First attempt to measure macroplastic fragmentation in rivers.

Direct field measurements of macroplastic fragmentation during its transport in rivers are currently unavailable, and there is no established method to perform them. Previous studies have showed that macroplastic fragmentation results in the production of harmful microplastics, and river channels can be hotspots for this process. Therefore, obtaining information about this process is crucial for quantifying the production of secondary microplastics in rivers and assessing the related risks for riverine biota and human health. Here, we propose a simple low-cost methodology for quantifying riverine macroplastic fragmentation by conducting repeated measurements of the mass of tagged macroplastic items before and after their transport in the river. As a proof-of-concept for this method, we conducted a 52-65 day experiment that allowed us to measure a median fragmentation rate of 0.044 ± 0.012 g for 1-liter PET bottles during their transport at low to medium flow in the middle mountain Skawa River in the Polish Carpathians. Using the obtained data (n = 42), we extrapolated that during low to medium flows, the median yearly mass loss of PET bottles in the study section is 0.26 ± 0.012 g/year (0.78 ± 0.036 % of bottle mass), and the median rate of bottle surface degradation is 3.13 ± 0.14 µm/year. These estimates suggest a relatively high fragmentation rate for a PET bottle in a mountain river even under low to medium flow conditions without high-energy transport. We discuss how our simple and relatively low-cost methodology can be flexibly adapted and future optimized to quantify macroplastic fragmentation in various types of rivers and their compartments, informing future mitigation efforts about the rates of formation and dispersion of secondary microplastics.

Macroplastic fragmentation in rivers.

The process of macroplastic (>0.5 cm) fragmentation results in the production of smaller plastic particles, which threaten biota and human health and are difficult to remove from the environment. The global coverage and long retention times of macroplastic waste in fluvial systems (ranging from years to centuries) create long-lasting and widespread potential for its fragmentation and the production of secondary micro- and nanoplastics. However, the pathways and rates of this process are mostly unknown and existing experimental data not fully informative, which constitutes a fundamental knowledge gap in our understanding of macroplastic fate in rivers and the transfer of produced microparticles throughout the environment. Here we present a conceptual framework which identifies two types of riverine macroplastic fragmentation controls: intrinsic (resulting from plastic item properties) and extrinsic (resulting from river characteristics and climate). First, based on the existing literature, we identify the intrinsic properties of macroplastic items that make them particularly prone to fragmentation (e.g., film shape, low polymer resistance, previous weathering). Second, we formulate a conceptual model showing how extrinsic controls can modulate the intensity of macroplastic fragmentation in perennial and intermittent rivers. Using this model, we hypothesize that the inundated parts of perennial river channels-as specific zones exposed to the constant transfer of water and sediments-provide particular conditions that accelerate the physical fragmentation of macroplastics resulting from their mechanical interactions with water, sediments, and riverbeds. The unvegetated areas in the non-inundated parts of perennial river channels provide conditions for biochemical fragmentation via photo-oxidation. In intermittent rivers, the whole channel zone is hypothesized to favor both the physical and biochemical fragmentation of macroplastics, with the dominance of the mechanical type during the periods with water flow. Our conceptualization aims to support future experimental and modelling works quantifying plastic footprint of different macroplastic waste in different types of rivers.

The unknown fate of macroplastic in mountain rivers.

Mountain rivers are typically seen as relatively pristine ecosystems, supporting numerous goods (e.g., water resources) for human populations living not only in the mountain regions but also downstream from them. However recent evidence suggests that mountain river valleys in populated areas can be substantially polluted by macroplastic (plastic item >25 mm). It is unknown how distinct characteristics of mountain rivers modulate macroplastic routes through them, which makes planning effective mitigation strategies difficult. To stimulate future works on this gap, we present a conceptual model of macroplastic transport pathways through mountain river. Based on this model, we formulate four hypotheses on macroplastic input, transport and mechanical degradation in mountain rivers. Then, we propose designs of field experiments that allow each hypothesis to be tested. We hypothesize that some natural characteristics of mountain river catchments can accelerate the input of improperly disposed macroplastic waste from the slope to the river. Further, we hypothesize that specific hydromorphological characteristics of mountain rivers (e.g., high flow velocity) accelerate the downstream transport rate of macroplastic and together with the presence of shallow water and coarse bed sediments it can accelerate mechanical degradation of macroplastic in river channels, accelerating secondary microplastic production. The above suggests that mountain rivers in populated areas can act as microplastic factories, which are able to produce more microplastic from the same amount of macroplastic waste inputted into them (in comparison to lowland rivers that have a different hydromorphology). The produced risks can not only affect mountain rivers but can also be transported downstream. The challenge for the future is how to manage the hypothesized risks, especially in mountain areas particularly exposed to plastic pollution due to waste management deficiencies, high tourism pressure, poor ecological awareness of the population and lack of uniform regional and global regulations for the problem.

Mountains of plastic: Mismanaged plastic waste along the Carpathian watercourses.

Plastic waste poses numerous risks to mountain river ecosystems due to their high biodiversity and specific physical characteristics. Here, we provide a baseline assessment for future evaluation of such risks in the Carpathians, one of the most biodiverse mountain ranges in East-Central Europe. We used high-resolution river network and mismanaged plastic waste (MPW) databases to map MPW along the 175,675 km of watercourses draining this ecoregion. We explored MPW levels as a function of altitude, stream order, river basin, country, and type of nature conservation in a given area. The Carpathian watercourses below 750 m a.s.l. (142,282 km, 81 % of the stream lengths) are identified as significantly affected by MPW. Most MPW hotspots (>409.7 t/yr/km2) occur along rivers in Romania (6568 km; 56.6 % of all hotspot lengths), Hungary (2679 km; 23.1 %), and Ukraine (1914 km; 16.5 %). The majority of the river sections flowing through the areas with negligible MPW (< 1 t/yr/km2) occur in Romania (31,855 km; 47.8 %), Slovakia (14,577 km; 21.9 %), and Ukraine (7492; 11.2 %). The Carpathian watercourses flowing through the areas protected at national level (3988 km; 2.3 % of all watercourses studied) have significantly higher MPW values (median = 7.7 t/yr/km2) than those protected at regional (51,800 km; 29.5 %) (median MPW = 1.25 t/yrkm2) and international levels (66 km; 0.04 %) (median MPW = 0 t/yr/km2). Rivers within the Black Sea basin (88.3 % of all studied watercourses) have significantly higher MPW (median = 5.1 t/yr/km2, 90th percentile = 381.1 t/yr/km2) than those within the Baltic Sea basin (median = 6.5 t/yr/km2, 90th percentile = 84.8 t/yr/km2) (11.1 % of all studied watercourses). Our study indicates the locations and extent of riverine MPW hotspots in the Carpathian Ecoregion, which can support future collaborations between scientists, engineers, governments, and citizens to better manage plastic pollution in this region.

First insight into the macroplastic storage in a mountain river: The role of in-river vegetation cover, wood jams and channel morphology.

Macroplastic storage in mountain rivers remains unexplored and it is unknown how river morphology and different surface types of river areas modulate this process. Therefore, we sampled macroplastic debris stored on the surface of emergent river areas with different vegetation cover and on wood jams in a channelized, single-thread reach and an unmanaged, multi-thread reach of the Dunajec River in the Polish Carpathians. Total amounts of macroplastic debris retained in these reaches were then estimated on the basis of mean mass of macroplastic deposited on unit area of each surface type and the area of this surface type in a given reach. Exposed river sediments and areas covered with herbaceous vegetation stored significantly lower amounts of macroplastic debris (0.6 and 0.9 g per 1 m2 on average) than wooded islands and wood jams (respectively 6 g and 113 g per 1 m2). The amounts of macroplastic debris stored on wood jams exceeded 19, 129 and 180 times those found on wooded islands, areas covered with herbaceous vegetation and exposed river sediments. Wooded islands and wood jams covering 16.7% and 1.5% of the multi-thread reach stored 43.8% and 41.1%, respectively, of the total amount of macroplastic stored in that reach, whereas these surface types were practically absent in the channelized reach. Consequently, the unmanaged, multi-thread reach, 2.4 times wider than the neighbouring channelized reach, stored 36 times greater amount of macroplastic per 1 km of river length. Our study demonstrated that the storage of macroplastic debris in a mountain river is controlled by channel management style and resultant river morphology, which modulate river hydrodynamics and a longitudinal pattern of the zones of transport and retention of macroplastic conveyed by river flow.

Changes in the hydrodynamics of a mountain river induced by dam reservoir backwater.

Upstream from a dam reservoir, river hydrodynamics may be directly changed by temporary inundation driven by the reservoir. This triggers morphological river changes which may additionally modify the initial hydrodynamics, even at the time when backwater inundation does not occur (indirect effects of backwater). We verified these hypotheses, applying two-dimensional hydraulic modelling of flood flows to a section of the mountainous Dunajec River upstream from the Czorsztyn Reservoir. The modelling was performed for small, medium and large floods, and hydraulic conditions were compared between the scenarios with lacking and maximum backwater inundation and between the river reaches subjected to backwater inundation and unaffected by backwater fluctuations. Direct effects of reservoir level fluctuations were limited to the reach subjected to backwater inundation during floods and comprised: significantly increased water depth and decreased flow velocity and bed shear stress in the channel and on the floodplain, as well as a re-established hydrological connectivity between the channel and floodplain during small and medium floods. Indirect effects of backwater inundation reflected channel widening and bed aggradation that triggered a positive feedback with changes in hydrodynamics, mostly by reducing the velocity of flood flows in the channel zone. These latter changes occurred on a longer distance upstream from the reservoir than the backwater reach itself, and they modified the river hydrodynamics even when backwater inundation did not occur. We propose a conceptual model which indicates that changes of mountain rivers upstream from dam reservoirs are driven by modified hydrodynamics and lead to different morphological adjustments than those induced by waters underloaded with sediment downstream from dams. Changes in hydrodynamics and the associated morphological and sedimentary adjustments of mountain rivers recorded upstream from dam reservoirs may locally mitigate impacts of channelization and channel incision on riverine and riparian ecosystems of these rivers.

Dam reservoir backwater as a field-scale laboratory of human-induced changes in river biogeomorphology: A review focused on gravel-bed rivers.

Only in the years 2007-2016 about 8000 large dams were constructed all over the world, adding to >50,000 previously built dams. These structures disturb abiotic and biotic components of rivers, but to date the knowledge of their impacts has been mainly derived from observations of downstream river reaches. Upstream from dams, however, backwater fluctuations induce sediment deposition, cause more frequent and higher valley-floor inundation, increase groundwater level, and change channel morphology and riparian vegetation. Little is known on the effects of these disturbances on the river biogeomorphological processes. In this review I synthesized knowledge on backwater effects on rivers into a model of backwater-induced abiotic-biotic interactions in the fluvial system. This model is next used to propose new hypotheses and research tasks concerning the biogeomorphology of gravel-bed rivers in the temperate climatic zone. Implications for flow-sediment-morphology-vegetation interactions and feedbacks are conceptualized in a river cross-section based on recent biogeomorphological insights and methodological approaches allowing to explore them in future studies. The model highlights that backwater-induced changes in abiotic and biotic components of river system trigger further feedbacks between them that additionally influence these components even without a direct backwater influence. Backwater-induced changes in hydrodynamics and sediment transport favour seed germination and growth of plants and decrease their mortality during floods, but also eliminate plants intolerant to prolonged inundation and intensive fine sediment deposition. These impacts may change the biogeomorphical structure of river system by modifying trajectories of biogeomorphic succession cycles and related zones of vegetation-hydromorphology interactions in the river corridor. Specifically, backwater effects may promote the development of more stable channel morphology and a less diverse mosaic of riparian vegetation and animals habitats, contrasting with those occurring in free-flowing rivers of the temperate zone.

Island development in a mountain river subjected to passive restoration: The Raba River, Polish Carpathians.

In the twentieth century the heavily channelized Raba River incised deeply in its mountain course. Abandonment of channelization structures in a 2.3-km-long reach within a forested corridor was followed by considerable channel widening during floods of 30- and 35-year recurrence interval, re-establishment of a multi-thread channel pattern and island development. Morphological and botanical surveys were conducted annually between 2011 and 2017 to determine the processes and patterns governing development of islands and their floristic complexity. Hydraulic conditions promoting establishment and persistence of islands were determined with one-dimensional hydraulic modelling of flood flows for 8 unmanaged river cross-sections with islands and 8 cross-sections in the adjacent channelized reaches. Average age, number of islands and their average and total area in the reach markedly increased over the study period. However, the increase was not steady but moderated by island erosion by flood flows, island establishment shortly after major floods and island coalescence in the years without such floods. Hydraulic modelling indicated that river cross-sections with islands are typified by significantly lower values of mean water depth, flow velocity, unit stream power and bed shear stress at flood flows than cross-sections in the adjacent, channelized reaches. Such conditions promote deposition of living driftwood on channel bars, initiating island development, and reduce the probability of erosion of existing islands. The total number of plant species on islands varied highly and either exceeded or was similar to that recorded on riparian forest plots in particular years. This study indicates that (i) island re-establishment in the river was initiated by substantial channel widening, (ii) variation in flood magnitudes exerts a considerable influence on the trajectory of island development, and (iii) the contribution of islands to the overall species richness of plant communities in the river corridor at early stages of island re-establishment may be highly varied.