A depth-invariant index to map floating algae: a conceptual design

The concept of floating algae index (FAI), a measure of the red-edge reflectance of floating vegetation, has been used to map floating macroalgae in global oceans. However, when the algae mats are submerged in water due to winds and other factors, the reduced reflectance (and thus reduced FAI value) is interpreted as being caused by reduced areal density (or biomass density) of algae, leading to underestimated algae amount. Here, a depth invariant FAI is proposed to overcome this difficulty. The depth invariant FAI is formulated using three spectral bands whose selective water attenuations can compensate for the reduced reflectance, resulting in FAI values that are nearly independent of the submerged depth of up to 30 cm with underestimates of the algae amount reduced from >50% to a few percent. In this study, the concept of depth invariant FAI is demonstrated through simulations only. With the new Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) mission and other hyperspectral satellite missions in the near future, such a concept is expected to be tested and evaluated with satellite data, from which the optimal bands can be selected to reduce uncertainties in mapping floating macroalgae.