Abstract
Phytoplankton in the ocean are extremely diverse. The abundance of various intracellular pigments are often used to study phytoplankton physiology and ecology, and identify and quantify different phytoplankton groups. In this study, phytoplankton absorption spectra ( a p h ( λ ) ) derived from underway flow-through AC-S measurements in the Fram Strait are combined with phytoplankton pigment measurements analyzed by high-performance liquid chromatography (HPLC) to evaluate the retrieval of various pigment concentrations at high spatial resolution. The performances of two approaches, Gaussian decomposition and the matrix inversion technique are investigated and compared. Our study is the first to apply the matrix inversion technique to underway spectrophotometry data. We find that Gaussian decomposition provides good estimates (median absolute percentage error, MPE 21–34%) of total chlorophyll-a (TChl-a), total chlorophyll-b (TChl-b), the combination of chlorophyll-c1 and -c2 (Chl-c1/2), photoprotective (PPC) and photosynthetic carotenoids (PSC). This method outperformed one of the matrix inversion algorithms, i.e., singular value decomposition combined with non-negative least squares (SVD-NNLS), in retrieving TChl-b, Chl-c1/2, PSC, and PPC. However, SVD-NNLS enables robust retrievals of specific carotenoids (MPE 37–65%), i.e., fucoxanthin, diadinoxanthin and 19 ′ -hexanoyloxyfucoxanthin, which is currently not accomplished by Gaussian decomposition. More robust predictions are obtained using the Gaussian decomposition method when the observed a p h ( λ ) is normalized by the package effect index at 675 nm. The latter is determined as a function of “packaged” a p h ( 675 ) and TChl-a concentration, which shows potential for improving pigment retrieval accuracy by the combined use of a p h ( λ ) and TChl-a concentration data. To generate robust estimation statistics for the matrix inversion technique, we combine leave-one-out cross-validation with data perturbations. We find that both approaches provide useful information on pigment distributions, and hence, phytoplankton community composition indicators, at a spatial resolution much finer than that can be achieved with discrete samples.
Highlights
Phytoplankton account for approximately half of global primary production via photosynthesis [1] and form the base of the marine food web
We demonstrated the retrieval of high spatially resolved phytoplankton pigment concentrations in the Fram Strait and its vicinity from underway hyperspectral aph(λ) (400–700 nm, ~3.5 nm wavelength resolution) by the application of Gaussian decomposition [22] and the matrix inversion technique [27]
For the matrix inversion technique, singular value decomposition (SVD) cannot guarantee the derivation of non-negative pigment-specific absorption spectra, it generates more accurate pigment estimates compared to the non-negative least squares (NNLS) derived spectra or the measured spectra from pigments in solution
Summary
Phytoplankton account for approximately half of global primary production via photosynthesis [1] and form the base of the marine food web. Intracellular pigments of phytoplankton, composed of chlorophylls (a, b and c), carotenoids (carotenes and xanthophylls) and phycobiliproteins (phycoerythrin, phycocyanin and allophycocyanin) [2], play a vital role in photoprotection and the light-driven part of photosynthesis. Some xanthophylls such as violaxanthin (Viola), zeaxanthin (Zea), diadinoxanthin (Diadino) and diatoxanthin (Diato) are involved in the xanthophyll cycle, one of the most important photoprotective mechanisms that drives the non-radiative dissipation of the excess light energy to prevent photoinhibition [4,5]. Their relative abundance can be used as a tracer of photoacclimation processes [5]
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