Estimate Sargassum wet-weight, nutrients, and pigments from satellite images
Sargassum moves around quite a bit, carried from place to place by winds and currents. Although she is not with us on this cruise (because she just successfully wrapped up her PhD defense!), Mengqiu Wang (USF) provides us with a look at how satellite imagery can help us locate and quantify Sargassum habitats.
Sargassum has distinctive pigment compositions, which control the reflected light intensity at different wavelengths or the “color”. The reflectance properties have been used to develop remote sensing algorithms to detect and quantify Sargassum from satellite images. However, currently we are only able to infer the area coverage or relative abundance, but not the associated wet-weight or biomass, which are very important for understanding their impacts on the ocean’s biology and chemistry.
To derive Sargassum biomass from satellite data, field measurements of the Sargassum density (biomass per area) and the corresponding reflectance become necessary. During previous cruises, we measured the natural density of several individual Sargassum patches and estimated a mean value of ~ 3 kg/m2 by referencing to a 1 m2 quadrat. We also designed experiments to measure the reflectance signals at different densities. These data were applied to build a model to link the remote sensing signal and Sargassum wet-weight. Generally, higher reflectance signal strength would indicate a larger Sargassum biomass density. Besides that, we also measured the nutrients and pigments concentrations per unit wet-weight in the lab with the Sargassum samples collected from the cruises. Using those relationships, we can calculate not only Sargassum wet-weight, but also their nutrients and pigments directly from satellite images.