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.

Scale-dependent effects of land cover on water physico-chemistry and diatom-based metrics in a major river system, the Adour-Garonne basin (South Western France).

The scale dependence of ecological phenomena remains a central issue in ecology. Particularly in aquatic ecology, the consideration of the accurate spatial scale in assessing the effects of landscape factors on stream condition is critical. In this context, our study aimed at assessing the relationships between multi-spatial scale land cover patterns and a variety of water quality and diatom metrics measured at the stream reach level. This investigation was conducted in a major European river system, the Adour-Garonne river basin, characterized by a wide range of ecological conditions. Redundancy analysis (RDA) and variance partitioning techniques were used to disentangle the different relationships between land cover, water-chemistry and diatom metrics. Our results revealed a top-down "cascade effect" indirectly linking diatom metrics to land cover patterns through water physico-chemistry, which occurred at the largest spatial scales. In general, the strength of the relationships between land cover, physico-chemistry, and diatoms was shown to increase with the spatial scale, from the local to the basin scale, emphasizing the importance of continuous processes of accumulation throughout the river gradient. Unexpectedly, we established that the influence of land cover on the diatom metric was of primary importance both at the basin and local scale, as a result of discontinuous but not necessarily antagonist processes. The most detailed spatial grain of the Corine land cover classification appeared as the most relevant spatial grain to relate land cover to water chemistry and diatoms. Our findings provide suitable information to improve the implementation of effective diatom-based monitoring programs, especially within the scope of the European Water Framework Directive.

Modeling the stream water nitrate dynamics in a 60,000-km2 European catchment, the Garonne, southwest France.

The spatial and temporal dynamics in the stream water NO(3)-N concentrations in a major European river-system, the Garonne (62,700 km(2)), are described and related to variations in climate, land management, and effluent point-sources using multivariate statistics. Building on this, the Hydrologiska Byråns Vattenbalansavdelning (HBV) rainfall-runoff model and the Integrated Catchment Model of Nitrogen (INCA-N) are applied to simulate the observed flow and N dynamics. This is done to help us to understand which factors and processes control the flow and N dynamics in different climate zones and to assess the relative inputs from diffuse and point sources across the catchment. This is the first application of the linked HBV and INCA-N models to a major European river system commensurate with the largest basins to be managed under the Water Framework Directive. The simulations suggest that in the lowlands, seasonal patterns in the stream water NO(3)-N concentrations emerge and are dominated by diffuse agricultural inputs, with an estimated 75% of the river load in the lowlands derived from arable farming. The results confirm earlier European catchment studies. Namely, current semi-distributed catchment-scale dynamic models, which integrate variations in land cover, climate, and a simple representation of the terrestrial and in-stream N cycle, are able to simulate seasonal NO(3)-N patterns at large spatial (>300 km(2)) and temporal (> or = monthly) scales using available national datasets.