A blue-green ocean signal points to possible microplastic buildup
Satellite observations show simultaneous brightening in blue and green light near the Great Pacific Garbage Patch. A peer-reviewed study found that small suspended plastics could produce this optical pattern. Natural changes remain possible, and the researchers want water samples to test the connection before ocean colour can serve as a reliable measure of microplastic accumulation.
Science··Night
Three satellite observations find a blue-green change
Waters near the Great Pacific Garbage Patch show a simultaneous increase in blue and green reflectance in satellite observations. The patch is a diffuse accumulation of plastic held within the North Pacific’s circulating currents. A peer-reviewed study in Communications Earth & Environment compared the observed colour change with the optical effects that small suspended microplastics could produce. The team includes Shea Cheatham, Daniel Koestner and Børge Hamre at Norway’s University of Bergen.[1], [2]
The researchers examined three separate satellite reflectance products over 20 years, from August 2002 to September 2022. Near the accumulation zone, combined blue and green brightening differed from changes in surrounding waters. The products did not agree across the entire region: atmospheric corrections, the merging of measurements from different sensors and changes in sensor performance can alter the resulting maps.[1]
Plastics scatter light differently from algae
Microplastics absorb relatively little visible light and can scatter it strongly. Phytoplankton instead absorb blue and red wavelengths, while coloured dissolved organic matter absorbs more blue light. Radiative-transfer calculations model how these materials affect light passing through seawater. In simulations, adding microplastics made the observed combination of blue and green brightening more frequent than natural changes alone. Mineral dust can scatter light too, but a nearby station’s measurements showed stable dust deposition over the period studied.[1], [2]
The team also tested changes in natural suspended particles and phytoplankton communities. Some combinations of natural conditions generated a similar colour shift, so plastic remains a possible explanation rather than an established sole driver. The lack of direct measurements of the smallest particles in this area prevents a firm conversion from the satellite signal to a measured plastic concentration.[1]
Water samples are needed to test the optical link
Larger fragments caught in surface nets do not represent all small particles suspended through the water column. Researchers therefore want optical measurements of water samples alongside direct counts of microplastics in the patch. Those observations would test whether the modelled signature tracks real accumulation. Differences between satellite products and uncertainty in the natural optical background remain important for developing pollution monitoring from ocean colour.[1], [2]
