Q2. What are the future works in this paper?
It is necessary to carry out multiple steps of particle extraction to account for dense particles therefore the suggested protocol for future samples would be to carry out a flotation using a concentrated ZnCl2 solution, followed by a timed manual sort of the remaining sediment to remove any unfloated plastic particles. Further research needs to be done at these locations to include the small scale particles ( < 1 mm ) and also particles within the water column and on the surface. The characteristics and chemical nature of particles found ( e. g. predominantly coloured, angular fragments ) suggest that many of these particles found were locally-derived secondary microplastics rather than primary microplastics from consumer products or secondary artificial fibres introduced by sewage effluent. The abundance of fibres at all sites suggests the influence of sewage effluent, even for the Leach where there is only one upstream STW and negligible effluent input ( Fig. 2 ).
Q3. What was the effective method of particle removal?
The most effective method of particle removal was flotation, which extracted between 51% (The Cut site 1) and 82% (Lambourn) of the total particles removed combining all three steps.
Q4. What is the importance of density of polymers?
The density of polymers is an important consideration given that the particles observed in sediment are likely to be of denser polymers; in flowing waters buoyant particles may have been transported downstream before they could become biofouled and dense enough to sink (Andrady, 2011; Van Cauwenberghe et al., 2013).
Q5. How many sediment samples were collected at each site?
At each site four sediment samples were collected at 1 m intervals along a 3 m transect running parallel to the bank at 1 m distance, therefore giving four replicate samples per site.
Q6. What factors contributed to the relatively high plastic fragment input at The Cut site 1?
Factors contributing to the relatively high plastic fragment input at The Cut site 1 are likely to be the presence of a storm drain immediately upstream from the sampling location carrying local urban runoff to the watercourse and the urban nature of the site, on the outskirts of a large town.
Q7. What was the first step in the sorting process?
The first sorting step was a visual inspection of the entire sample using a binocular lightmicroscope at 6x magnification (Wild Heerbrugg, Switzerland, with Photonic PL2000 cold light source), in order to determine to what extent this step could remove all microplastics and potentially eliminate the necessity for flotation in future analyses.
Q8. What type of polymer was found to be the least abundant?
The types of polymer identified were polyester/polyethylene terephthalate (PET, 14 particles) polypropylene (PP, five particles), polyarylsulphone thermoplastic (five particles), polyethylene (PE, two particles), polystyrene (PS, one particle), and poly vinylchloride (PVC, one particle).
Q9. What is the main reason for sampling in different weather conditions?
Therefore sampling in different weather and seasonal conditions would help develop understanding of the degree to which rivers act as a sink of microplastics and a source to the marine environment.
Q10. What is the software for spectral matching?
This software carries out optimised corrections for spectral matching including interdependent corrections of the baseline, intensity distortion and axis shift with further manual correction possible for noise and baseline correction.
Q11. What is the role of rivers in the study?
This study highlights the importance of rivers as a source of microplastics and other anthropogenic litter to the ocean, but also as a sink for dense plastics and anthropogenic particles with potential for environmental and ecological impacts.
Q12. What is the likely seasonal change in river flow?
It is also highly likely that seasonal changes in river flow will affect the presence and transport of microplastics within riverine systems.
Q13. How many particles were removed in the initial sorting step?
However an initial manual sort by hand and microscope through an amount of dry sediment alone appears to be ineffective, as a maximum of 37% particles were removed in this sorting step.