In New Caledonia, one third of the Island is composed of ultramafic rocks, and lateritic soils enriched in Fe, Ni and Cr. Open-cast mining occurs all around the Island, and processes of erosion and sedimentation, which occur naturally along the coastline, are strongly amplified by mining activities. Due to their position, at the interface between land and sea, mangroves receive extensive amounts of particles emanating from rivers through estuaries. The purpose of this study is to understand the distribution and partitioning of some transitional metals (Fe, Mn, Ni, Cr) in sediments and pore-waters in a mangrove swamp, which is situated downstream of a catchment characterized by lateritic soils that were exploited a century ago. Quantitative analyses on bulk and after selective extraction, were carried out on cores collected along a transect within the intertidal zone, i.e. beneath a Rhizophora stylosa stand, an Avicennia marina stand, and within a salt flat. Mean metal concentrations were (μmol g − 1): Fe (1997) > Ni (44.2) > Cr (31.9) > Mn (8.8). Thus, Ni, Cr and Fe concentrations in this study are substantially higher than mangrove world average. In addition, Ni concentrations are 10 to 100 times higher than in other New Caledonian mangrove developing downstream of a catchment not composed of ultramafic rocks. The studied mangrove is characterized by gradients of water and organic contents with depth and along the intertidal zone, which induced different redox conditions, and thus different metals partitioning. Transitional metals are deposited in the mangrove mainly as oxides and/or oxy-hydroxides, that are subsequently dissolved by bacteria for the decomposition of organic matter, and which leads to a strong increase of metals in the dissolved phase. Then, dissolved metals were precipitated with organic and sulfide compounds. To conclude, organic diagenesis in mangrove sediments leads to the transfer of transitional metals from oxide form to organic and sulfide forms.

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Pyrite contribution to Ni and Fe speciation is low and restricted to the sediments close to the shore Clay minerals are the major host for Fe and Ni across the shore-to-reef gradient Fe-rich smectite, glauconite, chrysotile and greenalite/berthierine are the major Fe and Ni-bearing clay minerals identified Greenalite/berthierine is the most Ni-rich clay mineral identified and it is considered to have formed in-situ upon early diagenesis Green clay authigenesis might represent a major process for trace metals cycling in shallow lagoon sedimentary settings *Highlights (for review : 3 to 5 bullet points (maximum 85 characters including spaces per bullet point) Lagoon sediments, trace metals, speciation, green clays, TEM, XAS.

This review presents environmental and biological indicators of the impact of three major categories of inputs in coral reef lagoons i.e. particles, nutrients and metals. Information was synthesized to extract well established indicators together with some interesting new concepts currently under development, and to provide the reader with an assessment of their respective advantages and drawbacks. The paper has been organized according to the capacity of three categories of indicators to respond either in a specific or a non specific way to a given source of input. The first section focuses on abiotic indicators which main interest is to respond instantaneously and in a truly specific way to a given source of input. The second and third sections present informations on bioindicators either at the sub-individual level or at the individual to community level, indicator specificity generally decreasing as a direct function of biological or ecological complexity. This review showed that even though significant work has already been done on coral reef ecosystems, much more scientific studies are still needed to answer the growing local demands for simple and truly validated tools to be used in environmental surveys. It is further stressed that, due to the biological and environmental diversity of coral reef lagoons, a preliminary step of on-site validation must be considered as an absolute prerequisite when indicators are planned to be used in the frame of a local environmental monitoring programme.

An iminodiacetate chelating resin was optimised for the rapid determination of Co, Cu, Fe, Mn and Ni in seawater. Using inexpensive, high‐capacity, reusable cartridges allowed high flow rates of up to 25 ml min−1. High preconcentration factors, of up to 500, were obtained in order to analyse samples using an ICP‐OES. The requirement for a buffer was eliminated due to the high tolerance of the ICP‐OES to interfering matrix elements, thereby further reducing the potential for contamination. Quantification limits in seawater were: Co = 6 ng l−1, Cu = 8 ng l−1, Fe = 6 ng l−1, Mn = 5 ng l−1 and Ni = 6 ng l−1. The method was verified by the analysis of near shore seawater (CASS‐4) and open ocean seawater (NASS‐5) reference materials. In order to satisfy the high sampling demands using the iminodiacetate cartridges, a portable off‐line preconcentration unit was developed for routine analysis. The multi‐channel preconcentration unit, was capable of treating up to eight samples simultaneously with concentrating times as little as 30 minutes. The technique was also used to determine dissolved metals in fresh and interstitial waters. The technique has been successfully used in a number of environmental studies and impact assessments to evaluate the effects of mining on the New Caledonian lagoon.