Sargassum mat or plastic windrow? Reading the spectral difference
If you've pulled up a true-color satellite scene and squinted at a pale brown streak wondering whether it's a sargassum raft or a plastic windrow, you're not missing something obvious. In visible bands, they can look almost identical. Both collect along the same surface convergence lines, both show up as thin, curling filaments against darker open water, and both get wider and more tangled near river mouths and current fronts. The real tell sits in spectral bands most people never open, not in the shape your eyes pick out.
Why true color fools you
Sargassum and plastic debris end up in the same place for the same physical reason: Langmuir circulation and current shear push floating material into long, narrow lines regardless of what that material is. A windrow of bottle caps and fishing line gets herded exactly like a windrow of brown algae. So if you're scanning RGB composites looking for a visual tell, a straight or serpentine line, you'll flag both. Field teams who've gone out on a sargassum call and come back with a net full of microplastic fragments (or the reverse) know this already. The line itself tells you where the water converges, not what's floating in it.
What the spectral bands actually show
This is where multispectral data earns its keep. Sargassum is a photosynthesizing organism. It absorbs strongly in the blue and red bands for chlorophyll-a and reflects hard in the near-infrared, the same red-edge jump you'd expect from a floating leaf canopy. That NIR bump is the signature behind the Floating Algae Index and similar vegetation-style indices, and it's consistent and strong even at coarse resolution.
Plastic debris doesn't carry a chlorophyll signature. Depending on polymer type, weathering, and whether it's fouled with biofilm, it can read anywhere from spectrally flat to faintly bright across the visible bands, but it will not produce the sharp NIR reflectance jump that live algae does. At wide-swath resolution you're not resolving individual bottles or nets. You're looking for a bright or anomalous streak sitting along a convergence line that does not carry the biological signature. If the NIR response is muted or absent where you'd expect it for a "plant" target, that line is a stronger candidate for debris than for sargassum, biofouling aside.
Shape, timing, and the context check
Spectral signature is the primary tell, but a few secondary cues help narrow it down before anyone commits a vessel. Sargassum mats tend to be patchier and wider, with scalloped edges where smaller rafts have merged, and they track with known bloom windows. If you're in the Caribbean or the Gulf during peak sargassum season, a chlorophyll-positive streak in that belt is doing exactly what sargassum does every year. Plastic windrows are typically thinner and more linear, hew tightly to the current front with less width variation, and don't follow a seasonal bloom calendar. They spike instead after storm runoff, river discharge events, or near shipping lanes and port approaches.
None of these cues work alone. A patchy shape near a river mouth after heavy rain could be either sargassum pulled off a reef or a storm-flushed debris field, and only the band math separates the two reliably. That's also why distinguishing biological rafts from debris lines on a single pass is a read, not a certainty. Cloud cover, sun glint, and partial pixel mixing at 10-30 m GSD all blur the signal, and ground truth from a planned detour or a reconnaissance pass still matters before a cleanup vessel changes course.
What this kind of event-driven read is actually good for is narrowing the search before fuel and crew hours go into a dead end. Instead of chasing every brown streak a true-color scene throws up, a team can prioritize the ones whose spectral profile lines up with debris rather than algae, and treat the rest as sargassum until proven otherwise.
If your team is trying to work out which windrows are worth a detour this week, Plastic Detection builds an early-access map of where those debris signatures are aggregating, ahead of the vessel.