Intercomparison of Ocean Color Algorithms for Picophytoplankton Carbon in the Ocean

• Published in: Frontiers in Marine Science Vol. 4
• Main Authors: Martínez-Vicente, Víctor, Evers-King, Hayley, Roy, Shovonlal, Kostadinov, Tihomir S., Tarran, Glen A., Graff, Jason R., Brewin, Robert J. W., Dall'Olmo, Giorgio, Jackson, Tom, Hickman, Anna E., Röttgers, Rüdiger, Krasemann, Hajo, Marañón, Emilio, Platt, Trevor, Sathyendranath, Shubha
• Format: Journal Article
• Language: English
• Published: Lausanne Frontiers Research Foundation 11-12-2017; Frontiers Media S.A
• Subjects: Algorithms; Carbon; carbon-to-chlorophyll; Chlorophyll; Chlorophylls; Climate change; Color; Data; Datasets; Flow cytometry; Geography; Intercomparison; Laboratories; ocean color remote sensing; Ocean colour; optical water class; Phytoplankton; phytoplankton carbon; picophytoplankton; Plankton; Remote sensing; Satellites; United Kingdom > UK; English Channel; United States > US
• ISSN: 2296-7745; 2296-7745
• DOI: 10.3389/fmars.2017.00378

The differences among phytoplankton carbon ($C_{phy}$) predictions from six ocean colour algorithms are investigated by comparison with \textit{in situ} estimates of phytoplankton carbon. The common satellite data used as input for the algorithms is the Ocean Colour Climate Change Initiative merged product. The matching \textit{in situ} data are derived from flow cytometric cell counts and per-cell carbon estimates for different types of pico-phytoplankton. This combination of satellite and \textit{in situ} data provides a relatively large matching dataset (N$>$500), which is independent from most of the algorithms tested and spans almost two orders of magnitude in $C_{phy}$. Results show that not a single algorithm outperforms any of the other when using all matching data. Concentrating on the oligotrophic regions ($B$ \textless 0.15 mg\,Chl\,m$^{-3}$), where flow cytometric analysis captures most of the phytoplankton biomass, reveals significant differences in algorithm performance. The bias ranges from -35\% to +150\% and RMSD (unbiased) from 5 to 10 mg\,C\,m$^{-3}$ among algorithms, with chlorophyll-based algorithms performing better than the rest. The backscattering-based algorithms produce different reasults at the clearest waters and these differences are discussed in terms of the different algorithms used for $b_{bp}$ retrieval.