The Problem

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).

Figure 1. A) Dynamics of MP along the river continuum. The largest accumulation of MP on the riverbed is expected in depositional zones. At watershed scale, deposition zones increase downstream, and at reach scale, they are expected to increase in habitats of low flow. Smaller MP size is expected downstream where velocity slows down.  B) The range of sizes of MP includes particles small as some viruses and most prokaryotic cells (0.1-5 µm); most eukaryotic cells (10-100 µm); free-living protozoa such as paramecium and amoebas (100-1000 µm) up to the macroscopic aquatic macroinvertebrates.

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).

Figure 2. Fragmentation of big plastic items and aggregation of small particles; microbial colonization of MP in the water column: the plastisphere; deposition of colonized MP on an epilithic biofilm and embedment inside of the EPS matrix.
Figure 3. Picture of a riffle located in the middle part of the Ter River, NE Spain (a); detail of epilithic biofilms (b and c); cobbles before and after biofilm sampling (d); biofilm sample (e); biofilm samples with MP (f, g, h).
Figure 4. Conceptual model of biofilm-mediated transient storage of MP in fluvial systems. MP tendency to sink and to be trapped within the biofilm matrix will vary depending on the density, size, shape microbial colonization of the particles and hydraulics that occur at the micro-scale level, while their retention will be affected by biofilm characteristics. MP degradation in the biofilm could occur by chemical, physical and biological degradation. Finally, MP can be remobilized by several detachment processes (scouring, biofilm self-detachment by senescence, or grazing by primary consumers). A reciprocal interaction between MP pollution and the structure and function of biofilms is therefore expected. Source: Guasch et al. in press.

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.

Modified form Guasch et al., in press.


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