Abstract
In this study we develop an analytical model for spectral backscattering and ocean colour remote sensing of blooms of the calcifying phytoplankton species Emiliania huxleyi. Blooms of this coccolithophore species are ubiquitous and particularly intense in temperate and subpolar ocean waters. We first present significant improvements to our previous analytical light backscattering model for E. huxleyi coccoliths and coccospheres by accounting for the elliptical shape of coccoliths and the multi-layered coccosphere architecture observed on detailed imagery of E. huxleyi liths and coccospheres. Our new model also includes a size distribution function that closely matches measured E. huxleyi size distributions. The model for spectral backscattering is then implemented in an analytical radiative transfer model to evaluate the variability of spectral remote sensing reflectance with respect to changes in the size distribution of the coccoliths and during a hypothetical E. huxleyi bloom decay event in which coccospheres shed their liths. Our modeled remote sensing reflectance spectra reproduced well the bright milky turquoise colouring of the open ocean typically associated with the final stages of E. huxleyi blooms, with peak reflectance at a wavelength of 0.49 μm. Our results also show that the magnitude of backscattering from coccoliths when attached to or freed from the coccosphere does not differ much, contrary to what is commonly assumed, and that the spectral shape of backscattering is mainly controlled by the size and morphology of the coccoliths, suggesting that they may be estimated from spectral backscattering.
Highlights
Coccolithophores are phytoplankton that form an exoskeleton of calcite scales called coccoliths (Figure 1)
Using our new coccolithophore and coccolith backscattering model and an algebraic radiative transfer model we investigate the variability in spectral backscattering and ocean color remote sensing reflectance (i) due to changes in E. huxleyi coccolith size distribution and morphology, (ii) due to changes in the ratio of free to attached coccoliths, and (iii) during a hypothetical bloom decay event in which E. huxleyi sheds its attached liths
Accounting for the elliptical shape of coccoliths using the equivalent radius aeq: aeq = a0b0 = 1.25 × 1.25/1.2 ≈ 1.14 we find that aeq/rc is 0.52, which brings the coccolithophore model of Zhai et al (2013) even closer to the experimental architecture of E. huxleyi coccospheres and liths observed by Hoffmann et al (2015)
Summary
Coccolithophores are phytoplankton that form an exoskeleton of calcite scales called coccoliths (Figure 1). They are major oceanic calcite producers and carbon exporters in the ocean (IglesiasRodríguez et al, 2002; Broecker and Clark, 2009). The backscattering of light by coccoliths and coccospheres forms the basis of the remote sensing algorithm for the retrieval of the concentration of particulate inorganic carbon, PIC (see Table 1 for a list of symbols and abbreviations), or calcite (Gordon et al, 2001; Balch et al, 2005). Sensed PIC has been widely used to study coccolithophore bloom extent, occurrence, and timing in the global ocean (Balch et al, 2011; Hopkins et al, 2015), as well as their poleward expansion in response to climate change (Winter et al, 2014; Neukermans et al, 2018)
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