Transfer of metallic trace elements by dissolved organic matter to the ocean


Project leader : Cécile Dupouy

The Team : Farid Juillot (IMPMC, Paris IRD), Hugues Lemonnier (IFREMER), Peggy Gunkel-Grillon (Université de Nouvelle-Calédonie), Laura Boher (VSC CRESICA), Marie Bessard (IRD), Léocadie Jamet (US IMAGO LAMA, IRD), Philippe Gérard (US IMAGO LAMA, IRD), Stéphane Mounier (MIO, Université de Toulon), Benjamin Oursel (MIO, Université de Toulon), Marc Tedetti (MIO, Marseille).

Project duration : March 2018-March 2021

Collaborating partners

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River in the north of New Caledonia lined with mangroves
Copyright : Laura Boher


The new INSU EC2CO TREMOR project aims to round out research work recently initiated by MIO in New Caledonia, with a thesis on the “Dynamics of coloured and fluorescent dissolved organic matter in tropical lagoon areas in the South Pacific - New Caledonia: climatic and anthropogenic influences” (Martias, 2018) dealing with FDOM in the waters of New Caledonia’s lagoon, as part of the INSU EC2CO TREMOLO project (2012-2015), and as part of the CNRT DYNAMINE project (2015-2018). It is the continuation of the DYNAMINE project (DYNAmique des métaux de la MINE au lagon) (2015-2017), funded by the CNRT, which focused on the dynamics of metals along the mining-mine-river-lagoon continuum in the Koné region (Koniambo massif). TREMOR focuses on the river-mangrove-lagoon interface and complements previous studies on mangroves conducted by IRD, IAC and UNC.


New Caledonia's coastal soils are characterised by the fact that they are essentially made up of laterites, which have been mined for decades for their content of Trace Metal Elements (TMEs), such as nickel, chromium, cobalt and manganese. Because of their high organic matter productivity, mangroves are a major reservoir of Dissolved Organic Matter (DOM) that can be transported to the ocean over long distances.

The observations carried out lead us to question the sink/source role of these ecosystems in relation to the dynamics of DOM at the land-sea interface, downstream of the New Caledonian mining massifs. The aim of the TREMOR project (2018-2020) is to provide answers based on the fluorescence properties of a fraction of the MOD, called Fluorescent Dissolved Organic Matter (FDOM).

TREMOR aims more particularly to better understand the origin, sources and sinks of the MODF identified in the rivers and lagoon of New Caledonia and to assess the influence of this MODF on the dissemination and toxicity of TMEs within the coastline downstream of mining sites. Comparisons with mangroves in other regions (Brazil, Fiji...) in relation to their geodynamics are envisaged due to the involvement of some team members in other regions of the world. TREMOR is also contributing to the development of a standardized method (spectrofluorometry) that can be used to quickly and cheaply identify certain traces of anthropization (sewage, fires...) in fresh and marine waters.


Approach, method and skills mobilized

This project mobilizes various skills in biogeochemistry, biology, geochemistry, chemistry and optics. Water samples are taken from the river and the lagoon, from the sediments (pore water), filtered online (micropore 0.2 μm) and stored until their analysis by spectrofluorometry. The 3D fluorescence emission/excitation spectra are analysed using a software developed (PARAFAC) to digitally extract the contributions of different fluorophores (natural, anthropic...) and to compare them with those found in other environments or periods.

Quenching laboratory experiments (fluorescence attenuation by complexation effect by successive addition in logarithmically increasing concentration) with some ETMs (Cu and Ni) allow the calculation of fluorophore complexation constants and finally give an indication of the capacity of the fluorescent CDMO to form complexes and trap ETMs.


Field of study

The study site is made up of two rivers: the Temala River, which drains volcano-sedimentary terrain, and the Coco River, which drains the ultrabasic lateritic massif of Koniambo). For each river, the samples correspond to a continuum river (stations T1 and C1) - mangrove (stations T2/T3 and C2/C3) - mouth (stations T4 and C4) - lagoon (stations T5 and C5) in the bays of Chasseloup and Vavouto, respectively.

First results

The first available results show that the Coco and Temala rivers have different geochemical signatures related to the geological and pedological nature of the drained land. For both sites, the concentrations of ETM increase strongly between the river and the mangrove, and are very low in the lagoon. Among the identified fluorophores, three are common to the two rivers studied (humic type in the rivers and mangroves, fulvic type at the mouths and lagoon). The complexing properties of fluorophores with respect to Ni and Cu were confirmed. All these results suggest that MODF certainly plays a role in the dispersion (river dilution effect), or non-dispersion (creation of autochthonous fluorophores in the mangrove), of the ETMs at the river-mangrove-lagoon interface downstream of the ultrabasic massifs. However, a particular phenomenon of aggregation/flocculation of MODF seems to occur during the mixing of fresh and salt water at the Coco site, which remains to be studied.


Outlook for TREMOR

The mechanisms of aggregation/flocculation of MODF will be more particularly studied in the laboratory at the end of 2019 (thanks to the support of a 3.5-month fixed-term contract). A campaign in March 2020 (thanks to the support of a lecturer from the University of Toulon, MIO, by CRESICA) will enable further study of the particular behaviour of MODF at the Coco site, by sampling the salinity gradient and focusing more specifically on the phase-particulate dissolved phase relationship. A final mission in October 2020 will be used to characterize the specific fluorophore of the mangrove, and valorize all the results.


Map of stations sampled in the Temala and Coco rivers flowing into the Koné lagoon in the Northern Province of New Caledonia
Copyright : Farid Juillot

References to go further


Bessard, M., 2019. Complexation de la MOD avec les ETM Analyse par extinction de fluorescence sur un continuum rivière-mangrove-lagon dans la province Nord de la Nouvelle-Calédonie, Rapport de Master 2 Univ. Perpignan, Directions C. Dupouy et S. Mounier, 42 pages avec annexes.

Boher , L., 2018. Analyse par quenching de fluorescence de la complexation de la MOD le long d’ un continuum rivière-mangrove-lagon à Koné (Province Nord de la Nouvelle-Calédonie), Rapport de Master 2, Univ. Brest Labex Mer, 23 pages avec annexes.

Boher L., Mounier, S., Juillot, F., Jamet, L., Dupouy, C., 2018. Saltmarsh organic matter fluorescence and metal interaction, Speed talk WOMS 2018, Carqueiranne, octobre 2018.

Dupouy C., Röttgers, R., Tedetti M., Frouin R., Rodier, Martias, C., M., Goutx, 2020, Influence of meteorological conditions on the distribution of CDOM and biogeochemical parameters in the Eastern lagoon of New Caledonia, Frontiers, Biogeochemical Responses of Tropical Ecosystems to Environmental Changes (sous presse).

Dupouy, C., Baumann, F., Bouard, S., Goirant, C., Lemonnier, H., Letourneur, Y., Meyer, M., Tatin, C., 2019. The “Along the River” program: a research and innovation project focusing on the integrated management of water, its uses and its governance in New Caledonia, in Abs. Booklet, p.7, Conférence STAR 2019, 19-23 novembre, Suva, Fiji.

Dupouy, C., Juillot F., Lemonnier, H., Bessard, M., Boher L., Jamet, L., Mounier, 2020. Influence of salinity gradient on Ni complexation by Fluorescent Dissolved Organic Matter (FDOM) and dispersal across estuaries in New Caledonia, OS3-8, EGU Vienne (Autriche), 3-5 Mai 2020.

Juillot et al., 2019, DYNAMINE : Dynamique des métaux de la mine au lagon dans l'hydrosphère en Nouvelle-Calédonie. CNRT Nickel et son Environnement, Tome Nickel et Santé, Rapport final, octobre 2019, pp. 206.

Marchand, C., Allenbach, M., Lallier-Vergès, E., 2011. Geoderma relationships between heavy metals distribution and organic matter cycling in mangrove sediments (Conception Bay, New Caledonia). Geoderma 160:444–456. j.geoderma.2010.10.015.

Martias, Chloé. 2018. “Thèse de Doctorat Dynamique de La Matière Organique Dissoute Colorée et Fluorescente En Zone Lagonaire Tropicale Dans Le Pacifique Sud ( Nouvelle Calédonie ): Influences Climatiques et Anthropogéniques.” 235 pages

Martias, C., Tedetti M., Lantoine, F., Jamet L., Dupouy, C., 2018. Spatial distribution of chromophoric dissolved organic matter and trace metals in the New Caledonia tropical lagoon (South Pacific Ocean), Science of the Total Environment, 616-617, 438-452, doi: 10.1016/j.scitotenv.2017.10.261.

Mounier, S., Zhao, H., Garnier, C., Redon, R., 2011. Copper complexing properties of dissolved organic matter: PARAFAC treatment of fluorescence quenching. Biogeochemistry 106, 107–116.

Stedmon, C.A., Markager, S., Bro, R., 2003. Tracing dissolved organic matter in aquatic environments using a new approach to fluorescence spectroscopy. Mar. Chem. 82, 239–254.