Interactive effect of urbanization and flood in modulating microplastic pollution in rivers.

Freshwater ecosystems play an important role in transporting and accumulating microplastics. Spatial and temporal variability in microplastic pollution can create critical spots and moments of elevated pollution, however, the consequences of their interaction are still poorly understood. This study aimed to assess the interaction between urbanization and flood episodes on river microplastic pollution. The water surface was sampled in two sites of the Garonne River, upstream and downstream a large urban area, during two flood episodes. Samples were chemically digested to facilitate particles isolation, and microplastics (700 µm-5 mm) were characterized through infrared spectroscopy (ATR-FTIR). Microplastic concentration increased by 5-8 fold during flood episodes, driven by river discharge. This increase was more significant in the downstream site. During the flood, there was an overall increase of larger particles on water surface, but only in the downstream site microplastic colours and polymeric compositions significantly varied. Principal component analysis of infrared spectra from polyethylene microplastics revealed that the main variance in the spectral region corresponded to hydroxyl and carbonyl groups. The carbonyl content in microplastics was significantly higher for particles collected during the flood, likely indicating a higher level of degradation. Urbanization modulates freshwater microplastic pollution during floods, and changes in microplastic physicochemical profile should be further integrated within toxicity studies to evaluate risks potentially elevated to animal and human health.

Protocol for microplastic pollution monitoring in freshwater ecosystems: Towards a high-throughput sample processing - MICROPLASTREAM.

Robust and reproducible quantification of microplastic pollution in freshwater ecosystems requires the processing of a large amount of samples collected in varying environmental conditions. Such samples are characterized by a high amount of organic matter compared to microplastics and are highly variable in terms of the quantity and the composition of matrices, requiring a standardized analytical protocol for sample treatment and analysis. However, two important and time-consuming steps for microplastic recovery are the elimination of organic matter and microscopic inspection of samples. Here, we developed and validated a protocol, targeting particles with length ranging from 700 µm to 5 mm, that includes a double-step digestion of organic matter, consisting of incubation with potassium hydroxide followed by hydrogen peroxide solutions, and two stereomicroscopic analyses. In addition, we developed several technical improvements allowing reducing the time needed to process samples, such as the design of an adapted filter-cap to improve the content transfer. The absence of physical and chemical alterations in the investigated microplastic pellets and the average reduction of 65.8% (± 9.59 SD) of organic matter in real samples demonstrated that our protocol is fit for purpose. We recommend a second stereomicroscopic analysis to avoid underestimating microplastic concentration and particle size distribution biased towards larger particles. When used for a large-scale monitoring of microplastic pollution, this protocol resulted in an estimated time of 38 h for one person for the treatment of a batch of 24 samples, allowing a higher throughput sample processing and reproducible quantification. • Protocol customization towards high-throughput sample processing • Double step digestion to improve organic matter elimination • Importance of stereomicroscopic analysis for microplastic recovery.

Urbanization and hydrological conditions drive the spatial and temporal variability of microplastic pollution in the Garonne River.

Microplastic (MP) pollution represents a novel environmental pressure acting on freshwater ecosystems. Improving our understanding of the dynamics of MP pollution in freshwater ecosystems is therefore a prerequisite for managing and limiting this pollution. In this study, we quantified the spatial and temporal variability of MP (size range 700 µm - 5 mm) pollution in surface water in 14 sites located across the Garonne river catchment (Southwestern France, 6 in the main river and 8 tributaries). MP concentration averaged 0.15 particles.m-3 (± 0.46 SD) and strongly varied both in space and in time. We found that the spatial variation in MP concentration was driven by urbanization and that the temporal variation in MP concentration and MP size was driven by hydrological conditions, with higher concentrations and smaller particles sizes in warm seasons with low discharge. Polyethylene (44.5%), polystyrene (30.1%) and polypropylene (18.2%) were the main polymers and their proportion did not vary significantly across sampled sites. Particle color was associated with polymer type, with a high proportion of white particles in polystyrene. We also found a significant and negative relationship between MP size and the distance to the source in sites located in the main stream. MP pollution across watershed, from headwater tributaries to lowland rivers, is dynamic, and further studies are needed to improve the resolution of our knowledge of spatial and temporal patterns of MP pollution in freshwater ecosystems.

Stable Isotope Insights into Microplastic Contamination within Freshwater Food Webs.

Microplastic pollution and ingestion are ubiquitous phenomena in freshwater ecosystems. However, our understanding of the role of trophic niche in microplastic ingestion is still limited. Here, we quantified the level of microplastic (700 µm to 5 mm) contamination for macroinvertebrates and fish within the Garonne river. We then used stable isotope analyses (δ13C and δ15N) to quantify trophic niches. We first demonstrated that the abundance of ingested microplastics differed between macroinvertebrates and fish and was not significantly related to microplastic pollution. We then found that microplastic characteristics (shape, color, size, and polymer composition) differ between the abiotic (surface waters and sediments) and biotic (ingested by macroinvertebrates and fish) compartments. The abundance of ingested microplastics increased with the size of organisms in both fish and macroinvertebrates and tended to increase with trophic position in macroinvertebrates only. Finally, the origin of the resources consumed by fish significantly affected the abundance of microplastics ingested. Altogether, these results suggest the absence of microplastic bioaccumulation in freshwater food webs and the dominance of direct consumption, most likely accidentally. The use of stable isotope analyses is therefore crucial to improve our understanding of microplastic ingestion by wild organisms.