The intensification of human activities all around the globe has led to the spread of micropollutants in high-mountain freshwater environments. We therefore aimed to assess the geospatial distribution and determine the potential sources of (total-) mercury (THg) and microplastics (MPs) in mountain freshwater ecosystems. To do so, we analyzed THg and MP concentrations in brown trout, biofilm, and sediments from lotic and lentic ecosystems in the Pyrenees - all subjected to different types of human pressure. Additionally, we assessed the potential impacts of these pollutants on fish, and explored the bioindication capacity of brown trout (Salmo trutta fario) and biofilm regarding THg and MP pollution. For the first time, we measured concentrations of MPs trapped in the matrix of freshwater biofilm. Our results suggest that THg in the Pyrenees might be explained by both legacy (regional) and distant sources, in combination with environmental characteristics such as the presence of peatlands or streamwater physicochemistry, while MPs in fish are linked to recent local pollution sources such as single-use plastics. In contrast, MPs in biofilm matrix and sediments indicate a combination of distant (i.e., atmospheric deposition) and recent local pollution sources. Moreover, hydrodynamics and plastic density likely control MP distribution in rivers. Based on Fulton's condition factor, we also found that higher THg concentrations caused a negative impact on fish health (K < 1), while no impact of MPs could be seen. Therefore, we suggest that brown trout and biofilm can serve as bioindicators of atmospheric deposition of THg in high-altitude lakes and that biofilm is a reliable bioindicator to assess MP pollution in remote environments. Brown trout may also act as a bioindicator of MP pollution, but only efficiently in more polluted areas.

Plastics, especially microplastics (<5 mm in length), are anthropogenic polymer particles that have been detected in almost all environments. Microplastics are extremely persistent pollutants and act as long-lasting reactive surfaces for additives, organic matter, and toxic substances. Biofilms are microbial assemblages that act as a sink for particulate matter, including microplastics. They are ubiquitous in freshwater ecosystems and provide key services that promote biodiversity and help sustain ecosystem function. Here, we provide a conceptual framework to describe the transient storage of microplastics in fluvial biofilm and develop hypotheses to help explain how microplastics and biofilms interact in fluvial ecosystems. We identify lines of future research that need to be addressed to better manage microplastics and biofilms, including how the sorption and desorption of environmental contaminants in microplastics affect biofilms and how microbial exchange between microplastics and the biofilm matrix affects biofilm characteristics like antibiotic resistance, speciation, biodiversity, species composition, and function. We also address the uptake mechanisms of microplastics by consumers and their propagation through the food web.

Plastic pollution is a worldwide environmental issue, which affects all natural habitats, even in the most pristine areas. Recent works suggest that rivers may act as temporary sinks for land-based plastic pollution while also serving as transport “highways” for plastic waste to the ocean. Until now though, studies on airborne microplastics (MPs) have mainly been focused on urban areas, with much less attention given towards remote and pristine systems. Further, studies have also stated that atmospheric circulation is responsible for the re-emission and return of MP particles from marine environments back to terrestrial habitats. Here, we present the results derived from the cooperation between the PLASTICOPYR and ATMO-PLASTIC projects, both focused on the impact of plastic pollution in the Pyrenees under different levels of human activity. The PLASTICOPYR and ATMO-PLASTIC projects aim to quantify MPs in different river habitats and the emission of MPs to the atmospheric compartment, respectively. Both projects aim to illustrate that MP pollution is not restricted uniquely to urban sites. The main goal of this collaboration is to acquire a holistic understanding of the link between human activity and plastic pollution in fluvial systems and the atmospheric compartment, in order to identify the main sources of MP pollution in pristine areas of the Pyrenees, including the site of Bernadouze OHM Vicdessos. Our results will allow us to identify the most impacted areas, and potential MP pollution hotspots, which in turn will further our understanding of plastic dynamics between rivers and the atmosphere.