Abstract
Sun-induced fluorescence (SIF) retrieved from satellite measurements has been widely used as proxy for chlorophyll-a concentration and as indicator of phytoplankton physiological status in oceans. The practical use of this naturally occurring light signal in environmental research is, however, under-exploited, particularly in research focusing on optically complex waters such as inland and coastal waters. In this study, we investigated methodological and knowledge gaps in remote sensing of chlorophyll-a SIF in optically complex waters by reviewing the theory behind SIF occurrence, the availability of existing and upcoming instrumentation, the availability of SIF retrieval schemes, and the applications for aquatic research.Starting with an overview of factors that influence SIF leaving the water body, we further investigated available and upcoming observational capacity by in situ, airborne and satellite sensors. We discuss requirements for spatial, spectral, temporal, and radiometric resolution of observing systems in the context of SIF dynamics. We assessed viable retrieval techniques able to disentangle SIF from non-SIF contribution to the upwelling radiance, ranging from the established multispectral Fluorescence Line Height algorithm (FLH) approach to hyperspectral approaches including model inversion, spectral fitting methods and machine learning regression procedures. Finally, we provide an overview of applications, which could potentially benefit from improved SIF emission estimates such as biomass estimation, algal bloom investigation and primary productivity modelling.
Highlights
Sun-induced fluorescence (SIF) emission from chlorophyll a pigments in phytoplankton is a convenient reporter signal of phyto plankton biomass and primary productivity in aquatic environments (Falkowski and Kiefer, 1985)
Studies from past decades enhanced our understanding on the dynamics of phytoplankton SIF and demonstrated the possible use of SIF as an optical indicator of ecological status in aquatic environments
We propose to invest in the development of new SIF retrieval schemes that are optimized for the rich information content provided by new sensor data
Summary
Sun-induced fluorescence (SIF) emission from chlorophyll a (chl a) pigments in phytoplankton is a convenient reporter signal of phyto plankton biomass and primary productivity in aquatic environments (Falkowski and Kiefer, 1985). Chl-a fluorescence emission occurs with a quantum yield that does not exceed few percent of the absorbed light (Zhou et al, 2008). This emitted signal is characterized by an emission peak at 685 nm (λem) (IOCCG, 2000) that adds to other upwelling radiance components, resulting in a total top of atmosphere radiance signal (LTOA(λem)) when detected by a satellite sensor: LTOA(λem) = La(λem) + Le(λem) + Lr(λem) + SIF(λem)
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