What do we know about plastic pollution in fluvial ecosystems?
Plastics are one of the most used materials worldwide because of its low cost of production and useful technical characteristics, including elasticity, lightless, resistance to corrosion, and ease of processing. However, plastic residues can cause serious environmental problems due to their low degradability.
These problems will probably be exacerbated in the near future as plastic production has been predicted to further increase (Lau et al. 2020). Plastic litter, especially microplastics (MP) particles (<5 mm) are emerging contaminants of global concern due to their possible interactions with biota, their potentially toxic chemical constituents, and persistence (Hartman et al. 2019).
Plastic pollution has a high prevalence in freshwater ecosystems. The main sources of MP are the runoff from urban, industrial, touristic and agricultural areas, and waste disposal sites such as waste water treatment (WWT) effluents and WWT sludge spreading, as well as atmospheric deposition. Downstream transport is the most important pathway of MP movement from river networks to the sea/ocean (Horton and Dixon 2018).
However, an important fraction of MP is retained instream, where the environmental conditions will establish the physical, chemical and biological interactions that affect their retention, movement and breakdown (Figure 1 A).
Microplastics are a universe of small plastic particles of environmental concern
Microplastics are not made of only one material, but encompass a diverse array of substances that differ not only in its chemical composition and density, but also in size and shape (Figure 1 B). This variability leads to differences in their potential partitioning between the water column and the river bed (i.e. benthic habitats) and the associated environmental impacts.
The most abundant MP are low-density (<1 g/cm3) materials such as polyethylene (PE) and polypropylene (PP), as well as other materials with intermediate density (1 g/cm3), such as polystyrene (PS). Other materials that are also common are polyethylene terephthalate (PET), polyester and polyacrylonitrile (PAN) and polyamide (PA) that have higher density (>1 g/cm3) and are generally more abundant in sediments (Schwarts et al. 2019).
Moreover, all plastic products contain some degree of reactive chemicals. For example, 12.285 compounds have been indicated in food packaging, more than 600 of which are potentially hazardous (Groh et al. 2020).
Some of these MP compounds are receiving increasing attention since they have been classified as carcinogenic, mutagenic, and endocrine disrupting (Erkes-Medrano and Thompson 2018).
How many microplastics will biofilms embed in their extracellular matrix?
Fluvial biofilms are benthic communities composed of bacteria, algae, fungi and meiofauna embedded in a matrix of extracellular polymeric substances (EPS) that develop on any submerged streambed substrata (Mora-Gomez et al. 2016).
This complex set of microbial organisms plays an important role in aquatic ecosystems in primary production, carbon and nutrient cycling (Battin et al. 2003). From headwaters to mid-order streams, microbial communities in biofilms are considered as the main primary producers (Vannote et al. 1980); and thus, biofilms are key basal resources of the trophic food webs in these ecosystems.
Thick biofilms are common on stable substrata in environments with high light and nutrient availability (Romaní 2010). Owing to the thickness and stickiness of the biofilm EPS matrix (Flemming and Wingender 2010), MP transported in the stream water column may be trapped and accumulated within this matrix (Figures 2, 3 and 4).
Research questions
Based on discussions among the components of the consortium, we identified certain critical key questions regarding MP in fluvial ecosystems. We formulate set of research questions still unsolved and needed to bring new insights on the presence, fate and effects of MP in fluvial biofilms and on the potential repercussion on the food web, which will in turn improve management strategies.
References
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