New algorithms for MODIS sun-induced chlorophyll fluorescence and a comparison with present data products

Abstract

The launch of the MODIS instrument (Esaias et al. 1998), with a waveband dedicated to the measurement of sun-induced chlorophyll fluorescence, has taken remote sensing of marine phytoplankton in a new direction. In theory, it is now possible to obtain a global quasi-synoptic assessment of near-surface fluorescence emission and its quantum yield. While the remote measurement of in vivo fluorescence emitted by phytoplankton is, in principle, straightforward (but see Letelier and Abbott 1996), obtaining an estimate of the quantum yield of fluorescence (the ratio of fluoresced to absorbed photons by phytoplankton) requires a complex algorithm (Abbott and Letelier 1999). Yet, this physiological measurement could foster a major leap in our understanding of the ocean by providing global coverage of a parameter linked to algal physiology (Kiefer and Reynolds 1992) and species composition (Loftus and Seliger 1975;Heaney 1978). In this study, we review the sources of variability of sun-induced fluorescence as they affect the MODIS data products and examine how they relate to the retrieval of the quantum yield of fluorescence and chlorophyll concentration. We propose new algorithms based on semi-empirical relationships from the bio-optical literature and compare our results to MODIS data products.Nature of the MODIS fluorescence measurement-Quantum yield of chlorophyll fluorescence. In this study, we define the quantum yield of chlorophyll a fluorescence in vivo (ϕ, dimensionless; see Table 1 for symbols and definitions) as the ratio of photons fluoresced by chlorophyll a over the whole fluorescence band to the photons absorbed by all cellular pigments. Others have referred to this as the apparent quantum yield of chlorophyll a fluorescence, limiting the term quantum yield of chlorophyll a fluorescence to the ratio of photons fluoresced by chlorophyll a (or by chlorophyll a associated with photosystem II [PSII]) to those absorbed only by photosynthetic pigments associated with PSII (e.g., Gilmore and Govindjee 1999). This distinction is important for the physiological interpretation of remote sensing data and for the comparison with laboratory measurements (see Web Appendix 2). New algorithms for MODIS sun-induced chlorophyll fluorescence and a comparison with present data products AbstractWe discuss important sources of variability in sun-induced chlorophyll fluorescence and examine difficulties in deriving fluorescence data products from satellite imagery, with a focus on the MODerate-resolution Imaging Spectroradiometer (MODIS) sensor. Our results indicate that there are limitations in the present MODIS algorithms that could lead to biases in the interpretation of the fluorescence products across gradients of chlorophyll concentration. To avoid some of these limitations, we suggest replacing the calculation of absorbed radiation by phytoplankton (ARP) over a finite depth with integration over the entire water column, and including a term accounting for cellular reabsorption of fluoresced light. These suggestions are in...

A mesocosm tool to optically study phytoplankton dynamics

“…The aim of this paper is to describe and test an indoor mesocosm facility for the study of phytoplankton dynamics under controlled conditions by using both in situ techniques for algal biomass and physiology estimations, as well as water-leaving radiance measurements. Such a facility would be essential to study the quantitative relations between fluorescence, biomass, and growing conditions to interpret variability in remotely sensed algal fluorescence (Huot et al 2005), and in examining the effect of nutrient perturbations on phytoplankton dynamics and associated optical properties.A widely used proxy for phytoplankton biomass in both in situ and remote-sensing studies is the chlorophyll a concentration. Chlorophyll a (Chl a) is estimated from remotely sensed water-leaving radiance in a number of ways (Huot et al 2005;Kirk 1994;Van Der Woerd and Pasterkamp 2008).…”

“…Such a facility would be essential to study the quantitative relations between fluorescence, biomass, and growing conditions to interpret variability in remotely sensed algal fluorescence (Huot et al 2005), and in examining the effect of nutrient perturbations on phytoplankton dynamics and associated optical properties.…”

“…The aim of this paper is to describe and test an indoor mesocosm facility for the study of phytoplankton dynamics under controlled conditions by using both in situ techniques for algal biomass and physiology estimations, as well as water-leaving radiance measurements. Such a facility would be essential to study the quantitative relations between fluorescence, biomass, and growing conditions to interpret variability in remotely sensed algal fluorescence (Huot et al 2005), and in examining the effect of nutrient perturbations on phytoplankton dynamics and associated optical properties.…”

“…Chlorophyll a (Chl a) is estimated from remotely sensed water-leaving radiance in a number of ways (Huot et al 2005;Kirk 1994;Van Der Woerd and Pasterkamp 2008). First, by measuring the water-leaving radiance absorption spectrum that is influenced by the absorption characteristics of the phytoplankton species, phytoplankton biomass, non-phytoplankton seston, and gilvin (yellow substances) (Kirk 1994 …”

“…Chlorophyll a (Chl a) is estimated from remotely sensed water-leaving radiance in a number of ways (Huot et al 2005;Kirk 1994;Van Der Woerd and Pasterkamp 2008). First, by measuring the water-leaving radiance absorption spectrum that is influenced by the absorption characteristics of the phytoplankton species, phytoplankton biomass, non-phytoplankton seston, and gilvin (yellow substances) (Kirk 1994 dependent on the Chl a concentration, but also on other radiation-absorbing phytoplankton pigments (Huot et al 2005). In water samples, these phytoplankton pigments are accurately measured by high-performance liquid chromatography (HPLC) (Gieskes and Kraay 1986;Zapata et al 2004).…”