Effects of biofouled plastics on phytoplankton community assembling and water chemistry: pilot study and implications for freshwater environments

The presence of plastic litter and microplastics in freshwaters has raised concern about their potential transport and accumulation in water and sediments over time. However, several direct and indirect environmental consequences are still not well understood. In this study, we investigated the role of plastic as a carrier of algal species favouring their dispersal in freshwaters, and we assessed the potential effects on water biodiversity and chemical features in a pilot, laboratory-based study. We simulated an algal community using 5 algal species commonly found in freshwaters and we exposed this community to both pristine and biofouled polypropylene fragments of 5 × 5 × 0.3 mm in size. The biofouled plastic was generated by incubating similar polypropylene fragments with 2 other freshwater algal species, which were different from those in the simulated community. In addition, we evaluated the effect of the dispersal of algae from plastic without the presence of a simulated pelagic community. The experiment lasted 15 days in total. At different time intervals, we assessed: total algal growth and photosynthetic efficiency, algal community composition and the concentration of macronutrients and minor elements in the water. We observed changes in the algal community composition and marked chemical alterations driven by the presence of the biofouled plastics. The presence of pristine plastic, instead, did not show significant changes in the community composition and in the concentration of dissolved elements. The dispersal of algal species from the biofilm on plastic and competitive interactions between the plastic biofilm and the pelagic community were, therefore, likely responsible for the changes in the algal diversity. These results confirmed the hypothesis that plastic can influence dispersal and biodiversity of the algal community. Biofouled plastic from environmental samples and environmental communities selected from natural freshwater bodies will be used in future experiments. This will help to unravel the potential consequences of plastic pollution for ecosystem functioning and microbial biodiversity.