Seasonal and multi-decadal zinc isotope variations in blue mussels from two sites with contrasting zinc contamination levels.

Zinc (Zn) isotope compositions in soft mussel tissues help identify internal biological processes and track coastal Zn sources in coastal environments, thus aiding in managing marine metal pollution. This study investigated the seasonal and multi-decadal Zn isotope compositions of blue mussels (genus Mytilus) from two French coastal sites with contrasting Zn environmental contamination. Concurrently, we characterized the isotope ratios of sediments and plankton samples at each site to understand the associations between organisms and abiotic compartments. Our primary objective was to determine whether these isotope compositions trace long-term anthropogenic emission patterns or if they reflect short-term biological processes. The multi-decadal isotope profiles of mussels in the Loire Estuary and Toulon Bay showed no isotope variations, implying the enduring stability of the relative contributions of natural and anthropogenic Zn sources over time. At seasonal scales, Zn isotope ratios were also constant; hence, isotope effects related to spawning and body growth were not discernible. The multi-compartmental analysis between the sites revealed that Toulon Bay exhibits a remarkably lower Zn isotope ratio across all studied matrices, suggesting the upward transfer of anthropogenic Zn in the food web. In contrast, the Zn isotope variability observed for sediments and organisms from the Loire Estuary fell within the natural baseline of this element. In both sites, adsorptive geogenic material carrying significant amounts of Zn masks the biological isotope signature of plankton, making it difficult to determine whether the Zn isotope ratio in mussels solely reflects the planktonic diet or if it is further modified by biological homeostasis. In summary, Zn isotope ratios in mussels offer promising avenues for delineating source-specific isotope signatures, contingent upon a comprehensive understanding of the isotope fractionation processes associated with the trophic transfer of this element through the plankton.

Distribution of rare earth elements and assessment of anthropogenic gadolinium in estuarine habitats: The case of Loire and Seine estuaries in France.

Rare earth elements (REEs), attractive to society because of their applications in industry, agriculture and medicine, are increasingly released into the environment especially in industrialized estuaries. This study compared the REE distribution in the abiotic compartments: water (dissolved phase (<0.45 µm), suspended particulate matter (SPM)) and sediment of the Loire and Seine estuaries (France). A total of 8 and 6 sites were investigated in the Loire and Seine, respectively, as well as 5 additional offshore sites for the Loire. Total REE concentrations were higher in the Loire for the dissolved phase (93.5 ± 63.3 vs 87.7 ± 16.2 ng/L), SPM (173.9 ± 18.3 vs 114.0 ± 17.8 mg/kg dw) and sediments (198.2 ± 27.9 vs 73.2 ± 27.4 mg/kg dw), explained by higher geogenic inputs. Individual REE contributions along with normalization highlighted heavy REE enrichments and Gd positive anomalies in the dissolved phase of the two estuaries, whereas REE distributions in SPM and sediments followed the natural abundance of the REE classes. The calculated Gd anomalies in the dissolved phase were higher in the Seine (9.7 ± 3.4) than in the Loire (3.0 ± 0.8), corresponding to 88.3 ± 5.1 % and 64.4 ± 11.1 % of anthropogenic Gd. This demonstrates a higher contamination of the Seine estuary, certainly due to the difference in the number of inhabitants between both areas involving different amounts of Gd used in medicine. The offshore sites of Loire showed lower total REE concentrations (55.8 ± 5.8 ng/L, 26.7 ± 38.2 mg/kg dw and 100.1 ± 11.7 mg/kg dw for the dissolved phase, SPM and sediments, respectively) and lower Gd anomalies (1.2 ± 0.2) corresponding to only 13.3 ± 3.9 % of anthropogenic Gd, confirming a contamination from the watershed. This study comparing two major French estuaries provides new data on the REE distribution in natural aquatic systems.

High inter-species variability in elemental composition of the twilight zone fauna varies implications for predators and exploitation by humans.

While the importance of oceanic micronektonic species in biogeochemical cycles and in the transfer of matter in food webs is globally recognized, specific knowledge on elemental concentrations and their variability within this community is still poorly documented. Here, we report for the first time in the Bay of Biscay, North-East Atlantic, the body composition in various biological parameters and chemical elements of a meso-to bathypelagic micronektonic community. Stable carbon and nitrogen isotope compositions (δ13C, δ15N), C:N ratios, energy density, as well as the concentrations in 6 macro-minerals and 13 trace elements including essential (micro-nutrients) and non-essential elements (undesirables, with no know biological function) were measured in whole organisms of 4 crustacean and 11 fish species caught simultaneously around 800 m depth. The results showed a low variability of δ13C values, confirming that all studied species share the same habitat. On the contrary, large differences were observed among species for several elements. Trace elements showed the greatest variability (i.e. larger range of values), especially silver (Ag), arsenic (As), cadmium (Cd), cobalt and vanadium. Significant differences were also revealed among taxa for Ag, As, Cd, copper and strontium concentrations (with crustaceans > fish), as well as for δ15N values and phosphorus concentrations (with fish > crustaceans). Although concentrations varied greatly among species, they could be grouped according to their energy density and composition in 19 chemical elements, through hierarchical clustering analysis. Six functional groups of species have been thus identified, reflecting contrasted nutritional benefit and/or exposure to undesirables for predators feeding on this deep pelagic community. Finally, the concentrations measured for the potentially toxic trace elements (undesirables) exceeded the existing European thresholds for Cd and to a lesser extent mercury (Hg), which point out potential risks in the perspective of a future exploitation of these deep living resources by humans.

Metal stable isotopes in transplanted oysters as a new tool for monitoring anthropogenic metal bioaccumulation in marine environments: The case for copper.

Metal release into the environment from anthropogenic activities may endanger ecosystems and human health. However, identifying and quantifying anthropogenic metal bioaccumulation in organisms remain a challenging task. In this work, we assess Cu isotopes in Pacific oysters (C. gigas) as a new tool for monitoring anthropogenic Cu bioaccumulation into marine environments. Arcachon Bay was taken as a natural laboratory due to its increasing contamination by Cu, and its relevance as a prominent shellfish production area. Here, we transplanted 18-month old oysters reared in an oceanic neighbor area into two Arcachon Bay mariculture sites under different exposure levels to continental Cu inputs. At the end of their 12-month long transplantation period, the oysters' Cu body burdens had increased, and was shifted toward more positive δ65Cu values. The gradient of Cu isotope compositions observed for oysters sampling stations was consistent with relative geographic distance and exposure intensities to unknown continental Cu sources. A binary isotope mixing model based on experimental data allowed to estimate the Cu continental fraction bioaccumulated in the transplanted oysters. The positive δ65Cu values and high bioaccumulated levels of Cu in transplanted oysters support that continental emissions are dominantly anthropogenic. However, identifying specific pollutant coastal source remained unelucidated mostly due to their broader and overlapping isotope signatures and potential post-depositional Cu isotope fractionation processes. Further investigations on isotope fractionation of Cu-based compounds in an aqueous medium may improve Cu source discrimination. Thus, using Cu as an example, this work combines for the first time a well-known caged bivalve approach with metal stable isotope techniques for monitoring and quantifying the bioaccumulation of anthropogenic metal into marine environments. Also, it states the main challenges to pinpoint specific coastal anthropogenic sources utilizing this approach and provides the perspectives for further studies to overcome them.

Rare Earth Element in Bivalves' Soft Tissues of French Metropolitan Coasts: Spatial and Temporal Distribution.

Rare earth elements (REE) are becoming an environmental pollutant of emerging concern, linked to their use in various anthropic processes. Because REE bioconcentrate in marine organisms throughout their food webs, a better understanding of biogeochemical processes leading to REE concentrations found in coastal species is necessary. This study was designed to assess REEs concentrations in various common bivalves from the French coastline to identify possible geographic, taxonomic, or temporal variations of concentrations. Based on the French Mussel Watch program, three species of bivalves (oyster Crassostrea gigas and mussels Mytilus edulis and Mytilus galloprovincialis) were collected all along the French metropolitan coast and soft tissues were analyzed for REE concentrations. Results have shown higher REE concentrations in bivalve soft tissues near estuaries without taxonomic nor national geographic differences. The highest levels have been observed in the Gironde estuary with total REE concentrations (∑REE) in oysters up to 10.94 µg g-1 d.w. The REE distribution pattern in both mussel species described a particle-like (inverse V-shape) pattern, whereas C. gigas REE distribution pattern changes from a particle-like to a dissolved-like pattern with a heavy REE (HREE) enrichment. However, no environmental parameter could be linked to these pattern changes. Finally, neither Gd anomalies nor an evolution of REE concentrations over a 30-year period have been detected in bivalves' soft tissues.

Differences in Copper Isotope Fractionation Between Mussels (Regulators) and Oysters (Hyperaccumulators): Insights from a Ten-Year Biomonitoring Study.

Copper (Cu) isotope compositions in bivalve mollusks used in marine-monitoring networks is a promising tool to monitor anthropogenic Cu contamination in coastal and marine ecosystems. To test this new biomonitoring tool, we investigated Cu isotope variations of two bivalves-the oyster Crassostrea gigas and the mussel Mytilus edulis-over 10 years (2009-2018) in a French coastal site contaminated by diffuse Cu anthropogenic sources. Each species displayed temporal concentration profiles consistent with their bioaccumulation mechanisms, that is, the Cu-regulating mussels with almost constant Cu concentrations and the Cu-hyperaccumulating oysters with variable concentrations that track Cu bioavailability trends at the sampling site. The temporal isotope profiles were analogous for both bivalve species, and an overall shift toward positive δ65Cu values with the increase of Cu bioavailabilities was associated with anthropogenic Cu inputs. Interestingly, mussels showed wider amplitudes in the isotope variations than oysters, suggesting that each species incorporates Cu isotopes in their tissues at different rates, depending on their bioaccumulation mechanisms and physiological features. This study is the first to demonstrate the potential of Cu isotopes in bivalves to infer Cu bioavailability changes related to anthropogenic inputs of this metal into the marine environment.

Assessment of the metal contamination evolution in the Loire estuary using Cu and Zn stable isotopes and geochemical data in sediments.

In this work, a multi-elemental approach combining Cu and Zn stable isotopes is used to assess the metal contamination evolution in the Loire estuary bulk sediments. Elemental geochemical data indicate an increase of metal concentrations from the beginning of the industrial period peaking in the 1990s, followed by an attenuation of metal contamination inputs to the estuary. Zinc isotope compositions suggest a binary mixing process between Zn derived from terrigenous material and multi-urban anthropogenic sources. Copper isotope systematics indicate a single natural dominant source represented by weathered silicate particles from soils and rocks. This work demonstrates the applicability of Zn isotopes to identify anthropogenic Zn sources in coastal systems, even under a low to moderate degree of contamination. Further studies are required to constrain Cu sources and to elucidate possible effects of grain-size and mineralogy in the Cu isotope composition of sediment in the Loire estuary.