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Nutrient enrichment mediates the effect of biodegradable and conventional microplastics on macroinvertebrate communities
Summary
A semi-natural mesocosm experiment compared the effects of biodegradable PLA microplastics and conventional HDPE microplastics on freshwater macroinvertebrate communities, finding that overall community effects were weak but that HDPE reduced invertebrate diversity more than PLA — except when nutrient enrichment was present, which erased this difference. The findings suggest that nutrient pollution interacts with microplastic type in complex ways, and that assessments of bioplastics as \"safer\" alternatives need to account for real-world co-stressors. More large-scale, long-term experiments are needed before drawing firm conclusions.
There is growing concern regarding the lack of evidence on the effects bioplastics may have on natural ecosystems, whilst their production continues to increase as they are considered as a greener alternative to conventional plastics. Most research is limited to investigations of the response of individual taxa under laboratory conditions, with few experiments undertaken at the community or ecosystem scale, either investigating microplastics independently or in combination with other pollutants, such as nutrient enrichment. The aim of this study is to experimentally compare the effects of oil-based (high density polyethylene - HDPE) with those of bio-based biodegradable (polylactic acid - PLA) microplastics and their interaction with nutrient enrichment on freshwater macroinvertebrate communities under seminatural conditions. There were no significant differences in total abundance, alpha and beta diversities, or community composition attributable to the type of microplastics, their concentration, or nutrient enrichment compared with the control. However, there was a significant difference in macroinvertebrate alpha diversity between high concentrations of both microplastic types under ambient nutrient conditions, with lower diversity in communities exposed to HDPE compared with PLA. Nutrient enrichment mediated the effect of microplastic type, such that the diversity of macroinvertebrate communities exposed to HDPE were similar to those communities exposed to PLA. These findings suggest that the effects of microplastic pollution on macroinvertebrate communities are very weak at large-scale settings under seminatural conditions and that these effects might be mediated by the nutrient status of freshwater ecosystems. More research under large-scale, long-term, seminatural settings are needed in order to elucidate the impact of both conventional plastics and bioplastics on natural environments and their interactive effect with other occurring stressors and pollutants.
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