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Tracking PFAS in ecosystems is challenging. In this context, monitoring programs are crucial to fill data gaps, especially in marine environments, which are the ultimate outlets for these forever chemicals. The 2021 chemical contamination monitoring campaign along the French Mediterranean coast established a baseline for PFAS concentrations in mussels, with 90 % of measurements below quantification limits. When detected, long-chain PFCA's were predominant. Spatial distribution patterns suggested continuous PFAS inputs and complex dynamics, shaped by the influence of large watersheds and rivers (Rhône, Aude, Huveaune). Lapeyrade shallow lagoon stood out as the most contaminated site. Similar PFAS profiles in connected sites implied shared sources but raised questions about accumulation processes in mussels. While certain sites had evident sources (e.g., military airbase for Palo lagoon), others remained uncertain (e.g., Toulon bay). Coastal stations (Banyuls, Cap Agde, Brégançon, Pampelonne) showed PFAS contamination without clear onshore sources, possibly due to insufficient transportation process understanding.

Après un rappel des objectifs, du fonctionnement et de la méthode d’interprétation des résultats des réseaux de surveillance microbiologique (REMI) et d’observation des contaminants chimiques (ROCCH), ce rapport effectue un bilan national et décrit le programme annuel des départements des Bouches-du-Rhône, du Var et de la Haute-Corse. Il présente l’ensemble des résultats obtenus, en particulier l’estimation de la qualité des zones de production de coquillages classées et l’évolution de leur qualité pour la période 2020-2022.

Active biomonitoring of chemical contamination (e.g., Cd, Hg, Pb, DDT, PCB, PAH) in French Mediterranean coastal waters has been performed for more than two decades. This study aimed at presenting the current contamination in 2021 and the temporal evolution of concentrations from 2000. Based on a relative spatial comparison, low concentrations were measured in 2021 at most sites (>83 %). Also, several stations with moderate to high levels were highlighted in the vicinity of major urban industrial centers (e.g., Marseille, Toulon) and near river mouths (e.g., Rhône, Var). Over the last 20 years, no major trend was revealed, mostly, especially for the relative high-level sites. This likely constant contamination over time, plus slight increases of metallic elements at a few sites, still raise questions on the efforts that remain to be made. The decreasing trends of organic compounds, in particular PAH, provide evidence of the efficiency of some management actions.

L’Ifremer coordonne, sur l’ensemble du littoral métropolitain, la mise en œuvre des réseaux d’observation et de surveillance de la mer côtière. Ce bulletin contient une synthèse et une analyse des données collectées par l’ensemble des réseaux pour les départements des Alpes maritimes, Bouches du Rhône, Var et Haute Corse. Des représentations graphiques homogènes, assorties de commentaires, donnent des indications sur les niveaux et les tendances des paramètres mesurés.

En Méditerranée, des campagnes de surveillance sont mises en œuvre tous les 3 ans depuis 2006 pour répondre aux objectifs de surveillance des eaux côtières et de transition. Ce rapport présente les résultats de la sixième évaluation des descripteurs opérés par IFREMER au titre de la Directive Cadre sur l’Eau, qui contribue à l’évaluation du bon état du bassin Rhône Méditerranée Corse menée par l’Agence de l’Eau. L’état des lieux 2021 met en évidence le bon état chimique de l’ensemble des masses d’eau côtières avec potentiellement un déclassement d’eau de la masse d’eau Côte Bleue (FRDC05) en raison d’une concentration en tributylétain (TBT) supérieure à la valeur guide considérée. Sur le plan biologique, à travers les éléments de qualité physico-chimique et phytoplancton, les masses d’eau côtières sont évaluées dans état écologique bon à très bon. Parmi les 22 masses d’eau de transition investiguées, les signes de mauvais état chimique et/ou écologique (au regard des descripteurs physico-chimie, phytoplancton et macrophytes) pointent essentiellement 4 masses d’eau : étangs de Canet (FRDT01), de l’Or (FRDT11a), Palavasiens est (FRDT11b) et Camargue Complexe Vaccarès (FRDT14a). Par rapport aux précédentes évaluations, les signes d’amélioration de l’état écologique ont pu être mis en évidence principalement sur les étangs de Berre (FRDT15a) et de Vaïne (FRDT15b).

Diel cycles of dissolved cationic metal concentrations commonly occur in freshwater streams in apparent response to coincident cycles in water quality parameters (pH, O2, temperature). Hourly sampling of the Cd-contaminated Riou Mort (France) revealed large diel cycles in “total” dissolved Cd (232−357 nM; <0.45 μm) and “truly” dissolved Cd (56−297 nM; <0.02 μm) which were strongly correlated with changes in water pH. Using measured fluxes, a dissolved O2 model was constructed that indicated that benthic metabolic activities, respiration and photosynthesis, were responsible for the diel O2 (and thus, CO2 and pH) variation in the stream. However, microsensor measurements also showed that the pH changes occurred at the biofilm interface earlier than in the bulk water column. This difference in timing was reflected in the Cd dynamics, where pH-controlled sorption effects caused Cd partitioning from the truly dissolved pool onto the biofilm in the morning, and from the truly dissolved pool onto large colloids (0.02−0.45 μm) later in the day. Because this process causes large changes in the bioavailable Cd fraction, it has significant implications for Cd toxicity in freshwater streams. This study demonstrates the profound control of benthic microbiological processes on the cycling of heavy metals in aquatic systems.

The light response curve methodology for microphytobenthic biofilms was studied by comparing the two most usual approaches used in pulse amplitude modulated (PAM) fluorometry. The non-sequential light curve (N-SLC) method is characterized by independent measures of the photosynthetic activity across a light gradient whereas the rapid light curve (RLC) method consists of successive measures on the same sample exposed to a stepwise increase of light intensities. Experiments were carried out on experimental microphytobenthic biofilms prepared from natural assemblages and acclimated to dark conditions. In preliminary experiments, N-SLCs were constructed from fluorescence induction curves performed at 12 different photon flux densities (PFDs). A minimum of 50 s of illumination was necessary to reach a stable light response curve; shorter illumination times resulted in underestimating the physiological parameters (alpha the light utilization coefficient in light-limited conditions and rETR(max) the maximum rate of photosynthesis efficiency) of the light response curve. For the comparison between N-SLCs and RLCs, the same time of illumination (50 s) was used for each light step of RLCs so that N-SLCs differed from RLCs only by the way the amount of light was delivered, i.e., a light dose accumulation for RLC. The experimental results showed the difference between the two photobiological response curves. In the lower range of PFDs, RLCs exhibited a larger value of alpha; in this light-limited part of the response curve the incremental increase of PFDs limited the development of NPQ and resulted in a better optimization of electron transport rate for RLC. In the higher range of PFDs, the trend was reversed and the RLC showed a lower value of rETR(max) than the N-SLC did; this is attributed to the light dose accumulation which likely led to a more efficient dispersion of energy, as illustrated by a higher non-photochemical quenching (NPQ). In conclusion, these results confirm

The microalgal biofilm, which is present on sediment surface, is the primary production unit on bare intertidal mudlfats. In order to improve our understanding of this primary production system, a study was undertaken at microscale to analyse the structural and functional properties of this biofilm. In the first part of this work, a detailed descriptive analysis lead to the characterisation of the spatiotemporal dynamics of the physical structure of the biofilm. It turns out that the biofilm exhibits its own dynamics at the sediment surface (as a discrete compartment in the top 200 µm), but still has a functional relationship with the total available biomass (PCB) distributed within the top 1 cm (linear relationship between the maximum size of the biofilm during an emersion and the total biomass). This descriptive survey also allowed to show the complexity of the "formation /dispersion" pattern of the biofilm during an emersion, and pointed out the fact that the biofilm tends to remain at the sediment surface all over the course of the emersion period. In the second part of this study, the aim was to assess the photosynthetic performance of in vitro biofilms. Pulse Amplitude Modulated (PAM) fluorometry proved to be a perfect tool to measure the photosynthetic activity of microalgae biofilms, especially by Rapid Light Curves (RLC) despite the complexity of the photobiological response which, in this instance, is a double function of light and time. RLC allowed to show that an increase in the size of the biofilm (the thickness) induces an under-estimation of the "true" physiological response of epipelic microalgae, but an increase of the resistance capacity of the biofilm as a whole to strong illuminations. These results clearly suggest a change in optical properties of the biofilm as a function of its size, and lead to an improvement of our representation of the biofilm during periods of photosynthetic activity., Le biofilm microalgal, qui se forme à la surface du sédiment, est l'entité de production primaire des vasières intertidales dépourvues de macrophytes. Dans l'optique d'une meilleure connaissance du fonctionnement de ce système de production, le biofilm a été étudié par deux approches complémentaires afin de déterminer ses propriétés structurelles et fonctionnelles à micro-échelle. Dans une première partie, l'étude descriptive a permis de caractériser le biofilm dans les dimensions verticale, horizontale et temporelle. Bien que le biofilm forme un compartiment différencié à la surface de la vase, doté de sa propre dynamique, il conserve un lien fonctionnel avec la biomasse disponible dans le 1er cm de sédiment, qui prend la forme d'une relation linéaire entre la taille maximum du biofilm au cours d'une émersion et la biomasse totale. Cette étude descriptive a également permis d'illustrer la complexité du schéma de formation / dispersion du biofilm au cours d'une émersion, notamment en mettant en évidence le maintien du biofilm à la surface du sédiment jusqu'à l'immersion. Dans une deuxième partie, l'étude fonctionnelle s'est attachée à sonder les performances photosynthétiques du biofilm in vitro. La fluorimétrie PAM (Pulse Amplitude Modulated) s'est avérée un outil tout à fait adapté à la mesure de l'activité photosynthétique de biofilms de microalgues, notamment par la méthode « Rapid Light Curve » (RLC), dont l'interprétation s'avère assez complexe du fait que la réponse photobiologique mesurée est une double fonction de la lumière et du temps. La RLC a permis de mettre en évidence que la structure même du biofilm induisait une sous-estimation de la réponse photosynthétique et une augmentation de la capacité de résistance à des éclairements intenses en fonction de la taille du biofilm. Ces résultats suggèrent une modification des propriétés optiques globales du biofilm en fonction de sa taille et permettent d'améliorer notre représentation fonctionnelle du biofilm

Many scientists consider that the top centimetre of the sediment on intertidal mudflats contains the photosynthetic competent biomass (PCB). Part of this biomass migrates upward to the surface of the sediment during diurnal emersion periods to form a temporary biofilm: the photosynthetic active biomass (PAB). The present study tests the hypothesis that the size of the biofilm (PAB) is functionally dependent on PCB. Therefore, we have plotted PAB as a function of PCB for a range of different environmental conditions: 3 seasons and 5 tidal situations, representing a total of 600 cores. This investigation points out that, in spring and autumn, there was a simple linear relationship between PAB and PCB, thus indicating that the maximum size of the biofilm represents a constant fraction (between 13 and 18 %) of the available biomass in the top centimetre of the mud. The relationship was more complex in winter, but the general trend remained the same. The dependence of the size of the biofilm on PCB might have important ecological implications in terms of primary productivity and also the monitoring of intertidal microalgal biomass at large spatial scales.

The spatio-temporal dynamics of the microphytobenthic biofilm was analysed at microscale over a complete day-time emersion period in an intertidal mudflat. The structures of the biofilm were monitored by analysing the variability of the epipelic diatom cover in the horizontal dimension and the kinetics of biomass dispersion in the vertical dimension. Colonisation of the sediment surface by diatoms was rapid (within 15 min); 75% of the biomass contained in the top 1 mm was concentrated in the upper 200 mum, but the cover never reached 100%. This biomass had a higher chlorophyll a/pheopigment ratio than deeper in the sediment, suggesting a better physiological state and a higher photosynthetic potential. The dynamics of the biomass in the upper 200 mum turned out to be independent of that deeper in the sediment, supporting the concept of an independent surface compartment, mainly responsible for primary production. In the second part of emersion, the horizontal dispersion of the microphytobenthic biofilm was concomitant with the onset of downward migration. The process was slow, showing that the diatoms were not phased in their downward movement. The migration process appeared to be asymmetrical: rapid upward migration and biofilm formation versus slow downward movement and dispersion of the biofilm. To take into account these structural characteristics of the biofilm and to simulate its dynamics, we developed a new mathematical model that provides simulations consistent with our observations. We have also shown for the first time that the constitution of the biofilm at the sediment surface (0 to 200 mum) is connected to an increase of biomass in the top 1 cm.