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article of the month – Page 4 – Laboratoire LEMAR UMR 6539

A Model of Mercury Distribution in Tuna from the Western and Central Pacific Ocean: Influence of Physiology, Ecology and Environmental Factors

Patrick Houssard, David Point, Laura Tremblay-Boyer, Valérie Allain, Heidi Pethybridge, Jeremy Masbou, Bridget E. Ferriss, Pascale A. Baya, Christelle Lagane, Christophe E. Menkes, Yves Letourneur, et Anne Lorrain



  • Information on ocean scale drivers of methylmercury levels and variability in tuna is scarce, yet crucial in the context of anthropogenic mercury (Hg) inputs and potential threats to human health. Here we assess Hg concentrations in three commercial tuna species (bigeye, yellowfin, and albacore, n = 1000) from the Western and Central Pacific Ocean (WCPO).
  • Models were developed to map regional Hg variance and understand the main drivers. Mercury concentrations are enriched in southern latitudes (10°S−20°S) relative to the equator (0°−10°S) for each species, with bigeye exhibiting the strongest spatial gradients. Fish size is the primary factor explaining Hg variance but physical oceanography also contributes, with higher Hg concentrations in regions exhibiting deeper thermoclines.
  • Tuna trophic position and oceanic primary productivity were of weaker importance. Predictive models perform well in the Central Equatorial Pacific and Hawaii, but underestimate Hg concentrations in the Eastern Pacific. A literature review from the global ocean indicates that size tends to govern tuna Hg concentrations, however regional information on vertical habitats, methylmercury production, and/or Hg inputs are needed to understand Hg distribution at a broader scale. Finally, this study establishes a geographical context of Hg levels to weigh the risks and benefits of tuna consumption in the WCPO.

Observed spatial variation in mercury concentrations (mg*kg −1 , dry weight) for bigeye, yellowfin, and albacore muscle samples captured in the Western and Central Pacific Ocean. Gray lines outline the five biogeochemical regions as defined in Houssard et al., 2017: NPTG (North Pacific Tropical Gyre), WARMm (Warm Pool modified), PEQD (Pacific Equatorial Divergence), SPSGm (South Pacific Subtropical Gyre modified) and ARCHm (Archipelagic deep basins modified) along with AUS-TAZ (Australia-Tasmania) and NZ (New Zealand).


Houssard, P., Point, D., Tremblay-Boyer, L., Allain, V., Pethybridge, H., Masbou, J., Ferriss, B.E., Baya, P.A., Lagane, C., Menkes, C.E., Letourneur, Y., and Lorrain, A. 2019. A Model of Mercury Distribution in Tuna from the Western and Central Pacific Ocean: Influence of Physiology, Ecology and Environmental Factors. Environmental Science & Technology. doi:10.1021/acs.est.8b06058.

Click here for the IRD news about this study (in French).

Modeling reproductive traits of an invasive bivalve species under contrasting climate scenarios from 1960 to 2100

Mélaine Gourault, Sébastien Petton,Yoann Thomas, Laure Pecquerie, Gonçalo M. Marques, Christophe Cassou, Élodie Fleury,Yves-Marie Paulet et Stéphane Pouvreau


  • The DEB model available for the Pacific oyster was applied in a new coastal environment: the bay of Brest (France).
  • This version was successfully calibrated using a new dataset covering 6 years (from 2009 to 2014) of field monitoring.
  • The model successfully predicted in detail the complex reproductive processes of C. gigas, especially its spawning behavior.
  • Hindcasting and forecasting simulations of the reproductive phenology of C. gigas were performed using IPCC scenarios.


Identifying the drivers that control the reproductive success of a population is vital to forecasting the consequences of climate change in terms of distribution shift and population dynamics. In the present study, we aimed to improve our understanding of the environmental conditions that allowed the colonization of the Pacific oyster, Crassostrea gigas, in the Bay of Brest since its introduction in the 1960s. We also aimed to evaluate the potential consequences of future climate change on its reproductive success and further expansion.

Three reproductive traits were defined to study the success of the reproduction: the spawning occurrence, synchronicity among individuals and individual fecundity. We simulated these traits by applying an individual-based modeling approach using a Dynamic Energy Budget (DEB) model. First, the model was calibrated for C. gigas in the Bay of Brest using a 6-year monitoring dataset (2009–2014). Second, we reconstructed past temperature conditions since 1960 in order to run the model backwards (hindcasting analysis) and identified the emergence of conditions that favored increasing reproductive success. Third, we explored the regional consequences of two contrasting IPCC climate scenarios (RCP2.6 and RCP8.5) on the reproductive success of this species in the bay for the 2100 horizon (forecasting analysis). In both analyses, since phytoplankton concentration variations were, at that point, unknown in the past and unpredicted in the future, we made an initial assumption that our six years of observed phytoplankton concentrations were informative enough to represent “past and future possibilities” of phytoplankton dynamics in the Bay of Brest. Therefore, temperature is the variable that we modified under each forecasting and hindcasting runs.

The hindcasting simulations showed that the spawning events increased after 1995, which agrees with the observations made on C. gigas colonization. The forecasting simulations showed that under the warmer scenario (RCP8.5), reproductive success would be enhanced through two complementary mechanisms: more regular spawning each year and potentially precocious spawning resulting in a larval phase synchronized with the most favorable summer period. Our results evidenced that the spawning dates and synchronicity between individuals mainly relied on phytoplankton seasonal dynamics, and not on temperature as expected. Future research focused on phytoplankton dynamics under different climate change scenarios would greatly improve our ability to anticipate the reproductive success and population dynamics of this species and other similar invertebrates.

Figure 4: Oyster growth and spawning simulations obtained by the DEB model compared with observed data from 2009 to 2014 (DFM = Dry Flesh Mass). Observed DFM is represented by black dots with standard deviation bars (n = 30). Grey lines represent individual growth trajectories simulated by the model. The dark red bold line represents the mean of the 30 trajectories.


Gourault, M., Petton, S., Thomas, Y., Pecquerie, L., Marques, G.M., Cassou, C., Fleury, E., Paulet, Y.-M., & Pouvreau, S. 2019. Modeling reproductive traits of an invasive bivalve species under contrasting climate scenarios from 1960 to 2100. Journal of Sea Research 143: 128–139. doi:10.1016/j.seares.2018.05.005.

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Oysters as sentinels of climate variability and climate change in coastal ecosystems


Yoann Thomas, Christophe Cassou, Pierre Gernez and Stéphane Pouvreau


Beyond key ecological services, marine resources are crucial for human food security and socio-economical sustainability. Among them, shellfish aquaculture and fishing are of primary importance but become more vulnerable under anthropogenic pressure, as evidenced by reported mass mortality events linked to global changes such as ocean warming and acidification, chemical contamination, and diseases. Understanding climate-related risks is a vital objective for conservation strategies, ecosystems management and human health. We provide here a comprehensive study of the historical mortality of adult oysters related to observed climate variability along the French Atlantic coast from 1986 to 2015, and we built on this knowledge to develop hindcast and forecast assessments of the oyster mortality risk from 1900 to 2100. We show that mortality events usually occur several months after winters dominated by the occurrence of positive North Atlantic oscillation (NAO+) atmospheric regimes of circulation. We explain the lagged response by the multiseasonal long-lasting imprint of wintertime NAO+ on biological and environmental factors, which partly structure oyster mortality etiology. Very high wintertime seawater temperature anomalies at the interannual timescale, which were mostly attributable to internal climate variability through NAO+ and which led to pronounced mortality over the observed period, are then treated as ‘analogs’ in a large ensemble of Intergovernmental Panel on Climate Change emission scenarios and models in order to anticipate future risks. Without any adaptive process, we provide evidence that actual exceptional mortality is likely to become the norm by ~2035, even if global warming is limited to +2 °C relative to pre-industrial levels.

Figure 1.

Figure 1. Time series for December–March (DJFM) winter NAO+ occurrences and annual oyster mortality rate. (a) Location of the six monitored stations along the French Atlantic coast: Arcachon (A), Breton Sounds (Br), Bourgneuf (B), Vilaine (Vi), Mont Saint-Michel (M) and Veys (V). (b) Centroid of the NAO+ weather regime obtained from daily anomalous sea-level pressure from NCEP/NCAR reanalysis (see Methods). The box shows the region of interest. (c) interannual oyster mortality rate averaged over the six stations (in %, black dots) and cumulated number of NAO+ days per winter (red triangles). Grey shading represents the inter-site standard deviation. The correlation coefficient between the two time series is equal to 0.77 (P < 0.01 based on r-test).

Article in pdf on IOP Science


Thomas, Y., Cassou, C., Gernez, P., and Pouvreau, S. 2018. Oysters as sentinels of climate variability and climate change in coastal ecosystems. Environ. Res. Lett. 13(10): 104009. doi:10.1088/1748-9326/aae254.

This article was the subject of two press releases and one article on the website of the daily newspaper Libération:

Influence of diatom diversity on the ocean biological carbon pump


Diatoms sustain the marine food web and contribute to the export of carbon from the surface ocean to depth. They account for about 40% of marine primary productivity and particulate carbon exported to depth as part of the biological pump. Diatoms have long been known to be abundant in turbulent, nutrient-rich waters, but observations and simulations indicate that they are dominant also in meso- and submesoscale structures such as fronts and filaments, and in the deep chlorophyll maximum. Diatoms vary widely in size, morphology and elemental composition, all of which control the quality, quantity and sinking speed of biogenic matter to depth. In particular, their silica shells provide ballast to marine snow and faecal pellets, and can help transport carbon to both the mesopelagic layer and deep ocean. Herein we show that the extent to which diatoms contribute to the export of carbon varies by diatom type, with carbon transfer modulated by the Si/C ratio of diatom cells, the thickness of the shells and their life strategies; for instance, the tendency to form aggregates or resting spores. Model simulations project a decline in the contribution of diatoms to primary production everywhere outside of the Southern Ocean. We argue that we need to understand changes in diatom diversity, life cycle and plankton interactions in a warmer and more acidic ocean in much more detail to fully assess any changes in their contribution to the biological pump.


Graphical abstract


Tréguer, P., Bowler, C., Moriceau, B., Dutkiewicz, S., Gehlen, M., Aumont, O., Bittner,L., Dugdale, R., Finkel, Z., Ludicone, D., Jahn,O., Guidi, L., Lasbleiz, M., Leblanc, K., Levy, M. & Pondaven, P. (2017). Influence of diatom diversity on the ocean biological carbon pump. Nature Geoscience 11, 27–37 (2017). doi:10.1038/s41561-017-0028-x