The ability of corals to modulate their nutrition strategy in response to variable nutrient suppl... more The ability of corals to modulate their nutrition strategy in response to variable nutrient supply remains poorly understood, limiting our understanding of energy flow in coral reef ecosystems and thus our comprehension of their resilience to global changes. We used a naturally occurring nutrient gradient along the reef flat of two seabird-inhabited islets in the SW Pacific to characterize spatiotemporal fluctuations in coastal nutrient availability, and how it modulates the trophic response of the mixotrophic coral Pocillopora damicornis. The clear gradients in dissolved [NOx] and δ15N values of macroalgae and both P. damicornis tissues and symbionts observed along the reef flat during the dry and the rainy season revealed that seabird-derived-N is supplied year-round to the reef flat. Yet, nitrogen isotope values of macroalgae show that the seabirds’ effect on coral reefs varies with sites and seasons. Metrics derived from the SIBER framework revealed that coral nutrition seasonal...
Tropical tunas are largely consumed worldwide, providing major nutritional benefits to humans, bu... more Tropical tunas are largely consumed worldwide, providing major nutritional benefits to humans, but also representing the main exposure to methylmercury, a potent neurotoxin that biomagnifies along food webs. The combination of ecological tracers (nitrogen and carbon stable isotopes, δ15N and δ13C) to mercury concentrations in tunas is scarce yet crucial to better characterise the influence of tuna foraging ecology on mercury exposure and bioaccumulation. Given the difficulties to get modern and historical tuna samples, analyses have to be done on available and unique samples. However, δ13C values are often analysed on lipid-free samples to avoid bias related to lipid content. While lipid extraction with non-polar solvents is known to have no effect on δ15N values, its impact on mercury concentrations is still unclear. We used white muscle tissues of three tropical tuna species to evaluate the efficiency and repeatability of different lipid extraction protocols commonly used in δ13C and δ15N analysis. Dichloromethane was more efficient than cyclohexane in extracting lipids in tuna muscle, while the automated method appeared more efficient but as repeatable as the manual method. Lipid extraction with dichloromethane had no effect on mercury concentrations. This may result from i) the affinity of methylmercury to proteins in tuna flesh, ii) the low lipid content in tropical tuna muscle samples, and iii) the non-polar nature of dichloromethane. Our study suggests that lipid-free samples, usually prepared for tropical tuna foraging ecology research, can be used equivalently to bulk samples to document in parallel mercury concentrations at a global scale.
Proceedings of the National Academy of Sciences, 2022
Significance Humans are exposed to toxic methylmercury mainly by consuming marine fish. New envir... more Significance Humans are exposed to toxic methylmercury mainly by consuming marine fish. New environmental policies under the Minamata Convention rely on a yet-poorly-known understanding of how mercury emissions translate into fish methylmercury levels. Here, we provide the first detailed map of mercury concentrations from skipjack tuna across the Pacific. Our study shows that the natural functioning of the global ocean has an important influence on tuna mercury concentrations, specifically in relation to the depth at which methylmercury concentrations peak in the water column. However, mercury inputs originating from anthropogenic sources are also detectable, leading to enhanced tuna mercury levels in the northwestern Pacific Ocean that cannot be explained solely by oceanic processes.
Merci Fred, pour essayer à chaque fois, sans te fâcher ! de me faire comprendre les stats… Merci ... more Merci Fred, pour essayer à chaque fois, sans te fâcher ! de me faire comprendre les stats… Merci à Olivier et Aude pour leur précieuse aide siliceuse et surtout pour la patience qu'ils m'ont accordée pour relire et améliorer le manuscrit. Merci mille fois ! Merci à tous ceux qui m'ont toujours aidé dans la panique : François Le Loc'h en premier, un énorme merci pour ton soutien au cours de ces 4 années (DEA + thèse), tu as toujours été là pour m'aider, tout le temps, quel talent ! Merci Monsieur. Merci aussi à Joëlle, qui m'a sauvée de la panique en cette fin de thèse en reprenant toutes mes manips, cadeau empoisonné que je t'ai fait là…sans rancune ? ! Merci Jacques Grall, pour m'avoir initiée à la recherche (et oui !) et pour m'avoir aidée ensuite à chaque fois que je suis venue gratter à ta porte. Et merci à Charlotte, Géraldine, Morgane (s), Agathe, Philippe, Rudolph et tous les autres…pour leur présence. Merci à Nicolas Dittert, le roi de PROTOOL (comprenne qui pourra !) toujours prêt à m'aider aussi. Un spécial merci à Alain Lemercier, l'indispensable Alain, merci pour tout, tes magnifiques photos, ta voie mélodieuse (hein, Anne ? !), tes talents d'informaticien.. les sauvetages de fichiers perdus…m'évitant bien des déprimes de fin de rédaction. Et merci Maryvonne de m'avoir acceptée (supportée ?) dans ton bureau malgré mes remuages incessants ! Merci à Servane Fixot, qui nous a abandonnés en chemin (malheureusement) mais dont je n'oublie pas l'aide précieuse, et merci à Mathias Rouan qui a repris le flambeau et nous a livré un beau programme de recalage ! Merci à tous les stagiaires qui ont contribué à ce travail de thèse : Jérome Joliff, Caroline Bouvier, Nicolas Lemarchand, Emilie Farcy et Tiscar Mellado. Merci à tous ceux qui ont réalisé les analyses isotopiques présentées dans la thèse, me permettant de compenser le manque brestois ! Norbert Naulet à l'Université de Nantes pour les filtres de MOP, Charlie Scrimgeour à Dundee pour les analyses de tissus de Pecten, David Mucciarone et Rob Dunbar, à Standford pour les analyses isotopiques de la calcite des coquilles Saint-Jacques, et Michel Fontugne à Gyf sur Yvette pour le carbone inorganique dissous. Et merci encore à Christophe Pécheyran, cette fois pour avoir analysé les éléments traces dans l'eau de mer, en maudissant certainement celui qui avait eu l'idée de faire un pas d'échantillonnage aussi soutenu ! ! Merci à tous les proches qui ont été là, tout près, les scientifiques et les autres, qui comprenaient moins ce qui pouvait bien m'empêcher de faire la fête ! Merci Doudou pour tes Tofifees, ton backgamon, ton poisson cru, tes pt'its dejs et bien d'autres choses encore…Merci Emilie, pour nos plongées, nos délires, notre cohabitation…Merci Thierry, Babeth, Armelle et Sylvain bien loin maintenant…(pourvu que j'arrive à vous rejoindre !), Mathias, mon binôme de maîtrise maintenant bien encré à l'IUEM. Merci les plongeurs…et spéciale dédicace à Marco mon moniteur préféré. Merci à ceux qui sont loin mais bien présents : Hélène, Marion, Gilles (mon belge préféré), Raph, mon premier colloc …qui a réussi à soutenir sa thèse en même temps que moi ! ! Merci à tous ceux que j'ai rencontré à l'IUEM et qui ont partagé bien des pauses…Pierrot le premier, Rémi, Morgane, Rafael, Hughes et merci à tous ceux que j'oublie dans la panique du départ à Bruxelles… Merci à toute ma famille qui ne m'a pas vue beaucoup ces dernières années… Et enfin merci à celui qui a le plus souffert au quotidien, et qui a tenu le coup à mon grand bonheur… 2.4 Discussion.
Large marine predators exhibit high concentrations of mercury (Hg) as neurotoxic methylmercury, a... more Large marine predators exhibit high concentrations of mercury (Hg) as neurotoxic methylmercury, and the potential impacts of global change on Hg contamination in these species remain highly debated. Current contaminant model predictions do not account for intraspecific variability in Hg exposure and may fail to reflect the diversity of future Hg levels among conspecific populations or individuals, especially for top predators displaying a wide range of ecological traits. Here, we used Hg isotopic compositions to show that Hg exposure sources varied significantly between and within three populations of white sharks (Carcharodon carcharias) with contrasting ecology: the northeastern Pacific, eastern Australasian, and southwestern Australasian populations. Through Δ200Hg signatures in shark tissues, we found that atmospheric Hg deposition pathways to the marine environment differed between coastal and offshore habitats. Discrepancies in δ202Hg and Δ199Hg signatures among white sharks provided evidence for intraspecific exposure to distinct sources of marine methylmercury, attributed to population and ontogenetic shifts in foraging habitat and prey composition. We finally observed a strong divergence in Hg accumulation rates between populations, leading to three times higher Hg concentrations in large Australasian sharks compared to northeastern Pacific sharks, and likely due to different trophic strategies adopted by adult sharks across populations. This study illustrates the variety of Hg exposure sources and Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site. bioaccumulation patterns that can be found within a single species and suggests that intraspecific variability needs to be considered when assessing future trajectories of Hg levels in marine predators.
Climate change is expected to affect marine mercury (Hg) biogeochemistry and biomagnification. Re... more Climate change is expected to affect marine mercury (Hg) biogeochemistry and biomagnification. Recent modeling work suggested that ocean warming increases methylmercury (MeHg) levels in fish. Here, we studied the influence of El Niño Southern Oscillations (ENSO) on Hg concentrations and stable isotopes in time series of seabird blood from the Peruvian upwelling and oxygen minimum zone. Between 2009 and 2016, La Niña (2011) and El Niño conditions (2015-2016) were accompanied by sea surface temperature anomalies up to 3 °C, oxycline depth change (20-100 m), and strong primary production gradients. Seabird Hg levels were stable and did not co-vary significantly with oceanographic parameters, nor with anchovy biomass, the primary dietary source to seabirds (90%). In contrast, seabird Δ199Hg, proxy for marine photochemical MeHg breakdown, and δ15N showed strong interannual variability (up to 0.8 and 3‰, respectively) and sharply decreased during El Niño. We suggest that lower Δ199Hg during El Niño represents reduced MeHg photodegradation due to the deepening of the oxycline. This process was balanced by equally reduced Hg methylation due to reduced productivity, carbon export, and remineralization. The non-dependence of seabird MeHg levels on strong ENSO variability suggests that marine predator MeHg levels may not be as sensitive to climate change as is currently thought. Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site.
Deep Sea Research Part II: Topical Studies in Oceanography, 2020
Please note that this is an author-produced PDF of an article accepted for publication following ... more Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site. factors and ecological (e.g. diet variability) processes. However, this study illustrates that model-corrected δ13C values are a valuable, relatively cost-effective tool for identifying potential areas of mixing across management zones, particularly when electronic tagging studies are limited or absent. Stable isotope analyses of tuna tissues can therefore be an additional tool for guiding spatial stock assessments on top predator movement, dispersal patterns, and how they may be altered under a changing climate.
Two species breeding in sympatry are more likely to coexist if their ecological niches are segreg... more Two species breeding in sympatry are more likely to coexist if their ecological niches are segregated either in time, space or in trophic habits. Here, we combined GPS-tracking, stable isotope analysis and DNA metabarcoding analysis to understand how the rare Tahiti petrel Pseudobulweria rostrata (TP) copes with the very abundant (i.e. 500,000 breeding pairs) wedge-tailed shearwater Ardenna pacifica (WTS) when breeding in sympatry in a tropical area. WTS foraged in restricted areas along their path, while TP predominantly foraged using extensive search behavior, suggesting a more opportunistic foraging strategy. Interspecific overlap of foraging areas was higher than intraspecific overlap. Breeding seasons largely overlap between species during the study, but TP seems to be asynchronous breeders. TP fed upon prey with higher δ15N values than WTS, and their diet was mainly composed of deep-sea organisms. TP could feed upon dead prey floating at the surface while WTS preyed mainly upo...
Delineating the stock structure of highly-mobile, wide-ranging fishes subject to exploitation is ... more Delineating the stock structure of highly-mobile, wide-ranging fishes subject to exploitation is a challenging task, yet one that is fundamental to optimal fisheries management. A case in point are stocks of skipjack tuna (Katsuwonus pelamis), yellowfin tuna (Thunnus albacares), bigeye tuna (Thunnus obesus) and albacore tuna (Thunnus alalunga) in the Pacific Ocean, which support important commercial, artisanal, subsistence, and recreational fisheries, and contribute roughly 70 % of global commercial tuna catches. Although some spatial and temporal structuring is recognised within these stocks, growing evidence from a range of approaches suggests that the stock structure of each tuna species is more complex than is currently assumed in both stock assessment and climate change models, and in management regimes. In a move towards improving understanding of the stock structure of skipjack, yellowfin, bigeye and South Pacific albacore tunas in the Pacific Ocean, an
Deposit-feeding sea cucumbers are efficient nutrient recyclers and have the potential to contribu... more Deposit-feeding sea cucumbers are efficient nutrient recyclers and have the potential to contribute to the limitation of organic matter load in polyculture or integrated aquaculture systems. Assessing how they assimilate organic matter originating from other farmed species is therefore important for the development of such multi-species farming systems. Here, a coupled stable isotope − fatty acid approach was used to characterize the assimilation of organic matter from shrimp (Penaeus stylirostris) farming by Holothuria scabra in an experimental culture system. H. scabra were reared in mesocosms on shrimp farming-originating sediment with and without additional food sources (maize and fish meals). Although fatty acid results did indicate that shrimp-farming sediment was assimilated by holothurids, we found no evidence of maize waste and fish meal contribution to H. scabra organic carbon (no effect on δ13C, no accumulation of meal-specific fatty acids). However, a strong effect of fi...
Journal of Experimental Marine Biology and Ecology, 2020
Lunar phase and illumination are known to affect nocturnal behavior of many organisms, particular... more Lunar phase and illumination are known to affect nocturnal behavior of many organisms, particularly through predator-prey interactions. Visual predators can benefit from higher light levels to increase their activity, while prey may decrease their activity to avoid predation. The lower number of nocturnal seabirds observed on colonies during full moon nights has been mostly interpreted as a predation avoidance strategy. However, it is also possible that shearwaters take advantage of the moon's illumination to feed also at night, and stay at sea to forage during full moon nights. We used miniaturized GPS-loggers to obtain 179 tracks from 99 wedge-tailed shearwaters breeding in New Caledonia, to investigate moonlight effects on individual behavior. Lunar phase significantly predicted self-provisioning trip duration, with individuals performing longer trips around the full moon. However, this relationship was not significant during chick-provisioning trips when adults have to frequently return to the colony. Adults mostly returned to the colony during moonlit periods, refuting the predation avoidance theory. Tracked individuals showed an unexpectedly high amount of nocturnal foraging activity (28% of total activity), positively influenced by the presence of the moon. δ 15 N stable isotope values were significantly related to the percentage of nocturnal foraging, but with a weak relationship, impeding our ability to confirm that wedge-tailed shearwaters fed on different prey when foraging at night. This study suggests that reduced colony attendance around the full moon may be linked to greater at-sea foraging opportunities in distant oceanic areas than to increased predation risk on land. persistent behavior inherited from co-evolution with predators (Bretagnolle et al., 2000). However, reduced activity at the colony during moonlit nights may
Considerable uncertainty remains over how increasing atmospheric CO2 and anthropogenic climate ch... more Considerable uncertainty remains over how increasing atmospheric CO2 and anthropogenic climate changes are affecting open‐ocean marine ecosystems from phytoplankton to top predators. Biological time series data are thus urgently needed for the world's oceans. Here, we use the carbon stable isotope composition of tuna to provide a first insight into the existence of global trends in complex ecosystem dynamics and changes in the oceanic carbon cycle. From 2000 to 2015, considerable declines in δ13C values of 0.8‰–2.5‰ were observed across three tuna species sampled globally, with more substantial changes in the Pacific Ocean compared to the Atlantic and Indian Oceans. Tuna recorded not only the Suess effect, that is, fossil fuel‐derived and isotopically light carbon being incorporated into marine ecosystems, but also recorded profound changes at the base of marine food webs. We suggest a global shift in phytoplankton community structure, for example, a reduction in 13C‐rich phytop...
Sharks are a diverse group of mobile predators that forage across varied spatial scales and have ... more Sharks are a diverse group of mobile predators that forage across varied spatial scales and have the potential to influence food web dynamics. The ecological consequences of recent declines in shark biomass may extend across broader geographic ranges if shark taxa display common behavioural traits. By tracking the original site of photosynthetic fixation of carbon atoms that were ultimately assimilated into muscle tissues of 5,394 sharks from 114 species, we identify globally consistent biogeographic traits in trophic interactions between sharks found in different habitats. We show that populations of shelf-dwelling sharks derive a substantial proportion of their carbon from regional pelagic sources, but contain individuals that forage within additional isotopically diverse local food webs, such as those supported by terrestrial plant sources, benthic production and macrophytes. In contrast, oceanic sharks seem to use carbon derived from between 30° and 50° of latitude. Global-scale...
Seabirds concentrate nutrients from large marine areas on their nesting islands playing an import... more Seabirds concentrate nutrients from large marine areas on their nesting islands playing an important ecological role in nutrient transfer between marine and terrestrial ecosystems. Here we investigate the role of guano on corals reefs across scales by analyzing the stable nitrogen isotopic (δ(15)N) values of the scleractinian coral Pocillopora damicornis on fringing reefs around two Pacific remote islets with large seabird colonies. Marine stations closest to the seabird colonies had higher nitrate + nitrite concentrations compared to more distant stations. Coral and zooxanthellae δ(15)N values were also higher at these sites, suggesting that guano-derived nitrogen is assimilated into corals and contributes to their nitrogen requirements. The spatial extent of guano influence was however restricted to a local scale. Our results demonstrate that seabird-derived nutrients not only spread across the terrestrial ecosystem, but also affect components of the adjacent marine ecosystem. Fur...
Deep Sea Research Part II: Topical Studies in Oceanography, 2015
Tuna catches represent a major economic and food source in the Pacific Ocean, yet are highly vari... more Tuna catches represent a major economic and food source in the Pacific Ocean, yet are highly variable. This variability in tuna catches remains poorly explained. The relationships between the distributions of tuna and their forage (micronekton) have been mostly derived from model estimates. Observations of micronekton and other mid-trophic level organisms, and their link to regional oceanography, however are scarce and constitute an important gap in our knowledge and understanding of the dynamics of pelagic ecosystems. To fill this gap, we conducted two multidisciplinary cruises (Nectalis1 and Nectalis2) in the New Caledonian Exclusive Economic Zone (EEZ) at the southeastern edge the Coral Sea, in 2011 to characterize the oceanography of the region during the cool (August) and the hot (December) seasons. The physical and biological environments were described by hydrology, nutrients and phytoplankton size structure and biomass. Zooplankton biomass was estimated from net sampling and acoustics and micronecton was estimated from net sampling, the SEAPODYM ecosystem model, a dedicated echosounder and non-dedicated acoustics. Results demonstrated that New Caledonia is located in an oligotrophic area characterized by low nutrient and low primary production which is dominated by a high percentage of picoplankton cyanobacteria Prochlorococcus (>90%). The area is characterized by a large-scale north-south temperature and salinity gradient. The northern area is influenced by the equatorial Warm Pool and the South Pacific Convergence Zone and is characterized by higher temperature, lower salinity, lower primary production and micronekton biomass. The southern area is influenced by the Tasman Sea and is characterized by cooler temperature, higher salinity, higher primary production and micronekton biomass. Interactions between the dynamic oceanography and the complex topography creates a myriad of mesoscale eddies, inducing patchy structures in the frontal area. During the cool season, a tight coupling existed between the ocean dynamics and primary production, while there was a stronger decoupling during the hot season. There was little difference in the composition of mid-trophic level organisms (zooplankton and micronekton) between the two seasons. This may be due to different turnover times and delays in the transmission of primary production to upper trophic levels. Examination of various sampling gears for zooplankton and micronekton showed that net biomass 4 estimates and acoustic-derived estimates compared reasonably well. Estimates of micronekton from net observations and the SEAPODYM model were in the same range. The non-dedicated acoustics adequately reproduced trends observed in zooplankton from nets, but the acoustics could not differentiate between zooplankton and micronekton and absolute biomasses could not be calculated. Understanding the impact of mesoscale features on higher trophic levels will require further investigation and patchiness induced by eddies raises the question of how to best sample highly dynamic areas via sea experiments.
Background: Cephalopods play a major role in marine ecosystems, but knowledge of their feeding ec... more Background: Cephalopods play a major role in marine ecosystems, but knowledge of their feeding ecology is limited. In particular, intra-and inter-individual variations in their use of resources has not been adequatly explored, although there is growing evidence that individual organisms can vary considerably in the way they use their habitats and resources. Methodology/Principal Findings: Using d 13 C and d 15 N values of serially sampled gladius (an archival tissue), we examined high resolution variations in the trophic niche of five large (.60 cm mantle length) jumbo squids (Dosidicus gigas) that were collected off the coast of Peru. We report the first evidence of large inter-individual differences in jumbo squid foraging strategies with no systematic increase of trophic level with size. Overall, gladius d 13 C values indicated one or several migrations through the squid's lifetime (,8-9 months), during which d 15 N values also fluctuated (range: 1 to 5%). One individual showed an unexpected terminal 4.6% d 15 N decrease (more than one trophic level), thus indicating a shift from higher-to lower-trophic level prey at that time. The data illustrate the high diversity of prey types and foraging histories of this species at the individual level. Conclusions/Significance: The isotopic signature of gladii proved to be a powerful tool to depict high resolution and ontogenic variations in individual foraging strategies of squids, thus complementing traditional information offered by stomach content analysis and stable isotopes on metabolically active tissues. The observed differences in life history strategies highlight the high degree of plasticity of the jumbo squid and its high potential to adapt to environmental changes.
To test the potential of diet switching experiments in ecophysiological studies of marine inverte... more To test the potential of diet switching experiments in ecophysiological studies of marine invertebrates, stable carbon isotope ratios were measured at different seasons in the gonad, adductor muscle, digestive gland and gills of scallops (Pecten maximus) and oysters (Crassostrea gigas) held for 15 days on a constant diet of phytoplankton depleted in 13 C. The aim of this study was to determine if differences in carbon incorporation could be detected among species, seasons and organs, and if so, whether it was consistent with their known energy-allocation patterns. After offering the new diet, isotope values of the different organs gradually shifted and significant differences among organs, seasons and species were found. A carbon incorporation index (CII) was calculated to compare the metabolic activity of each organ of the two species between day 0 and day 15. For both species, the digestive gland had the highest CII, the adductor muscle the lowest, while gonad and gills had intermediate values. The CII was generally much higher in P. maximus than in C. gigas, suggesting higher metabolic activity in this species. Seasonal differences in the CII were also observed for the two species and were interpreted as differences in metabolic activity in accordance with our energy allocation scenario. Therefore, stable isotope diet switching experiments appear to be of great value for assessing metabolic orientation in bivalves.
To determine the habitat and resource use of Dosidicus gigas in the Northern Humboldt Current Sys... more To determine the habitat and resource use of Dosidicus gigas in the Northern Humboldt Current System, we analysed carbon and nitrogen stable isotopes of 234 individuals collected during 2008-2010. Large variations in mantle stable isotope ratios were recorded, with values ranging from-19.1 to-15.1 % (d 13 C) and from 7.4 to 20.5 % (d 15 N). Most of the variation was explained by latitude, followed by distance to shelf break for carbon and by squid size for nitrogen. Latitudinal variations with increasing values from north to south were also found in zooplankton samples and were related to changes in isotope baseline values probably due to oxygen minimum zones that occur off Peru. This similar latitudinal trend in both zooplankton and D. gigas samples reveals that D. gigas is a relatively resident species at the scale of its isotopic turnover rate (i.e. a few weeks), even if this is not necessarily the case at the scale of its life. A small but significant size effect on d 13 C values suggests that jumbo squid perform offshore-onshore ontogenic migration, with juveniles distributed offshore. For nitrogen, the high interindividual variability observed with mantle length indicates that D. gigas can prey on a high variety of resources at any stage of their life cycle. This large-scale study off the coast of Peru provides further evidence that D. gigas have the capability to explore a wide range of habitats and resources at any stage of their life. Communicated by C. Harrod.
Environmental archives are useful tools for describing past and current climate variations and th... more Environmental archives are useful tools for describing past and current climate variations and they provide an opportunity to assess the anthropogenic contribution in coastal ecological changes. Along the West African coast, few studies have focused on such archives in coastal ecosystems. The bloody cockle Senilia senilis, an intertidal bivalve mollusk species, is widely distributed from Western Sahara to Angola, and has been harvested by humans over thousands of years. Therefore, this species appears to be a good candidate for assessing past variations of key environmental parameters such as temperature, primary production, and Saharan dust advection within West African coastal ecosystems. In the present paper, we focused (i) on the identification of growth rhythms of S. senilis shells in Mauritania (Banc d'Arguin), and (ii) on the potential of these shells as (paleo-)environmental archives. The method we used combined environmental survey, sclerochronology, and geochemical analyses of aragonite samples. We showed that microgrowth line formation was controlled by a tidal forcing, leading to the formation of two lines per lunar day. Brightness and thickness of these microgrowth lines progressively decreased from spring to neap tides (fortnightly cycle). Lunar daily growth rates displayed strong seasonal variations, with highest values (> 300 μm per lunar day) recorded in summer. The oxygen isotope composition of S. senilis shells (δ 18 O aragonite) accurately tracked seawater temperature seasonal variations, with a precision of 0.8°C. Finally, we discussed the opportunity to use Ba:Ca ratio in shells as a proxy for primary production or for Saharan dust transport. We also hypothesized that either Canary Currentvariations or, more probably, massive aerosol transfers from Sahara to the Atlantic Ocean could control uranium availability in coastal waters and explain the occurrence of U:Ca peaks within S. senilis shells.
In this study we test if calcite shells of the common mussel, Mytilus edulis, contain barium in p... more In this study we test if calcite shells of the common mussel, Mytilus edulis, contain barium in proportion to the water in which they grew. Similar to all bivalves analyzed to date, the [Ba/Ca] shell profiles are characterized by a relatively flat background [Ba/Ca] shell , interrupted by sharp [Ba/Ca] shell peaks. Previous studies have focused on these [Ba/Ca] shell peaks, but not on the background [Ba/Ca] shell. We show that in both laboratory and field experiments, there is a direct relationship between the background [Ba/Ca] shell and [Ba/Ca] water in M. edulis shells. The laboratory and field data provided background Ba/Ca partition coefficients (D Ba) of 0.10 ± 0.02 and 0.071 ± 0.001, respectively. This range is slightly higher than the D Ba previously determined for inorganic calcite, and slightly lower than foraminiferal calcite. These data suggest that M. edulis shells can be used as an indicator of [Ba/Ca] water , and therefore, fossil or archaeological M. edulis shells could be used to extend knowledge of estuarine dissolved Ba throughputs back in time. Moreover, considering the inverse relationship between [Ba/Ca] water and salinity, background [Ba/Ca] shell data could be used as an estuary specific indicator of salinity. The cause of the [Ba/Ca] shell peaks is more confusing, both the laboratory and field experiments indicate that they cannot be used as a direct proxy of [Ba/Ca] water or phytoplankton production, but may possibly be caused by barite ingestion.
The ability of corals to modulate their nutrition strategy in response to variable nutrient suppl... more The ability of corals to modulate their nutrition strategy in response to variable nutrient supply remains poorly understood, limiting our understanding of energy flow in coral reef ecosystems and thus our comprehension of their resilience to global changes. We used a naturally occurring nutrient gradient along the reef flat of two seabird-inhabited islets in the SW Pacific to characterize spatiotemporal fluctuations in coastal nutrient availability, and how it modulates the trophic response of the mixotrophic coral Pocillopora damicornis. The clear gradients in dissolved [NOx] and δ15N values of macroalgae and both P. damicornis tissues and symbionts observed along the reef flat during the dry and the rainy season revealed that seabird-derived-N is supplied year-round to the reef flat. Yet, nitrogen isotope values of macroalgae show that the seabirds’ effect on coral reefs varies with sites and seasons. Metrics derived from the SIBER framework revealed that coral nutrition seasonal...
Tropical tunas are largely consumed worldwide, providing major nutritional benefits to humans, bu... more Tropical tunas are largely consumed worldwide, providing major nutritional benefits to humans, but also representing the main exposure to methylmercury, a potent neurotoxin that biomagnifies along food webs. The combination of ecological tracers (nitrogen and carbon stable isotopes, δ15N and δ13C) to mercury concentrations in tunas is scarce yet crucial to better characterise the influence of tuna foraging ecology on mercury exposure and bioaccumulation. Given the difficulties to get modern and historical tuna samples, analyses have to be done on available and unique samples. However, δ13C values are often analysed on lipid-free samples to avoid bias related to lipid content. While lipid extraction with non-polar solvents is known to have no effect on δ15N values, its impact on mercury concentrations is still unclear. We used white muscle tissues of three tropical tuna species to evaluate the efficiency and repeatability of different lipid extraction protocols commonly used in δ13C and δ15N analysis. Dichloromethane was more efficient than cyclohexane in extracting lipids in tuna muscle, while the automated method appeared more efficient but as repeatable as the manual method. Lipid extraction with dichloromethane had no effect on mercury concentrations. This may result from i) the affinity of methylmercury to proteins in tuna flesh, ii) the low lipid content in tropical tuna muscle samples, and iii) the non-polar nature of dichloromethane. Our study suggests that lipid-free samples, usually prepared for tropical tuna foraging ecology research, can be used equivalently to bulk samples to document in parallel mercury concentrations at a global scale.
Proceedings of the National Academy of Sciences, 2022
Significance Humans are exposed to toxic methylmercury mainly by consuming marine fish. New envir... more Significance Humans are exposed to toxic methylmercury mainly by consuming marine fish. New environmental policies under the Minamata Convention rely on a yet-poorly-known understanding of how mercury emissions translate into fish methylmercury levels. Here, we provide the first detailed map of mercury concentrations from skipjack tuna across the Pacific. Our study shows that the natural functioning of the global ocean has an important influence on tuna mercury concentrations, specifically in relation to the depth at which methylmercury concentrations peak in the water column. However, mercury inputs originating from anthropogenic sources are also detectable, leading to enhanced tuna mercury levels in the northwestern Pacific Ocean that cannot be explained solely by oceanic processes.
Merci Fred, pour essayer à chaque fois, sans te fâcher ! de me faire comprendre les stats… Merci ... more Merci Fred, pour essayer à chaque fois, sans te fâcher ! de me faire comprendre les stats… Merci à Olivier et Aude pour leur précieuse aide siliceuse et surtout pour la patience qu'ils m'ont accordée pour relire et améliorer le manuscrit. Merci mille fois ! Merci à tous ceux qui m'ont toujours aidé dans la panique : François Le Loc'h en premier, un énorme merci pour ton soutien au cours de ces 4 années (DEA + thèse), tu as toujours été là pour m'aider, tout le temps, quel talent ! Merci Monsieur. Merci aussi à Joëlle, qui m'a sauvée de la panique en cette fin de thèse en reprenant toutes mes manips, cadeau empoisonné que je t'ai fait là…sans rancune ? ! Merci Jacques Grall, pour m'avoir initiée à la recherche (et oui !) et pour m'avoir aidée ensuite à chaque fois que je suis venue gratter à ta porte. Et merci à Charlotte, Géraldine, Morgane (s), Agathe, Philippe, Rudolph et tous les autres…pour leur présence. Merci à Nicolas Dittert, le roi de PROTOOL (comprenne qui pourra !) toujours prêt à m'aider aussi. Un spécial merci à Alain Lemercier, l'indispensable Alain, merci pour tout, tes magnifiques photos, ta voie mélodieuse (hein, Anne ? !), tes talents d'informaticien.. les sauvetages de fichiers perdus…m'évitant bien des déprimes de fin de rédaction. Et merci Maryvonne de m'avoir acceptée (supportée ?) dans ton bureau malgré mes remuages incessants ! Merci à Servane Fixot, qui nous a abandonnés en chemin (malheureusement) mais dont je n'oublie pas l'aide précieuse, et merci à Mathias Rouan qui a repris le flambeau et nous a livré un beau programme de recalage ! Merci à tous les stagiaires qui ont contribué à ce travail de thèse : Jérome Joliff, Caroline Bouvier, Nicolas Lemarchand, Emilie Farcy et Tiscar Mellado. Merci à tous ceux qui ont réalisé les analyses isotopiques présentées dans la thèse, me permettant de compenser le manque brestois ! Norbert Naulet à l'Université de Nantes pour les filtres de MOP, Charlie Scrimgeour à Dundee pour les analyses de tissus de Pecten, David Mucciarone et Rob Dunbar, à Standford pour les analyses isotopiques de la calcite des coquilles Saint-Jacques, et Michel Fontugne à Gyf sur Yvette pour le carbone inorganique dissous. Et merci encore à Christophe Pécheyran, cette fois pour avoir analysé les éléments traces dans l'eau de mer, en maudissant certainement celui qui avait eu l'idée de faire un pas d'échantillonnage aussi soutenu ! ! Merci à tous les proches qui ont été là, tout près, les scientifiques et les autres, qui comprenaient moins ce qui pouvait bien m'empêcher de faire la fête ! Merci Doudou pour tes Tofifees, ton backgamon, ton poisson cru, tes pt'its dejs et bien d'autres choses encore…Merci Emilie, pour nos plongées, nos délires, notre cohabitation…Merci Thierry, Babeth, Armelle et Sylvain bien loin maintenant…(pourvu que j'arrive à vous rejoindre !), Mathias, mon binôme de maîtrise maintenant bien encré à l'IUEM. Merci les plongeurs…et spéciale dédicace à Marco mon moniteur préféré. Merci à ceux qui sont loin mais bien présents : Hélène, Marion, Gilles (mon belge préféré), Raph, mon premier colloc …qui a réussi à soutenir sa thèse en même temps que moi ! ! Merci à tous ceux que j'ai rencontré à l'IUEM et qui ont partagé bien des pauses…Pierrot le premier, Rémi, Morgane, Rafael, Hughes et merci à tous ceux que j'oublie dans la panique du départ à Bruxelles… Merci à toute ma famille qui ne m'a pas vue beaucoup ces dernières années… Et enfin merci à celui qui a le plus souffert au quotidien, et qui a tenu le coup à mon grand bonheur… 2.4 Discussion.
Large marine predators exhibit high concentrations of mercury (Hg) as neurotoxic methylmercury, a... more Large marine predators exhibit high concentrations of mercury (Hg) as neurotoxic methylmercury, and the potential impacts of global change on Hg contamination in these species remain highly debated. Current contaminant model predictions do not account for intraspecific variability in Hg exposure and may fail to reflect the diversity of future Hg levels among conspecific populations or individuals, especially for top predators displaying a wide range of ecological traits. Here, we used Hg isotopic compositions to show that Hg exposure sources varied significantly between and within three populations of white sharks (Carcharodon carcharias) with contrasting ecology: the northeastern Pacific, eastern Australasian, and southwestern Australasian populations. Through Δ200Hg signatures in shark tissues, we found that atmospheric Hg deposition pathways to the marine environment differed between coastal and offshore habitats. Discrepancies in δ202Hg and Δ199Hg signatures among white sharks provided evidence for intraspecific exposure to distinct sources of marine methylmercury, attributed to population and ontogenetic shifts in foraging habitat and prey composition. We finally observed a strong divergence in Hg accumulation rates between populations, leading to three times higher Hg concentrations in large Australasian sharks compared to northeastern Pacific sharks, and likely due to different trophic strategies adopted by adult sharks across populations. This study illustrates the variety of Hg exposure sources and Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site. bioaccumulation patterns that can be found within a single species and suggests that intraspecific variability needs to be considered when assessing future trajectories of Hg levels in marine predators.
Climate change is expected to affect marine mercury (Hg) biogeochemistry and biomagnification. Re... more Climate change is expected to affect marine mercury (Hg) biogeochemistry and biomagnification. Recent modeling work suggested that ocean warming increases methylmercury (MeHg) levels in fish. Here, we studied the influence of El Niño Southern Oscillations (ENSO) on Hg concentrations and stable isotopes in time series of seabird blood from the Peruvian upwelling and oxygen minimum zone. Between 2009 and 2016, La Niña (2011) and El Niño conditions (2015-2016) were accompanied by sea surface temperature anomalies up to 3 °C, oxycline depth change (20-100 m), and strong primary production gradients. Seabird Hg levels were stable and did not co-vary significantly with oceanographic parameters, nor with anchovy biomass, the primary dietary source to seabirds (90%). In contrast, seabird Δ199Hg, proxy for marine photochemical MeHg breakdown, and δ15N showed strong interannual variability (up to 0.8 and 3‰, respectively) and sharply decreased during El Niño. We suggest that lower Δ199Hg during El Niño represents reduced MeHg photodegradation due to the deepening of the oxycline. This process was balanced by equally reduced Hg methylation due to reduced productivity, carbon export, and remineralization. The non-dependence of seabird MeHg levels on strong ENSO variability suggests that marine predator MeHg levels may not be as sensitive to climate change as is currently thought. Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site.
Deep Sea Research Part II: Topical Studies in Oceanography, 2020
Please note that this is an author-produced PDF of an article accepted for publication following ... more Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site. factors and ecological (e.g. diet variability) processes. However, this study illustrates that model-corrected δ13C values are a valuable, relatively cost-effective tool for identifying potential areas of mixing across management zones, particularly when electronic tagging studies are limited or absent. Stable isotope analyses of tuna tissues can therefore be an additional tool for guiding spatial stock assessments on top predator movement, dispersal patterns, and how they may be altered under a changing climate.
Two species breeding in sympatry are more likely to coexist if their ecological niches are segreg... more Two species breeding in sympatry are more likely to coexist if their ecological niches are segregated either in time, space or in trophic habits. Here, we combined GPS-tracking, stable isotope analysis and DNA metabarcoding analysis to understand how the rare Tahiti petrel Pseudobulweria rostrata (TP) copes with the very abundant (i.e. 500,000 breeding pairs) wedge-tailed shearwater Ardenna pacifica (WTS) when breeding in sympatry in a tropical area. WTS foraged in restricted areas along their path, while TP predominantly foraged using extensive search behavior, suggesting a more opportunistic foraging strategy. Interspecific overlap of foraging areas was higher than intraspecific overlap. Breeding seasons largely overlap between species during the study, but TP seems to be asynchronous breeders. TP fed upon prey with higher δ15N values than WTS, and their diet was mainly composed of deep-sea organisms. TP could feed upon dead prey floating at the surface while WTS preyed mainly upo...
Delineating the stock structure of highly-mobile, wide-ranging fishes subject to exploitation is ... more Delineating the stock structure of highly-mobile, wide-ranging fishes subject to exploitation is a challenging task, yet one that is fundamental to optimal fisheries management. A case in point are stocks of skipjack tuna (Katsuwonus pelamis), yellowfin tuna (Thunnus albacares), bigeye tuna (Thunnus obesus) and albacore tuna (Thunnus alalunga) in the Pacific Ocean, which support important commercial, artisanal, subsistence, and recreational fisheries, and contribute roughly 70 % of global commercial tuna catches. Although some spatial and temporal structuring is recognised within these stocks, growing evidence from a range of approaches suggests that the stock structure of each tuna species is more complex than is currently assumed in both stock assessment and climate change models, and in management regimes. In a move towards improving understanding of the stock structure of skipjack, yellowfin, bigeye and South Pacific albacore tunas in the Pacific Ocean, an
Deposit-feeding sea cucumbers are efficient nutrient recyclers and have the potential to contribu... more Deposit-feeding sea cucumbers are efficient nutrient recyclers and have the potential to contribute to the limitation of organic matter load in polyculture or integrated aquaculture systems. Assessing how they assimilate organic matter originating from other farmed species is therefore important for the development of such multi-species farming systems. Here, a coupled stable isotope − fatty acid approach was used to characterize the assimilation of organic matter from shrimp (Penaeus stylirostris) farming by Holothuria scabra in an experimental culture system. H. scabra were reared in mesocosms on shrimp farming-originating sediment with and without additional food sources (maize and fish meals). Although fatty acid results did indicate that shrimp-farming sediment was assimilated by holothurids, we found no evidence of maize waste and fish meal contribution to H. scabra organic carbon (no effect on δ13C, no accumulation of meal-specific fatty acids). However, a strong effect of fi...
Journal of Experimental Marine Biology and Ecology, 2020
Lunar phase and illumination are known to affect nocturnal behavior of many organisms, particular... more Lunar phase and illumination are known to affect nocturnal behavior of many organisms, particularly through predator-prey interactions. Visual predators can benefit from higher light levels to increase their activity, while prey may decrease their activity to avoid predation. The lower number of nocturnal seabirds observed on colonies during full moon nights has been mostly interpreted as a predation avoidance strategy. However, it is also possible that shearwaters take advantage of the moon's illumination to feed also at night, and stay at sea to forage during full moon nights. We used miniaturized GPS-loggers to obtain 179 tracks from 99 wedge-tailed shearwaters breeding in New Caledonia, to investigate moonlight effects on individual behavior. Lunar phase significantly predicted self-provisioning trip duration, with individuals performing longer trips around the full moon. However, this relationship was not significant during chick-provisioning trips when adults have to frequently return to the colony. Adults mostly returned to the colony during moonlit periods, refuting the predation avoidance theory. Tracked individuals showed an unexpectedly high amount of nocturnal foraging activity (28% of total activity), positively influenced by the presence of the moon. δ 15 N stable isotope values were significantly related to the percentage of nocturnal foraging, but with a weak relationship, impeding our ability to confirm that wedge-tailed shearwaters fed on different prey when foraging at night. This study suggests that reduced colony attendance around the full moon may be linked to greater at-sea foraging opportunities in distant oceanic areas than to increased predation risk on land. persistent behavior inherited from co-evolution with predators (Bretagnolle et al., 2000). However, reduced activity at the colony during moonlit nights may
Considerable uncertainty remains over how increasing atmospheric CO2 and anthropogenic climate ch... more Considerable uncertainty remains over how increasing atmospheric CO2 and anthropogenic climate changes are affecting open‐ocean marine ecosystems from phytoplankton to top predators. Biological time series data are thus urgently needed for the world's oceans. Here, we use the carbon stable isotope composition of tuna to provide a first insight into the existence of global trends in complex ecosystem dynamics and changes in the oceanic carbon cycle. From 2000 to 2015, considerable declines in δ13C values of 0.8‰–2.5‰ were observed across three tuna species sampled globally, with more substantial changes in the Pacific Ocean compared to the Atlantic and Indian Oceans. Tuna recorded not only the Suess effect, that is, fossil fuel‐derived and isotopically light carbon being incorporated into marine ecosystems, but also recorded profound changes at the base of marine food webs. We suggest a global shift in phytoplankton community structure, for example, a reduction in 13C‐rich phytop...
Sharks are a diverse group of mobile predators that forage across varied spatial scales and have ... more Sharks are a diverse group of mobile predators that forage across varied spatial scales and have the potential to influence food web dynamics. The ecological consequences of recent declines in shark biomass may extend across broader geographic ranges if shark taxa display common behavioural traits. By tracking the original site of photosynthetic fixation of carbon atoms that were ultimately assimilated into muscle tissues of 5,394 sharks from 114 species, we identify globally consistent biogeographic traits in trophic interactions between sharks found in different habitats. We show that populations of shelf-dwelling sharks derive a substantial proportion of their carbon from regional pelagic sources, but contain individuals that forage within additional isotopically diverse local food webs, such as those supported by terrestrial plant sources, benthic production and macrophytes. In contrast, oceanic sharks seem to use carbon derived from between 30° and 50° of latitude. Global-scale...
Seabirds concentrate nutrients from large marine areas on their nesting islands playing an import... more Seabirds concentrate nutrients from large marine areas on their nesting islands playing an important ecological role in nutrient transfer between marine and terrestrial ecosystems. Here we investigate the role of guano on corals reefs across scales by analyzing the stable nitrogen isotopic (δ(15)N) values of the scleractinian coral Pocillopora damicornis on fringing reefs around two Pacific remote islets with large seabird colonies. Marine stations closest to the seabird colonies had higher nitrate + nitrite concentrations compared to more distant stations. Coral and zooxanthellae δ(15)N values were also higher at these sites, suggesting that guano-derived nitrogen is assimilated into corals and contributes to their nitrogen requirements. The spatial extent of guano influence was however restricted to a local scale. Our results demonstrate that seabird-derived nutrients not only spread across the terrestrial ecosystem, but also affect components of the adjacent marine ecosystem. Fur...
Deep Sea Research Part II: Topical Studies in Oceanography, 2015
Tuna catches represent a major economic and food source in the Pacific Ocean, yet are highly vari... more Tuna catches represent a major economic and food source in the Pacific Ocean, yet are highly variable. This variability in tuna catches remains poorly explained. The relationships between the distributions of tuna and their forage (micronekton) have been mostly derived from model estimates. Observations of micronekton and other mid-trophic level organisms, and their link to regional oceanography, however are scarce and constitute an important gap in our knowledge and understanding of the dynamics of pelagic ecosystems. To fill this gap, we conducted two multidisciplinary cruises (Nectalis1 and Nectalis2) in the New Caledonian Exclusive Economic Zone (EEZ) at the southeastern edge the Coral Sea, in 2011 to characterize the oceanography of the region during the cool (August) and the hot (December) seasons. The physical and biological environments were described by hydrology, nutrients and phytoplankton size structure and biomass. Zooplankton biomass was estimated from net sampling and acoustics and micronecton was estimated from net sampling, the SEAPODYM ecosystem model, a dedicated echosounder and non-dedicated acoustics. Results demonstrated that New Caledonia is located in an oligotrophic area characterized by low nutrient and low primary production which is dominated by a high percentage of picoplankton cyanobacteria Prochlorococcus (>90%). The area is characterized by a large-scale north-south temperature and salinity gradient. The northern area is influenced by the equatorial Warm Pool and the South Pacific Convergence Zone and is characterized by higher temperature, lower salinity, lower primary production and micronekton biomass. The southern area is influenced by the Tasman Sea and is characterized by cooler temperature, higher salinity, higher primary production and micronekton biomass. Interactions between the dynamic oceanography and the complex topography creates a myriad of mesoscale eddies, inducing patchy structures in the frontal area. During the cool season, a tight coupling existed between the ocean dynamics and primary production, while there was a stronger decoupling during the hot season. There was little difference in the composition of mid-trophic level organisms (zooplankton and micronekton) between the two seasons. This may be due to different turnover times and delays in the transmission of primary production to upper trophic levels. Examination of various sampling gears for zooplankton and micronekton showed that net biomass 4 estimates and acoustic-derived estimates compared reasonably well. Estimates of micronekton from net observations and the SEAPODYM model were in the same range. The non-dedicated acoustics adequately reproduced trends observed in zooplankton from nets, but the acoustics could not differentiate between zooplankton and micronekton and absolute biomasses could not be calculated. Understanding the impact of mesoscale features on higher trophic levels will require further investigation and patchiness induced by eddies raises the question of how to best sample highly dynamic areas via sea experiments.
Background: Cephalopods play a major role in marine ecosystems, but knowledge of their feeding ec... more Background: Cephalopods play a major role in marine ecosystems, but knowledge of their feeding ecology is limited. In particular, intra-and inter-individual variations in their use of resources has not been adequatly explored, although there is growing evidence that individual organisms can vary considerably in the way they use their habitats and resources. Methodology/Principal Findings: Using d 13 C and d 15 N values of serially sampled gladius (an archival tissue), we examined high resolution variations in the trophic niche of five large (.60 cm mantle length) jumbo squids (Dosidicus gigas) that were collected off the coast of Peru. We report the first evidence of large inter-individual differences in jumbo squid foraging strategies with no systematic increase of trophic level with size. Overall, gladius d 13 C values indicated one or several migrations through the squid's lifetime (,8-9 months), during which d 15 N values also fluctuated (range: 1 to 5%). One individual showed an unexpected terminal 4.6% d 15 N decrease (more than one trophic level), thus indicating a shift from higher-to lower-trophic level prey at that time. The data illustrate the high diversity of prey types and foraging histories of this species at the individual level. Conclusions/Significance: The isotopic signature of gladii proved to be a powerful tool to depict high resolution and ontogenic variations in individual foraging strategies of squids, thus complementing traditional information offered by stomach content analysis and stable isotopes on metabolically active tissues. The observed differences in life history strategies highlight the high degree of plasticity of the jumbo squid and its high potential to adapt to environmental changes.
To test the potential of diet switching experiments in ecophysiological studies of marine inverte... more To test the potential of diet switching experiments in ecophysiological studies of marine invertebrates, stable carbon isotope ratios were measured at different seasons in the gonad, adductor muscle, digestive gland and gills of scallops (Pecten maximus) and oysters (Crassostrea gigas) held for 15 days on a constant diet of phytoplankton depleted in 13 C. The aim of this study was to determine if differences in carbon incorporation could be detected among species, seasons and organs, and if so, whether it was consistent with their known energy-allocation patterns. After offering the new diet, isotope values of the different organs gradually shifted and significant differences among organs, seasons and species were found. A carbon incorporation index (CII) was calculated to compare the metabolic activity of each organ of the two species between day 0 and day 15. For both species, the digestive gland had the highest CII, the adductor muscle the lowest, while gonad and gills had intermediate values. The CII was generally much higher in P. maximus than in C. gigas, suggesting higher metabolic activity in this species. Seasonal differences in the CII were also observed for the two species and were interpreted as differences in metabolic activity in accordance with our energy allocation scenario. Therefore, stable isotope diet switching experiments appear to be of great value for assessing metabolic orientation in bivalves.
To determine the habitat and resource use of Dosidicus gigas in the Northern Humboldt Current Sys... more To determine the habitat and resource use of Dosidicus gigas in the Northern Humboldt Current System, we analysed carbon and nitrogen stable isotopes of 234 individuals collected during 2008-2010. Large variations in mantle stable isotope ratios were recorded, with values ranging from-19.1 to-15.1 % (d 13 C) and from 7.4 to 20.5 % (d 15 N). Most of the variation was explained by latitude, followed by distance to shelf break for carbon and by squid size for nitrogen. Latitudinal variations with increasing values from north to south were also found in zooplankton samples and were related to changes in isotope baseline values probably due to oxygen minimum zones that occur off Peru. This similar latitudinal trend in both zooplankton and D. gigas samples reveals that D. gigas is a relatively resident species at the scale of its isotopic turnover rate (i.e. a few weeks), even if this is not necessarily the case at the scale of its life. A small but significant size effect on d 13 C values suggests that jumbo squid perform offshore-onshore ontogenic migration, with juveniles distributed offshore. For nitrogen, the high interindividual variability observed with mantle length indicates that D. gigas can prey on a high variety of resources at any stage of their life cycle. This large-scale study off the coast of Peru provides further evidence that D. gigas have the capability to explore a wide range of habitats and resources at any stage of their life. Communicated by C. Harrod.
Environmental archives are useful tools for describing past and current climate variations and th... more Environmental archives are useful tools for describing past and current climate variations and they provide an opportunity to assess the anthropogenic contribution in coastal ecological changes. Along the West African coast, few studies have focused on such archives in coastal ecosystems. The bloody cockle Senilia senilis, an intertidal bivalve mollusk species, is widely distributed from Western Sahara to Angola, and has been harvested by humans over thousands of years. Therefore, this species appears to be a good candidate for assessing past variations of key environmental parameters such as temperature, primary production, and Saharan dust advection within West African coastal ecosystems. In the present paper, we focused (i) on the identification of growth rhythms of S. senilis shells in Mauritania (Banc d'Arguin), and (ii) on the potential of these shells as (paleo-)environmental archives. The method we used combined environmental survey, sclerochronology, and geochemical analyses of aragonite samples. We showed that microgrowth line formation was controlled by a tidal forcing, leading to the formation of two lines per lunar day. Brightness and thickness of these microgrowth lines progressively decreased from spring to neap tides (fortnightly cycle). Lunar daily growth rates displayed strong seasonal variations, with highest values (> 300 μm per lunar day) recorded in summer. The oxygen isotope composition of S. senilis shells (δ 18 O aragonite) accurately tracked seawater temperature seasonal variations, with a precision of 0.8°C. Finally, we discussed the opportunity to use Ba:Ca ratio in shells as a proxy for primary production or for Saharan dust transport. We also hypothesized that either Canary Currentvariations or, more probably, massive aerosol transfers from Sahara to the Atlantic Ocean could control uranium availability in coastal waters and explain the occurrence of U:Ca peaks within S. senilis shells.
In this study we test if calcite shells of the common mussel, Mytilus edulis, contain barium in p... more In this study we test if calcite shells of the common mussel, Mytilus edulis, contain barium in proportion to the water in which they grew. Similar to all bivalves analyzed to date, the [Ba/Ca] shell profiles are characterized by a relatively flat background [Ba/Ca] shell , interrupted by sharp [Ba/Ca] shell peaks. Previous studies have focused on these [Ba/Ca] shell peaks, but not on the background [Ba/Ca] shell. We show that in both laboratory and field experiments, there is a direct relationship between the background [Ba/Ca] shell and [Ba/Ca] water in M. edulis shells. The laboratory and field data provided background Ba/Ca partition coefficients (D Ba) of 0.10 ± 0.02 and 0.071 ± 0.001, respectively. This range is slightly higher than the D Ba previously determined for inorganic calcite, and slightly lower than foraminiferal calcite. These data suggest that M. edulis shells can be used as an indicator of [Ba/Ca] water , and therefore, fossil or archaeological M. edulis shells could be used to extend knowledge of estuarine dissolved Ba throughputs back in time. Moreover, considering the inverse relationship between [Ba/Ca] water and salinity, background [Ba/Ca] shell data could be used as an estuary specific indicator of salinity. The cause of the [Ba/Ca] shell peaks is more confusing, both the laboratory and field experiments indicate that they cannot be used as a direct proxy of [Ba/Ca] water or phytoplankton production, but may possibly be caused by barite ingestion.
Uploads
Papers by anne lorrain