Fluorescence Component in the Reflectance Spectra from Coastal Waters. II. Performance of retrieval algorithms

Abstract

Retrieval of chlorophyll fluorescence magnitude using Fluorescence Height algorithms in coastal waters is more complicated than in the open ocean because of the strong deviations of elastic reflectance within the fluorescence band from the derived fluorescence baseline. We use results of our recently established relationship between fluorescence magnitude and concentrations of water constituents together with extensive HYDROLIGHT simulations, field and satellite data to analyze the performance and retrieval limitations of MODIS and MERIS FLH algorithms in the variety of coastal waters and to examine improvements for spectral band selection suitable for future sensors.

A Comparison between Local and Global Spaceborne Chlorophyll Indices in the St. Lawrence Estuary

“…The linear behavior between chl and FLH CM variations seemed to contradict empirical and theoretical parameterizations reported by other studies [9,20,33]. However, we attribute this deviation to the lack of 'packaging effect' and re-absorption of photons as a function of chl in the phytoplankton culture experiments.…”

“…Moreover and over detritus-rich waters, most of the reflected energy in the spectral range 670-690 nm is associated to elastic scattering contributions coming from sediments and/or organic non-living particulates. Thus, as TSM becomes enriched in detritus, nFLH is not longer a good indicator of chl [33] and an uncoupling between nFLH and TSM is anticipated resulting in m's that are near zero or negative.…”

“…Conversely, the leveling off of nFLH at higher TSM values could be interpreted as an increase in detritus mass with respect to microalgae (e.g., MTZ and north shore). The aforementioned impact of detritus on chlorophyll fluorescence peak and FLH has been investigated by Gilerson et al [9,33] who showed that chl explains one-third of FLH variability when concentrations of non-algal particulates (NAP) are relatively low (<2 g·m −3 ). However, FLH may not be an accurate proxy for chl when NAP concentrations are above 2 g·m −3 and becomes an useless index at NAP concentrations above 10 g·m −3 due to the overwhelming contribution of elastic scattering (>95%) with respect to fluorescence.…”

Evaluation of MODIS-Aqua Atmospheric Correction and Chlorophyll Products of Western North American Coastal Waters Based on 13 Years of Data

“…Further analysis is required with a larger in situ dataset to explore this method, given that the fluorescence signal can vary not only with chla concentration, but also due to photo-inhibition, phytoplankton species, and cell physiology [85]. In addition, effective fluorescence light height retrieval can be difficult in turbid coastal waters with TSM exceeding 5 mg m −3 [86,87] where NIR elastic scattering confounds the signal [88]. These are typical concentrations in the Salish Sea during spring/summer [16,39].…”

Hourly turbidity monitoring using Geostationary Ocean Color Imager fluorescence bands

“…Even though the FLH has many advantages over the bluegreen band ratio algorithms, it breaks down in highly scattering waters, where high red peak values in the reflectance are primarily due to contributions from elastic scattering modulated by chlorophyll absorption rather than the fluorescence, thus falsely indicating possible chlorophyll rich areas. [48][49][50] In contrast, the RBD technique easily differentiates between the two effects, giving positive values under true bloom conditions and negative values in highly scattering waters. 3 The RBD approach is for high chlorophyll concentration (>1 mg∕m 3 ) waters and it depends on the chlorophyll fluorescence quantum yield and the backscattering properties of the particles in the water.…”