Mismatch between the ecological processes driving early life-stage dynamics of bivalves at two contrasting French Polynesian lagoons.

The pearl-farming industry depends mostly on the natural recruitment of pearl oysters. Little is known about the relative influence of different ecological processes on the natural recruitment of pearl oysters across biogeographical scales. Spatio-temporal dynamics of bivalve larvae and spats were described at Ahe and Mangareva, 1500 km apart across French Polynesia. We quantified the effect of candidate environmental predictors on the dynamics of larvae. Both lagoons showed similar temporal dynamics with twice more larvae and 6 times more spat in Ahe. Pinctada maculata spat were more abundant than for P. margaritifera at both lagoons. While the temporal dynamics in larvae abundance were best explained by a positive effect of temperature in Ahe, the dynamics in Mangareva were poorly predicted by the environmental variables, meaning bivalve early-life stages perform better in Ahe than Mangareva suggesting a mismatch between the relevant environmental forces driving larval dynamics at these two contrasting lagoons.

Effects of elevated temperature and pCO2 on the respiration, biomineralization and photophysiology of the giant clam Tridacna maxima.

Many reef organisms, such as the giant clams, are confronted with global change effects. Abnormally high seawater temperatures can lead to mass bleaching events and subsequent mortality, while ocean acidification may impact biomineralization processes. Despite its strong ecological and socio-economic importance, its responses to these threats still need to be explored. We investigated physiological responses of 4-year-old Tridacna maxima to realistic levels of temperature (+1.5°C) and partial pressure of carbon dioxide (pCO2) (+800 µatm of CO2) predicted for 2100 in French Polynesian lagoons during the warmer season. During a 65-day crossed-factorial experiment, individuals were exposed to two temperatures (29.2°C, 30.7°C) and two pCO2 (430 µatm, 1212 µatm) conditions. The impact of each environmental parameter and their potential synergetic effect were evaluated based on respiration, biomineralization and photophysiology. Kinetics of thermal and/or acidification stress were evaluated by performing measurements at different times of exposure (29, 41, 53, 65 days). At 30.7°C, the holobiont O2 production, symbiont photosynthetic yield and density were negatively impacted. High pCO2 had a significant negative effect on shell growth rate, symbiont photosynthetic yield and density. No significant differences of the shell microstructure were observed between control and experimental conditions in the first 29 days; however, modifications (i.e. less-cohesive lamellae) appeared from 41 days in all temperature and pCO2 conditions. No significant synergetic effect was found. Present thermal conditions (29.2°C) appeared to be sufficiently stressful to induce a host acclimatization response. All these observations indicate that temperature and pCO2 are both forcing variables affecting T. maxima's physiology and jeopardize its survival under environmental conditions predicted for the end of this century.

Influence of temperature and pearl rotation on biomineralization in the pearl oyster, Pinctada margaritifera.

The objective of this study was to observe the impact of temperature on pearl formation using an integrative approach describing the rotation of the pearls, the rate of nacre deposition, the thickness of the aragonite tablets and the biomineralizing potential of the pearl sac tissue though the expression level of some key genes. Fifty pearl oysters were grafted with magnetized nuclei to allow the rotation of the pearls to be described. Four months later, 32 of these pearl oysters were exposed to four temperatures (22, 26, 30 and 34°C) for 2 weeks. Results showed that the rotation speed differed according to the movement direction: pearls with axial movement had a significantly higher rotation speed than those with random movement. Pearl growth rate was influenced by temperature, with a maximum between 26 and 30°C but almost no growth at 34°C. Lastly, among the nine genes implicated in the biomineralization process, only Pmarg-Pif177 expression was significantly modified by temperature. These results showed that the rotation speed of the pearls was not linked to pearl growth or to the expression profiles of biomineralizing genes targeted in this study. On the basis of our results, we consider that pearl rotation is a more complex process than formerly thought. Mechanisms involved could include a strong environmental forcing in immediate proximity to the pearl. Another implication of our findings is that, in the context of ocean warming, pearl growth and quality can be expected to decrease in pearl oysters exposed to temperatures above 30°C.

Trophic status of earthen ponds used for semi-intensive shrimp (Litopenaeus stylirostris, Stimpson, 1874) farming in New Caledonia (Pacific Ocean).

We have investigated temporal variability in the quantity and biochemical composition of sediment organic matter along with variables proxies of water eutrophication (e.g., inorganic nutrient and chlorophyll-a) at two shrimp farms located in the Southern coast of New Caledonia and characterised by clear differences in shrimp feeding practices and levels of initial trophic conditions. The results of our study reveal that the trophic status of the water column increased during the rearing cycle at both sites, determining a general, though moderated, eutrophication. However, the water column trophic descriptors did not allow to discriminate differences in the trophic status among the investigated sites or between sites in the same farming plant, even if they were subjected to different feeding practices and largely different initial characteristics of the sediment. Temporal variations in biopolymeric C and phytopigment sedimentary contents (used as proxies of benthic eutrophication) varied inconsistently among sites. The multivariate analyses did not identify significant temporal patterns in the benthic trophic status, but allowed discriminating the four investigated sites. The semi-intensive shrimp farming significantly contributed to changing the water column and sediments trophic status of the earthen ponds, but the extent of those changes was not consistently observed in all ponds. In any of the investigated ponds the trophic status exceeded concerning thresholds over which hypoxia or anoxia could occur. We conclude that the established semi-intensive practices adopted so far for shrimp farming activities in the earthen ponds of New Caledonia are able to maintain the status of the ponds below the eutrophication levels over which dystrophic crises could sharply abate most of the reared biomass.

Brachidontes variabilis and Patella sp. as quantitative biological indicators for cadmium, lead and mercury in the Lebanese coastal waters.

The mussel, Brachidontes variabilis, and the limpet, Patella sp., were used as indicators to monitor cadmium, lead and mercury concentrations along the Lebanese coast. Studies were carried out in order to define the best strategy for assessing and minimizing the effects of size and physiological condition on the metal contents of the molluscs, and corrective models were constructed. Metal concentrations in surface water were measured to estimate bioconcentration factors (BCFs). The BCFs varied from 8.3 x 10(3) to 3.4 x 10(4), from 7.5 x 10(3) to 8.0 x 10(3) and from 3.0 x 10(4) to 3.2 x10(4), for Cd, Pb and Hg, respectively. For limpets, BCFs varied from 1.7 x 10(4) to 7.4 x 10(4) for Cd, from 2.5 x 10(3) to 6 x 10(3) for Pb and remained fairly constant at around 10(4) for Hg. The highest BCFs were associated with lowest contamination levels. The results of the geographical survey exhibited a similar large-scale spatial pattern for the two species and followed the metal concentration distributions measured in the waters.

[Phytoplankton ecology along the French coasts: inter-site comparison]. / Ecologie du phytoplancton sur les côtes françaises: comparaison inter-sites.

The aim of this study is to characterize the large-scale geographical structures of microphytoplankton communities along French coasts. Once the temporal variability component shared by all sampled coastal sites was identified, and seasonal succession of microphytoplankton communities was described. A between-site analysis, using multiable comparison methods (STATIS), allowed the identification of three large areas according to the temporal variability patterns of microphytoplankton populations. The results indicate that the hydrodynamic characteristics of the different areas play a major role in the geographical structure of microalgal populations in mainland French coastal waters.

Comparison of polycyclic aromatic hydrocarbon levels in mussels and oysters in France and the United States.

This paper compares data of 15 individual polycyclic aromatic hydrocarbon (PAH) concentrations from two monitoring programs: the French Rdseau National d'Observation de la qualité du milieu marin (RNO) and the Mussel Watch project of the U.S. National Status and Trends (NST) program. Polycyclic aromatic hydrocarbon measurements in bivalve (mussels and oysters) are made from samples collected at 265 sites along the U.S. coastline and at 97 sites in the French coastal waters. Individual PAH patterns were found strikingly similar between the two countries with higher concentrations for high-molecular-weight (HMW) PAHs. Principal component analysis results for both RNO and NST show the variability to be dominated by just two components with HMW compounds contributing primarily to the first and low-molecular-weight (LMW) compounds to the second. This could imply a separation of petrogenic and pyrolytic sources with the latter being the more important in both nations accounting for the similarity in results.

Integrated biomarker response: a useful tool for ecological risk assessment.

A battery of biomarkers is often used to evaluate the effects of exposure to chemical contaminants and detect responses to environmental stress. Unfortunately, field application of biomarkers is subject to various constraints (e.g., the availability of living material) that can limit data acquisition and prevent the use of multivariate methods during statistical analysis. In these circumstances, a simple method is needed to summarize biomarker responses and simplify their interpretation in biomonitoring programs. The present study used star plots to display results for the panel of biomarkers used for each station and survey. Integrated biomarker response (IBR) was then computed as the star plot area. Star plots using IBR values instead of biomarker data make it possible to visualize between-site and/or between-survey differences for comparison with exposure conditions. This approach was applied to sites in the Baltic Sea and the Seine Estuary, English Channel. In both cases, IBR values were visually compared to polycyclic aromatic hydrocarbons (PAH) or polychlorobiphenyls (PCB) levels measured in mussel or fish tissues. The IBR, as an indicator of environmental stress, appears to be a useful tool for scientists and managers in assessing ecological risk.