Evidence for the ‘growth-dilution’ of microplastics and microfibers in urban stream fish populations

Here, we assessed patterns of microplastics (MP; particles <5 mm) and microfibers (MF; fibers <5 mm) within benthic macroinvertebrates and fishes collected from a polluted urban stream (10-km segment; n = 5 reaches; 200-m reach lengths). Our first objective was to describe patterns of MPs and MFs for four predominant macroinvertebrate taxa to estimate the quantities of plastics in the prey resource pool. Our second objective was to test whether plastic concentrations (#/g of fillet; wet mass) increase (i.e., bioaccumulation hypothesis) or decrease (i.e., growth-dilution hypothesis) in stream fishes across a gradient of body lengths. Both MF and MP were documented in all invertebrate and fish taxa examined (but not necessarily all individuals), with MF consistently outnumbering MP. Scaling plastic burdens to the area of a stream reach (800 m) revealed that the four predominant benthic invertebrates accounted for an estimated 100,000 MF and 23,120 MPs within the prey resource pool. For stream fishes, we found that most individuals had MPs and MF in their fillets. Furthermore, MP and MF concentrations declined with increasing body length providing evidence for the growth dilution hypothesis. However, we also found that MFs and MPs declined at different rates (i.e., test of heterogeneity of slopes) in one fish species, suggesting additional biological processes may expel plastics as fish grow larger. Overall, our findings revealed that bioaccumulation was not occurring because concentrations did not increase. Instead, growth-dilution appears to be a predominant factor, but our analysis also suggests other biological processes are causing the expulsion of MPs and MFs.