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Effects of drying on plastic fragmentation and microplastic size on the functional role of a shredder organism Gammarus fossarum.
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This study found that drying conditions in intermittent rivers accelerate plastic fragmentation, producing more and smaller microplastics as drying duration increases. Subsequent lab experiments showed these smaller particles were more lethal to the freshwater amphipod Gammarus fossarum, a key shredder organism, suggesting that intermittent rivers and seasonal streams — which make up a large fraction of global waterways — may be underappreciated hotspots of microplastic generation and ecological harm.
Microplastics (MPs), generally defined as plastic particles <5 mm, are raising global concern due to their ubiquitous presence in ecosystems, their potential toxicity to organisms as well as the threat they pose to ecosystem functioning and human health. Rivers have long been considered as pipes transporting MPs towards the ocean. However, recent studies have shown that MPs also impact these ecosystems. MPs can accumulate for long periods in rivers and negatively affect freshwater communities as well as the functioning of these ecosystems. Currently, most studies on the effects of microplastics in rivers are based on perennial rivers (i.e. rivers that never cease to flow), whereas intermittent rivers and ephemeral streams (i.e. watercourses that cease to flow or dry at some point during the year, IRES) have been poorly studied despite their significant contribution to watercourses worldwide. Therefore, the effects of drying on the transport, accumulation and fragmentation of MPs remain unknown. Here, we explored through a first laboratory experiment whether different drying durations had significant influences on plastic fragmentation (i.e. formation of smaller plastic particles). The plastic fragmentation was assessed in mesocosms under controlled conditions, simulating a gradient of drying duration to evaluate their effects on plastic fragmentation. We found that plastic fragmentation increased with drying duration, resulting in an increase in the abundance and size of formed MPs as well as in the mass loss of the initial plastic item. Based on these results, we developed a second laboratory experiment to evaluate the impacts of different sizes and concentrations of MPs on the sentinel species Gammarus fossarum, a common amphipod in temperate European rivers that plays a key functional role as a shredder organism. The effects of a 28 days exposure to MPs on the functional role of Gammarus fossarum were quantified by measuring feeding rate, assimilation efficiency and mortality. We found that MPs concentration had no effect on mortality whereas mortality increased with decreasing MPs size. Surprisingly, the feeding rate and assimilation efficiency were influenced by MPs concentration, while the size of the particles had no direct effect on these variables. Our results suggest that IRES could be considered as hotspot areas of plastic fragmentation and that the subsequently formed MPs could affect river ecosystems functioning through negative effects on key functional organisms.
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