Microplastics in the maximum chlorophyll layer along a north-south transect in the Mediterranean Sea in comparison with zooplankton concentrations.

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.

Zooplankton biomass, size structure, and associated metabolic fluxes with focus on its roles at the chlorophyll maximum layer during the plankton-contaminant MERITE-HIPPOCAMPE cruise.

Recent studies have demonstrated that plankton can be a key pathway for the uptake and transfer of contaminants entering the marine environment up to top predators. The plankton-contaminant MERITE-HIPPOCAMPE cruise was devoted to quantifying contaminants in water and the whole plankton size range (10 size fractions) at 10 stations along a north-south transect in the western Mediterranean Sea from the French to the Tunisian coasts through the Provençal and Algerian basins. Pumping and filtering devices and net sampling have been used for collecting very high amounts of small particles and planktonic organisms in the chlorophyll maximum layer (CML). The present paper characterizes the zooplankton components for which the contaminant measurements were carried out. At each station, a horizontal towed Hydro-Bios net with a 60 µm mesh-size net was used to discriminate 5 size-fractions from 60 µm to a few mm. For each size-fraction, one part of the sample was used for dry weight measurements and the other one for estimating the contribution to biomass of detritus, phytoplankton, and among zooplankton of the major taxonomic groups based on the imagery tools ZOOSCAN and FLOWCAM. In each zooplankton size fraction, metabolic rates were calculated from the size spectrum to estimate trophic and excretion fluxes flowing through this fraction. These observations were compared to a similar analysis of tows in the upper layer (vertical) and the surface layer (horizontal). The total sampled biomass concentration at the CML was higher than in the water column (COL) and much higher than at the surface (SURF) in most of the stations, but in the CML and COL a substantial contribution was due to detritus mostly concentrated in the smallest size-fractions (60-200 µm and 200-500 µm). Absolute values of zooplankton biomass show neither a clear spatial pattern nor a significant difference between strata. The CML layer was dominated by copepods similarly to COL and SURF, but presented a higher contribution of nauplii and a near absence of appendicularians. At some stations, crustaceans and gelatinous plankton could be important contributors to CML. The zooplankton biomass composition of the two smallest fractions (<500 µm) was dominated by nauplii, small copepods and, occasionally, by small miscellaneous organisms (mostly pteropodes). In contrast, clear differences between stations appeared for the largest fractions (>500 µm) due to large crustaceans, gelatinous organisms, and chaetognaths. These changes in biomass composition according to size fractions suggest a progressive trophic shift from dominant herbivory in the smallest fractions to more contrasted trophic structure (including carnivory) in the largest fractions. The daily carbon demand and the N and P excretion of zooplankton were on average higher at the CML but with no significant difference with COL. The zooplankton grazing represented 2.7 to 22.7 % of the phytoplankton stock per day, whereas its excretion represented a daily N and P recycling compared to dissolved inorganic nitrogen and phosphorus stocks ranging respectively from 0.2 to 19 % and from 0 to 21 %. This information should help in the interpretation of the content of various contaminants in zooplankton fractions.

Spatial variations of biochemical content and stable isotope ratios of size-fractionated plankton in the Mediterranean Sea (MERITE-HIPPOCAMPE campaign).

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 stations 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.

Contamination of planktonic food webs in the Mediterranean Sea: Setting the frame for the MERITE-HIPPOCAMPE oceanographic cruise (spring 2019).

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.