Detection and Dynamic Imaging of Ocean Microplastics from Space

Most ocean microplastic concentration data come from manta net trawling. Net trawling has a low sample rate that may be underestimating the true plastic concentrations. Plastic concentrations are also not well sampled outside the North Atlantic and North Pacific gyres. Ocean circulation models have been used to estimate the transport of plastics in the ocean and model predictions of their accumulation in ocean gyres have been validated to some degree by net trawl data. However, concentrations of plastic debris in close proximity can vary by orders of magnitude in the span of days, suggesting the presence of complex transport mechanisms and spatiotemporal variability that net trawl sampling and global circulation models may be unable to adequately resolve. Attempts have been made to use spaceborne remote sensing methods to detect areas of high oceanic plastic concentrations. Methods based on hyperspectral imaging are being successfully explored, but their coverage and timeliness are limited. An alternate approach is presented here which provides more global, time resolved, coverage. Spaceborne radar measurements of ocean surface roughness are used to infer the reduction in responsiveness to wind-driven roughening caused by microplastics and surfactant tracers. On a global scale over long time periods, the reduction correlates strongly with the mass density of microplastics near the surface measured by net trawl campaigns and predicted by ocean circulation models. On a global scale on shorter time scales, time lapse images derived from the radar observations reveal seasonal changes in the microplastic mass density which appear to be related to seasonal ocean circulation patterns. On smaller spatial and temporal scales, time lapse images reveal episodic bursts of microplastic outflow from major river discharges into the sea.