mining sites [10,51]. Compared to the western lagoons, the ELNC is deeper and has more restricted connections to the open ocean, through strait passages [9,10,47].…”

“…On the continental side, there are many small and medium size rivers that contribute to the system, as well as several mining sites [10,51]. Compared to the western lagoons, the ELNC is deeper and has more restricted connections to the open ocean, through strait passages [9,10,47]. The austral spring-summer is generally hot and rainy, with more intensive showers extending through January to March, whereas the austral autumn-winter (March to September) is generally dryer and marked by more intensive southeast trade winds [52,53].…”

“…In February 2016 it experienced massive coral bleaching [2] and is still under recovery. The east coast of New Caledonia is also rich in nickel and cobalt, and mining exploitation activities have accelerated soil erosion processes and impacted the water quality of the lagoon throughout the past years [9,10]. Hence, monitoring programs are essential to understand the processes affecting the water quality and coral reef ecosystem of the ELNC, to detect changes timelessly, and to improve protection programs.…”

“…Such algorithms should be more suitable for optically complex waters, with independent variation of the in the context of the TREMOLO project [9], with three in situ campaigns up to date: the CALIOPE 1 (October 2011), CALIOPE 2 (March 2014), and CALIOPE 3 (March 2016, [10]). …”