The effects of microplastics size and type on entrapment by freshwater macrophytes under vertical and lateral deposition

Marine and freshwater macrophytes are known to filter off microplastics from the water column; however, the effects of microplastic size and type on their retention by different macrophytes species have yet to be investigated. Here we tested the retention of different sizes and types of microplastics, introduced under two flow regimes (vertical deposition in still water and lateral deposition in a unidirectional current), by two submerged macrophyte species, Hydrilla verticillata and Mayaca fluviatilis, using ex situ experiments. Microplastics entrapment efficiency in macrophytes was determined by calibrating the dry weight (DW) of the plant and analysing the characteristics of each macrophyte species via a comparison of their leaf outer-edge perimeter-to-area ratio (P:A). The entrapment efficiency of macrophytes was higher in still water than in moving water. Hydrilla verticillata had greater average leaf surface area and retained the most polyethylene terephthalate (PET) of size 800-1000 μm under both lateral deposition in moving water (1.75±0.11 g) and vertical deposition in still water (2.85±0.24 g). Conversely, M. fluviatilis had greater P:A, surface area, and high surface cellulose and retained the most PET of size 600-800 μm in both moving (0.73±0.07 g) and still (0.92±0.159 g) water. Our findings highlight the influence of microplastic size and material type, macrophyte morphology and surface area, and water flow conditions in determining the entrapment rate of microplastics by macrophytes.