This paper looks at experiential feedback and the technical and scientific challenges tied to the MERITE-HIPPOCAMPE cruise that took place in the Mediterranean Sea in spring 2019. This cruise proposes an innovative approach to investigate the accumulation and transfer of inorganic and organic contaminants within the planktonic food webs. We present detailed information on how the cruise worked, including 1) the cruise track and sampling stations, 2) the overall strategy, based mainly on the collection of plankton, suspended particles and water at the deep chlorophyll maximum, and the separation of these particles and planktonic organisms into various size fractions, as well as the collection of atmospheric deposition, 3) the operations performed and material used at each station, and 4) the sequence of operations and main parameters analysed. The paper also provides the main environmental conditions that were prevailing during the campaign. Lastly, we present the types of articles produced based on work completed by the cruise that are part of this special issue.

Black pearl farming is the second source of French Polynesia income after tourism, and Gambier Islands are the main farming sites. Gambier main lagoon contains several sub-lagoons critical for pearl oyster rearing and spat collecting (SC). The Rikitea lagoon, traditionally had good SC rates in the warm season which ensured steady supplies of oysters for black pearl production. However, since 2018, SC has abruptly decreased. To assess the factors affecting SC, Gambier lagoon hydrodynamics was investigated in 2019–2020 to calibrate a hydrodynamic model and simulate larval dispersal around the SC areas. The model shows the strong wind influence on larval dispersal and accumulation patterns and suggests that windy months in the warm season as it can occur during La Niña episodes can explain recent poor SC. Larval dispersal scenarios also informed on best locations to perform adult oyster restocking, a practice that can also enhance SC on the long term.

The aim of this study was to characterize and quantify microplastics (MPs) at the chlorophyll maximum layer (CML), around 30 to 60 m depth, during a cruise dedicated to the study of contaminants in plankton, the MERITE-HIPPOCAMPE project, along a north-south transect in the western Mediterranean Sea (Tedetti et al., 2023). Plankton were collected by horizontal net tows in this layer using a multinet Hydrobios Midi equipped with 60 μm mesh-size nets. The collected plankton were fractionated through a sieve column for various later contaminant measurements and for zooplankton analysis (Fierro-González et al., 2023). For all stations, samples were also fully examined for microplastics (MPs) for fractions >300 μm. MPs were found at all stations in the CML layer (mean: 42.9 ± 45.4 MPs m−3), of which 96 ± 4 % were fibers. The ratios of mesozooplankton/MPs and detritus/MPs in this CML were respectively 223 ± 315 and 2544 ± 2268. These data are analyzed together with MPs concentrations from sea- surface sampled with a 300 μm net-size Manta net at the same stations. Overall, our observations highlight the very high density of fibers at the CML, mainly associated with aggregates, raising the hypothesis of their interactions with marine snow. Therefore, the importance of marine snow and vertical layering will have to be considered in future MP distribution modelling efforts.

The Anthropocene, defined by human-induced environmental transformations, presents a critical challenge: plastic pollution. This complex problem, particularly prominent in coastal and marine environments, requires integrated and adaptive responses. This opinion paper examines global efforts across policy interventions, scientific innovations, and public education, highlighting both advancements and hurdles in managing this problem. These include enforcement limitations in policy implementation, scalability and cost issues in scientific innovations, and challenges in effecting large-scale behavioral change through public education. The complexities inherent in managing plastic litter in coastal and marine environments are further discussed, emphasizing the necessity for an integrated approach. This approach involves interdisciplinary collaboration, adaptive management, stakeholder engagement, policy integration, sustainable financing, resilience building, capacity enhancement, technological innovation, policy reform, ecosystem-based management, disaster risk reduction, and advocacy. The management of plastic pollution in the Anthropocene requires strategic planning, innovative thinking, and unified global efforts, ultimately providing an opportunity to redefine our relationship with the planet and steer toward a more sustainable future.

This paper evaluates the use of a PBS/PBAT biodegradable rope to reduce the environmental impact of fishing gear lost at sea. The study aims to better understand the degradation mechanisms that the rope and its monofilaments may encounter due to the long term exposure to seawater. The monofilaments were immersed in natural seawater for up to 18 months, and rope samples were also immersed to study aging at a larger scale and evaluate the ability of a modelling tool to predict initial and aged states of the rope. At low temperatures, no loss of properties was observed for the monofilament and rope. However, at higher temperatures, biodegradation and hydrolysis processes were observed, leading to a faster loss of properties in the monofilament compared to the rope. The modelling tool provided conservative predictions due to severe mechanical test conditions of aged monofilament and a degradation gradient within the rope structure.

Pollution is one of the main anthropogenic threats to marine ecosystems. Studies analysing the accumulation and transfer of contaminants in planktonic food webs tend to rely on samples collected in discrete water bodies. Here, we assessed the representativeness of measurements at the chlorophyll-a maximum layer during the MERITE-HIPPOCAMPE cruise for the entire water column by investigating the vertical distribution of particles and plankton obtained by in-situ optical profilers at nine stations across the Mediterranean Sea. We identified specific conditions where the interpretation of results from contaminant analyses can be improved by detailing plankton size structure and vertical distributions. First, the presence of higher than usual plankton concentrations can result in sampling issues that will affect biomass estimation within each size class and therefore bias our understanding of the contaminant dynamics. Secondly, the presence of an unsampled water layer with high zooplankton biomass might imply non-resolved contaminant pathways along the trophic structure. This study lays the basis for optimizing sampling strategy in contaminant studies.

Total and dissolved concentrations of inorganic mercury (IHg) and methylmercury (MeHg) in water (Adour Estuary) were determined during three sampling campaigns and related to biogeochemical variables (nutrients, organic matter). Factors (sampling time, sample type) were included in analysis of covariance with effect separation. The urban estuary suffered historically from anthropogenic sources, however, decreased emissions have reduced Hg concentrations. Total IHg (0.51–3.42 ng L−1) and MeHg (25–81 pg L−1) concentrations are additively described by suspended particulate matter and particulate organic carbon. Higher total concentrations, carried by organic-rich particles, were found near specific discharge points (0.79–8.02 ng L−1 and 34–235 pg L−1 for IHg and MeHg, respectively). The associated high dissolved MeHg concentrations could not be explained only by biogeochemical variables. Better efficiency of the models is found for total than for dissolved concentrations. Models should be checked with other contaminants or with estuaries, suffering from downstream contamination.

Vast quantities of debris are beaching at remote islands in the western Indian Ocean. We carry out marine dispersal simulations incorporating currents, waves, winds, beaching, and sinking, for both terrestrial and marine sources of debris, to predict where this debris comes from. Our results show that most terrestrial debris beaching at these remote western Indian Ocean islands drifts from Indonesia, India, and Sri Lanka. Debris associated with fisheries and shipping also poses a major risk. Debris accumulation at Seychelles is likely seasonal, peaking during February–April. This pattern is driven by monsoonal winds and may be amplified during positive Indian Ocean Dipole and El-Niño events. Our results underline the vulnerability of small island states to marine plastic pollution, and are a crucial step towards improved management of the issue. The trajectories used in this study are available for download, and our analyses can be rerun under different parameter choices.

The concentrations and isotopic compositions of carbon (C), copper (Cu), zinc (Zn), and lead (Pb) in coastal sediments were analyzed to identify potential pollution sources. High concentrations of total organic carbon (TOC) and metals were found close to cities and industrial areas. The isotopic compositions of C, Cu, Zn, and Pb tended to decrease as their concentrations increased. Bi-plots between δ65Cu and δ66Zn showed that the isotopic compositions in most coastal sediments, except sediments around a smelter, were similar to the isotopic compositions of road dust in urban and industrial areas of Korea. Our results suggest that heavy metal pollution in coastal sediments is greatly influenced by the pollution source, such that most metals originate from traffic and industrial activities in the urban environment. This analysis of multiple isotopes provides insights concerning the transport mechanisms and clarifies potential sources of metal contamination in coastal environments.

Plankton represents the main source of carbon in marine ecosystems and is consequently an important gateway for contaminants into the marine food webs. During the MERITE– HIPPOCAMPE campaign in the Mediterranean Sea (April–May 2019), plankton was sampled from pumping and net tows at 10 from the French coast to the Gulf of Gabès (Tunisia) to obtain different size fractions in contrasted regions. This study combines various approaches, including biochemical analyses, analyses of stable isotope ratios (δ13C, δ15N), cytometry analyses and mixing models (MixSiar) on size-fractions of phyto- and zooplankton from 0.7 to >2000 μm. Pico- and nanoplankton represented a large energetic resource at the base of pelagic food webs. Proteins, lipids, and stable isotope ratios increased with size in zooplankton and were higher than in phytoplankton. Stable isotope ratios suggest different sources of carbon and nutrients at the base of the planktonic food webs depending on the coast and the offshore area. In addition, a link between productivity and trophic pathways was shown, with high trophic levels and low zooplankton biomass recorded in the offshore area. The results of our study highlight spatial variations of the trophic structure within the plankton size-fractions and will contribute to assess the role of the plankton as a biological pump of contaminants.