Sampling microfibres at the sea surface: The effects of mesh size, sample volume and water depth

Microfibres are one of the most ubiquitous particulate pollutants, occurring in all environmental compartments. They are often assumed to be microplastics, but include natural as well as synthetic textile fibres and are perhaps best treated as a separate class of pollutants given the challenges they pose in terms of identification and contamination. Microfibres have been largely ignored by traditional methods used to sample floating microplastics at sea, which use 300-500 μm mesh nets that are too coarse to sample most textile fibres. There is thus a need for a consistent set of methods for sampling microfibres in seawater. We processed bulk water samples through 0.7-63 μm filters to collect microfibres in three ocean basins. Fibre density increased as mesh size decreased: 20 μm mesh sampled 41% more fibres than 63 μm, and 0.7 μm filters sampled 44% more fibres than 25 μm mesh, but mesh size (20-63 μm) had little effect on the size of fibres retained. Fibre density decreased with sample volume when processed through larger mesh filters, presumably because more fibres were flushed through the filters. Microfibres averaged 2.5 times more abundant at the sea surface than in water sampled 5 m sub-surface. However, the data were noisy; counts of replicate 10-L samples had low repeatability (0.15-0.36; CV = 56%), suggesting that single samples provide only a rough estimate of microfibre abundance. We propose that sampling for microfibres should use a combination of <1 μm and 20-25 μm filters and process multiple samples to offset high within-site variability in microfibre densities.