Here we assessed the subsurface ultraphytoplanktonic (< 10 μm) community along a North-South round-trip Mediterranean transect as part of a MERITE-HIPPOCAMPE cruise campaign in April–May 2019. Temperature, salinity, and nutrient concentrations in subsurface waters (2–5 m depth) were also measured along the transect. The subsurface ultraphytoplankton community structure was resolved with a spatial resolution of few kilometers and temporal resolution of 30-min intervals using automated pulse shape recording flow cytometry. The subsurface waters were clustered into seven areas based on temperature and salinity characteristics. Synechococcus were by far the most abundant group in all prospected zones, and nanoeukaryotes were the main biomass component, representing up to 51 % of ultraphytoplanktonic carbon biomass. Apparent net primary productivity (NPP) followed a decreasing gradient along the transect from north to south and was mostly sustained by Synechococcus in all zones. These findings are likely to have implications in terms of the trophic transfer of contaminants in planktonic food webs, as they highlight the potential role of nanoplankton in contaminants bioaccumulation processes and the potential role of Synechococcus in a likely transfer via grazing activities.

Here we assessed the subsurface ultraphytoplanktonic (< 10 μm) community along a North-South round-trip Mediterranean transect as part of a MERITE-HIPPOCAMPE cruise campaign in April–May 2019. Temperature, salinity, and nutrient concentrations in subsurface waters (2–5 m depth) were also measured along the transect. The subsurface ultraphytoplankton community structure was resolved with a spatial resolution of few kilometers and temporal resolution of 30-min intervals using automated pulse shape recording flow cytometry. The subsurface waters were clustered into seven areas based on temperature and salinity characteristics. Synechococcus were by far the most abundant group in all prospected zones, and nanoeukaryotes were the main biomass component, representing up to 51 % of ultraphytoplanktonic carbon biomass. Apparent net primary productivity (NPP) followed a decreasing gradient along the transect from north to south and was mostly sustained by Synechococcus in all zones. These findings are likely to have implications in terms of the trophic transfer of contaminants in planktonic food webs, as they highlight the potential role of nanoplankton in contaminants bioaccumulation processes and the potential role of Synechococcus in a likely transfer via grazing activities.

Tires can release a large number of chemical compounds that are potentially hazardous for aquatic organisms. An ecophysiological system was used to do high-frequency monitoring of individual clearance, respiration rates, and absorption efficiency of juvenile oysters (8 months old) gradually exposed to four concentrations of tire leachates (equivalent masses: 0, 1, 10, and 100 μg tire mL−1). Leachates significantly reduced clearance (52 %) and respiration (16 %) rates from 1 μg mL−1, while no effect was observed on the absorption efficiency. These results suggest that tire leachates affect oyster gills, which are the organ of respiration and food retention as well as the first barrier against contaminants. Calculations of scope for growth suggested a disruption of the energy balance with a significant reduction of 57 %. Because energy balance directs whole-organism functions (e.g., growth, reproductive outputs), the present study calls for an investigation of the long-term consequences of chemicals released by tires.

Tires can release a large number of chemical compounds that are potentially hazardous for aquatic organisms. An ecophysiological system was used to do high-frequency monitoring of individual clearance, respiration rates, and absorption efficiency of juvenile oysters (8 months old) gradually exposed to four concentrations of tire leachates (equivalent masses: 0, 1, 10, and 100 μg tire mL⁻¹). Leachates significantly reduced clearance (52 %) and respiration (16 %) rates from 1 μg mL⁻¹, while no effect was observed on the absorption efficiency. These results suggest that tire leachates affect oyster gills, which are the organ of respiration and food retention as well as the first barrier against contaminants. Calculations of scope for growth suggested a disruption of the energy balance with a significant reduction of 57 %. Because energy balance directs whole-organism functions (e.g., growth, reproductive outputs), the present study calls for an investigation of the long-term consequences of chemicals released by tires.

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.

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.

Trace elements and δ15N values were analysed in micronekton (crustaceans, fishes and squids) sampled in the south-western Indian Ocean. Myctophids were associated with high concentrations of arsenic at La Pérouse and MAD-Ridge seamounts, and with lead and manganese at MAD-Ridge and in the Mozambique Channel. The difference in cadmium, copper and zinc concentrations between micronekton broad categories reflected differing metabolic and storage processes. When significant, negative relationships were found between micronekton body size and trace element concentrations, which can possibly be attributed to differing metabolic activity in young and old individuals, dietary shifts and/or dilution effect of growth. No relationships were found between trace element concentrations and δ15N values of micronekton (except cobalt which decreased with increasing δ15N values), since most trace elements are not biomagnified in food webs due to regulation and excretion processes within organisms. All trace element pairs were positively correlated in fishes suggesting regulation processes.

Trace elements and δ¹⁵N values were analysed in micronekton (crustaceans, fishes and squids) sampled in the south-western Indian Ocean. Myctophids were associated with high concentrations of arsenic at La Pérouse and MAD-Ridge seamounts, and with lead and manganese at MAD-Ridge and in the Mozambique Channel. The difference in cadmium, copper and zinc concentrations between micronekton broad categories reflected differing metabolic and storage processes. When significant, negative relationships were found between micronekton body size and trace element concentrations, which can possibly be attributed to differing metabolic activity in young and old individuals, dietary shifts and/or dilution effect of growth. No relationships were found between trace element concentrations and δ¹⁵N values of micronekton (except cobalt which decreased with increasing δ¹⁵N values), since most trace elements are not biomagnified in food webs due to regulation and excretion processes within organisms. All trace element pairs were positively correlated in fishes suggesting regulation processes.

The Mediterranean Sea water bodies are ones of the most polluted, especially with microplastics. As the seafloor is the ultimate sink for litter, it is considered a hotspot for microplastic pollution. We provide an original analytical development based on the coupling of tandem mass spectrometry to pyrolysis-gas chromatography to improve the detection of plastic contamination in marine organisms. Due to the high selectivity of the mass spectrometer, a straightforward sample preparation consists uniquely of potassium hydroxide digestion. The quantification of six common polymers is possible in one run. The method was applied to analyze the plastic content from 500 μm down to 0.7 μm in the whole body of seven benthic species with variable feeding modes. Plastic was detected in all samples, with an almost systematic detection of polypropylene and polyethylene. Our method presents a major development in determining the levels of plastic contaminations in samples with rich organic matter content.

The Mediterranean Sea water bodies are ones of the most polluted, especially with microplastics. As the seafloor is the ultimate sink for litter, it is considered a hotspot for microplastic pollution. We provide an original analytical development based on the coupling of tandem mass spectrometry to pyrolysis-gas chromatography to improve the detection of plastic contamination in marine organisms. Due to the high selectivity of the mass spectrometer, a straightforward sample preparation consists uniquely of potassium hydroxide digestion. The quantification of six common polymers is possible in one run. The method was applied to analyze the plastic content from 500 μm down to 0.7 μm in the whole body of seven benthic species with variable feeding modes. Plastic was detected in all samples, with an almost systematic detection of polypropylene and polyethylene. Our method presents a major development in determining the levels of plastic contaminations in samples with rich organic matter content.

Ingestion of microplastics by aquatic organisms is often harmful due to the dilution of their regular food with low-calorie microplastic particles, but can also be beneficial if nutritious biofilms are present on the microplastic surface. This begs the question: is ingestion of microplastic harmful or beneficial and can the net effect of the two mechanisms be quantified? Here, we quantified these harmful and beneficial effects on Daphnia magna, using dose-response tests with clean and biofouled microplastic respectively, and determined the trade-off between these counteracting effects. A population model was developed to calculate the isoclines for zero population growth, separating the regime where adverse food dilution dominated from that where the beneficial biofilm vector mechanism dominated. Our results show that the organisms grew better when exposed to biofouled microplastic compared to pristine microplastic. Very good model predictions (R2 = 0.868–0.991) of the effects of biofouled microplastic were obtained based on literature parameter values, with optimization required only for the two sub-model parameters driving the dose-effect relationships for pristine microplastic. These results contradict previous sudies were only pristine microplastic were used and demonstrate that the ruling paradigm of unambiguously adverse microplastic effects is not ecologically justifiable.

Ingestion of microplastics by aquatic organisms is often harmful due to the dilution of their regular food with low-calorie microplastic particles, but can also be beneficial if nutritious biofilms are present on the microplastic surface. This begs the question: is ingestion of microplastic harmful or beneficial and can the net effect of the two mechanisms be quantified? Here, we quantified these harmful and beneficial effects on Daphnia magna, using dose-response tests with clean and biofouled microplastic respectively, and determined the trade-off between these counteracting effects. A population model was developed to calculate the isoclines for zero population growth, separating the regime where adverse food dilution dominated from that where the beneficial biofilm vector mechanism dominated. Our results show that the organisms grew better when exposed to biofouled microplastic compared to pristine microplastic. Very good model predictions (R² = 0.868–0.991) of the effects of biofouled microplastic were obtained based on literature parameter values, with optimization required only for the two sub-model parameters driving the dose-effect relationships for pristine microplastic. These results contradict previous sudies were only pristine microplastic were used and demonstrate that the ruling paradigm of unambiguously adverse microplastic effects is not ecologically justifiable.