The role of benthic biofilms in trapping estuarine microplastics

Benthic biofilms are known for their ability to stabilise and trap fine estuarine sediments and contaminants. Due to their presence on the sediment surface, benthic biofilms interact with emerging contaminants such as microplastics (MPs) that settle to the bed, potentially influencing MP transport dynamics from land to sea. This study explores the influence of benthic biofilm development on the capture and retention of MPs under flow and how other chemical stressors adsorbed to the MPs, such as heavy metals, may influence biofilm retention of MPs. We hypothesised that i) higher biofilm development would increase MP capture and retention under flow and ii) that heavy metals associated with MPs would negatively affect the biofilm community and its ability to retain MPs.Sieved sediment (500µm to remove large fauna) was added to small 250mL chambers and inoculated with 30mL of biofilm-rich surface sediment (control-absent, low and high biofilm biomass). Tidal simulation in an outdoor greenhouse promoted biofilm development in chambers over 21 days. High-density MPs (polyamide) MPs were added to the chambers at ‘high tide’ on day 14. These MPs were mechanically- and UV-aged then exposed to heavy metals (control (no metal), copper (Cu) and lead (Pb)) prior to their use. MPs were also fluorescently stained with a non-toxic dye to aid in MP erosion measurements. At the end of the incubation period, intact inner cores were gently removed, flushed and placed level with the bed of a small benchtop recirculating flume and exposed to incremental increases in flow velocity to erode the MPs. All experimental runs were filmed under UV light to fluoresce MPs and image analysis was used to determine the critical erosion threshold for MP motion from the video footage based on the loss of coverage across the core area. The remaining sediment from the chambers was extracted for biochemical analysis. Significantly higher critical shear stresses were required to remove MPs from the bed when biofilm was present, while Cu and Pb contamination had minimal effects on MP resuspension. This suggests that benthic biofilms have the potential to mediate MP resuspension dynamics and therefore MP transport from land-to-sea. Comparing our MP erosion thresholds (for PA particles only) against the bed shear stresses from a 2-D hydrodynamic model of the macrotidal Humber estuary, UK, it was found that MP erosion thresholds would be exceed in ~76% of the modelled cells during a 10-day simulation period covering a late-neap, spring, early-neap tidal cycle in the absence of biofilms. However, the presence of biofilm reduced the area of critical shear stress exceedance to ~36%. Without biofilms, MP erosion thresholds would be exceeded in 80% of the permanently inundated cells and 11.3% of the intertidal cells. Again, with biofilms present, MP erosion thresholds would be exceeded in 38% of the permanently inundated cells and just 3.8% of the intertidal cells. These findings can help improve our understanding of MP fluxes across the sediment-water interface in estuaries and provides evidence that the role of benthic biofilms should be included when parameterising MP transport models.