Effects of plastic pollution on leaf litter decomposition in Swedish freshwater ecosystems : a field study addressing recently and globally emerging pollutants

Abstract: Plastic pollution is recognized as an increasing environmental problem, but few studies have addressed impacts on ecosystem functions, and very few have been conducted in the field. This study investigated the effects of polypropylene particles derived from disposable face masks on in-situ leaf litter decomposition processes in freshwater ecosystems. The field experiment followed a modified leaf litter bag protocol in a central-swedish urban pond and focussed on decomposition processes mediated by microbes and meiofauna. The litter bags with a mesh size of approximately 0.5mm in diameter were constructed from cotton and filled with 3.5g alder (Alnus glutinosa L.) leaves. Further, three different material treatments with different types of added material were applied to the litter bags: (1) plastic material was added, (2) saw dust as reference material was added and (3) no material was added (control group). Within the plastic treatment two different plastic particle size treatments (small/microplastic and big/ macroplastic) and two leaching treatments (unleached and pre-leached) were included. The experiment period lasted seven weeks and included five timepoints at which subsamples of litter bags were retrieved. Three main response variables were quantified: (1) ecosystem respiration as an indicator for the metabolic activity of the organism community, (2) leaf mass loss and (3) tensile strength loss of the cotton bags. The latter two variables functioned as indicator for the decomposition potential of organic material in the studied ecosystem. The findings suggest that mixing a material resistant to biodegradation within a leaf litter patch acts as a physical barrier to microbes and thus slows down the decomposition, with this effect overall being stronger for the saw dust reference material than the plastic. The presence of unleached plastic was found to increase ecosystem respiration as well as decomposition of the cotton bags compared to the pre-leached treatment. Being one of very few field experiments this study provides valuable insights on how the pervasive extent of plastic pollution from face masks may affect key functions associated with carbon cycling in freshwaters. The obtained results further illustrate how complex biological stress responses to anthropogenic pollution might be on ecosystem level