Renaud Fichez

Using geochronology to reconstruct the evolution of particulate phosphorus inputs during the past century in the Papeete Lagoon (French Polynesia)

Abstract During the past century, the sediment deposition rate has been assessed by measuring excess 210Pb activity in a sediment core sampled in the harbour area of the Papeete Lagoon (Tahiti, French Polynesia). Dated sediment samples were analysed following a sequential extraction method to quantify five sedimentary phosphorus pools: loosely sorbed or exchangeable phosphorus, ferric iron-bound phosphorus, authigenic calcium carbonate associated phosphorus, detrital apatite and detritic inorganic phosphorus, and organic phosphorus. Results from sediment dating showed the core to cover the period from 1865 to 1995, with a sharp change around 1957 corresponding to a doubling in sediment deposition rates. From the bottom to the top of the core, concentrations decreased for authigenic calcium carbonate-associated phosphorus, were almost constant for exchangeable phosphorus, and increased significantly for iron-bound phosphorus, detritic inorganic phosphorus and organic phosphorus. As a consequence, the calculated phosphorus total accumulation rate was multiplied by four from top to bottom. Those trends were related to modifications in the relative importance of the five phosphorus reservoirs, especially for organic phosphorus, which contribution increased to 30% of total phosphorus in recent sediments. From the sequential extraction approach it is possible to estimate that phosphorus with a terrestrial origin represented 54% of total phosphorus in recent sediment, while it only represented 30% at the beginning of the century. Present particulate phosphorus inputs were also assessed from a sediment trap approach and a budget of phosphorus burial efficiency is proposed.

Spatial variability in Sediment Oxygen Consumption under winter conditions in a lagoonal system in New Caledonia (South Pacific)

Sediment Oxygen Consumption (SOC) was investigated during a winter (Southern Hemisphere) cruise in the southwest lagoon of New Caledonia. Oxygen fluxes were measured at 11 sampling stations distributed along two coast to reef transects. Three different methods of flux measurements were used: diver-operated benthic chambers, laboratory incubation of sediment cores and oxygen microprofiles determinations. SOC values varied between 450 and 2250 μmol O2 m−2 h−1. The level of agreement between the three techniques strongly varied as a function of sediment type. Most of the SOC values from the grey sand zone in the middle part of the lagoon and the muddy bottoms of the bays did not show significant differences. A central station presenting a dense seagrass bed gave lower SOC determined by oxygen microprofiles compared to the two other methods. In coarse carbonated sands from the back reef area, SOC measured by in situ benthic chambers were higher than SOC measured by incubation techniques. This discrepancy could be explained by physical disturbance of the sediments, macroscale variability in benthic communities or technical efficiency of the sediment sampling device and probably by a combination of all three processes. Nevertheless, for the other sediment types that represented 85% of the lagoon bottoms, the results from the three techniques used for SOC determination were strongly convergent. Based on this assumption, it could be stated that the oxygen fluxes were essentially driven by microbial activity compared to biologically mediated vertical transport of solutes. The SOC values determined during this study were in agreement with budgets previously calculated for the lagoon. Regardless of the back reef area, spatial variability in SOC can be explained by the organic matter content of sediments which clearly showed a coast to reef gradient with higher organic carbon and nitrogen contents in the coastal sediments. The C/N ratios demonstrate the higher rate of freshly deposited organic matter near the coast compared to more central stations in the lagoon.

Impact of wind and freshwater inputs on phytoplankton biomass in the coral reef lagoon of New Caledonia during the summer cyclonic period: a coupled three-dimensional biogeochemical modeling approach

A coupled three-dimensional physical-biological model was developed in order to simulate the ecological functioning and potential impacts of land-derived inputs in the southwest lagoon of New Caledonia. This model considered pelagic biogeochemical cycling of organic matter, taking into account advection and diffusion processes driven mainly by local wind fields and freshwater discharges. Modeled phytoplankton dynamics were strongly correlated with both freshwater nutrient inputs and wind-driven hydrodynamic processes, the latter resulting in a large input of oceanic water from the southeast part of the lagoon under trade wind conditions. In situ data obtained during the summer (January 1998) under trade wind conditions supported predicted concentration gradients along several coast to reef transects and provided a validation of the coupled physical-biogeochemical model. An additional sensitivity analysis showed that the alteration of the biogeochemical parameters did not strongly affect the results of the model. Freshwater inputs of nutrients were simulated using a realistic scenario corresponding to the summer rainy season of 1997–1998 in New Caledonia. Despite occasional flooding events from the main rivers considered in these simulations, no significant meso-scale phytoplankton bloom was identified. Hydrodynamically driven dispersion and rapid uptake of nutrients by phytoplankton were sufficient to spatially constrain the impact of river inputs and maintain oligotrophic conditions. The fine spatial grid of our three-dimensional model demonstrated that eutrophication in the southwest lagoon of New Caledonia is confined to the most restricted coastal embayments, while most of the lagoon experiences sustained oligotrophic conditions.

The tropical brown alga Lobophora variegata as a bioindicator of mining contamination in the New Caledonia lagoon: a field transplantation study

Previous field and laboratory studies have identified the alga Lobophora variegata as a good candidate for biomonitoring metal contamination in the New Caledonia lagoon which is subjected to intensive and extensive metal inputs from land-based mining activities. The aim of this work was to further assess the bioindicative potential of this species by investigating, in the field, its bioaccumulation capacity for local key contaminants, i.e. Ag, As, Cd, Co, Cr, Cu, Mn, Ni and Zn. Algae from clean and contaminated sites were cross-transplanted for a period of three months in order to determine the in situ uptake and depuration kinetics of the nine elements. Results indicate that algae transplanted to the contaminated site displayed a significant linear increase in concentration with time for Ag, As, Cd, Co, Cr, Cu, Mn and Ni. In contrast, algae transplanted to the clean site did not show major depuration of these elements, except for Co. Overall, L. variegata showed a rapid temporal response in metal uptake, especially for the elements intensively released into the coastal environment of New Caledonia (viz., Co, Cr, Mn and Ni). This species appears therefore as an excellent bioindicator species of metal contamination in this area. Our results also provide background information necessary for using L. variegata under in situ experimental conditions so as to provide better quantitative information on ambient metal contamination levels. The wide distribution of L. variegata in tropical areas further enhances its potential as a bioindicator species of metal contamination in other tropical coastal environments.

Delineation of heavy metal contamination pathways (seawater, food and sediment) in tropical oysters from New Caledonia using radiotracer techniques

Bioaccumulation of Ag, Cd, Co, Cr and Zn was studied in the oysters Isognomon isognomon and Malleus regula, using highly sensitive radiotracer techniques. Metals were readily bioconcentrated from the dissolved phase. Sediment exposures indicated a low bioavailability of sediment-bound metals (3-5 orders of magnitude lower than dissolved metals). In both seawater and sediment experiments, the two oysters displayed similar bioaccumulation behaviour towards all metals but Ag. Indeed, Ag was much more efficiently incorporated and retained in I. isognomon. Metals ingested with food (phytoplankton) were efficiently assimilated (34-77%) and strongly retained in oyster tissues (T(b1/2)>or=20 d). Estimation of the relative contribution of each exposure pathway indicated that for both species sediment was the dominant pathway for Co and Cd, whereas food was the major source of Zn. Regarding Ag, seawater was the main source for I. isognomon (86%), whereas sediment was the predominant route for M. regula (92%).

Validation of two tropical marine bivalves as bioindicators of mining contamination in the New Caledonia lagoon: Field transplantation experiments

The bioaccumulation and retention capacities of some key local contaminants of the New Caledonia lagoon (Ag, As, Cd, Co, Cr, Cu, Mn, Ni and Zn) have been determined in the oyster Isognomon isognomon and the edible clam Gafrarium tumidum during transplantation experiments. In a first set of experiments, oysters and clams from a clean site were transplanted into contaminated sites. Uptake kinetics determined in the field indicated that for Cr and Cu in oysters and Co, Ni, and Zn in clams, concentrations in transplanted bivalves reached those of resident organisms after 100d, whereas for the other elements, it would require a longer time for transplanted bivalves to reach the same levels as in the resident populations (e.g., up to 3 years for Cd). However, the slow uptake rate for metals observed in the latter transplantation is rather related to low bioavailability of metals at the contaminated sites than to low bioaccumulation efficiency of the organisms. Indeed, results of a second transplantation experiment into two highly contaminated stations indicated a faster bioaccumulation of metals in both bivalves. Results of both transplantations point out that the clam G. tumidum is a more effective bioindicator of mining contamination than I. isognomon, since it is able to bioaccumulate the contaminants to a greater extent. However the very efficient metal retention capacity noted for most elements indicates that organisms originating from contaminated sites would not be suitable for monitoring areas of lower contamination. Hence, geographical origin of animals to be transplanted in a monitoring perspective should be carefully selected.

Influence of food on the assimilation of selected metals in tropical bivalves from the New Caledonia lagoon: Qualitative and quantitative aspects

The present study aimed at examining the influence of food quality and quantity on the assimilation efficiency (AE) of metals in two abundant bivalves in the New Caledonia lagoon, the oyster Isognomon isognomon and the clam Gafrarium tumidum. Bivalves were exposed via their food to the radiotracers of three metals of concern in New Caledonia ((54)Mn, (57)Co and (65)Zn) under different feeding conditions (phytoplankton species, cell density, and cell-associated metal concentration). When bivalves were fed Heterocapsa triquetra, Emiliania huxleyi and Isochrysis galbana, AE of Mn, Co and Zn was strongly influenced by the phytoplankton species and by the metal considered. In contrast, when fed one given phytoplankton species previously exposed to different concentrations of Co, phytoplankton-associated Co load had no influence on the AE and on the retention time of the metal in both bivalves. Metals ingested with I. galbana displayed generally the highest AE in both bivalve species, except for Mn in clams for which the highest AE was observed for H. triquetra. Influence of food quantity was investigated by exposing bivalves to different cell densities of I. galbana (5 x 10(3), 10(4) or 5 x 10(4) cell ml(-1)). As for food quality, food quantity was found to influence AE of Mn, Co and Zn, the highest AE being observed when bivalves were fed the lowest cell density. Overall, results indicate that the two bivalve species are able to adjust their feeding strategies according to the food conditions prevailing in their environment.

Geomorphology and hydrogeology of selected islands of French Polynesia : Tikehau (atoll) and Tahiti (Barrier reef)

The large geomorphological diversity of the Polynesian barrier and atoll reefs can be accommodated by a single heuristic model that is called as geothermal endo-upwelling. The model is based on the circulation of interstitial water driven by thermal convection and modulated at the reef surface by oceanic wave surge and by the circulation of recharge-driven meteoric water. The geothermal endo-upwelling model, which can be viewed as a form of low-energy hydrothermalism, impacts on a diversity of biogeochemical processes including (1) the productivity, calcification and cementation processes active in algal-coral reef ecosystems, (2) carbonate and phosphate diagenesis, and (3) degradation of organic matter. Barrier reefs and atolls of French Polynesia are surrounded by warm and clear oligotrophic water of the South Pacific gyre. Atolls forming the Tuamotu Archipelago show great geomorphologic and hydrologic diversity. Reef-lagoon systems of other archipelagoes (Society Islands, Austral Islands) are similarly diverse. In all these cases, however, barrier reefs maintain a set of uniform morphological features and must be viewed as the first-order structure.

Pore water geochemistry and mixing processes within the Tahiti barrier reef

Abstract We report a multi-parameter study of hydrological and geochemical tracers in a deep borehole (150m) of the Tahiti barrier reef. The principal objective was to characterize the origin of the reef interstitial waters and their exchange and mixing patterns with the surrounding ocean waters. The measurements show that the hydrological and geochemical properties within the borehole at any given level are distinct from the oceanic composition. The vertical distribution of the various tracers displays a coherent picture and identifies two major sources for the reef interstitial waters: first, a deep Pacific water recharge from at least 350 m depth and second, Pacific surface waters that penetrate from the top and the upper section of the reef flanks. The deep Pacific waters are thought to infiltrate at the periphery of the volcanic edifice. They move upward through the basaltic basement and on rising are further mixed, inside the carbonate pile, with the Pacific surface waters.