Abstract
Over the past years, a large number of new approaches in the domain of ocean-color have been developed, leading to a variety of innovative descriptors for phytoplankton communities. One of these methods, named PHYSAT, currently allows for the qualitative detection of five main phytoplankton groups from ocean-color measurements. Even though PHYSAT products are widely used in various applications and projects, the approach is limited by the fact it identifies only dominant phytoplankton groups. This current limitation is due to the use of biomarker pigment ratios for establishing empirical relationships between \textit{in-situ} information and specific ocean-color radiance anomalies in open ocean waters. However, theoretical explanations of PHYSAT suggests that it could be possible to detect more than dominance cases but move more towards phytoplanktonic assemblage detection. Thus, to evaluate the potential of PHYSAT for the detection of phytoplankton assemblages, we took advantage of the Continuous Plankton Recorder (CPR) survey, collected in both the English Channel and the North Sea. The available CPR dataset contains information on diatom abundance in two large areas of the North Sea for the period 1998-2010. {Using this unique dataset, recurrent diatom assemblages were retrieved based on classification of CPR samples}. Six diatom assemblages were identified \textit{in-situ}, \textcolor{blue}{each having indicators taxa or species}. Once this first step was completed, the \textit{in-situ} analysis was used to empirically associate the diatom assemblages with specific PHYSAT spectral anomalies. This step was facilitated by the use of previous classifications of regional radiance anomalies in terms of shape and amplitude, coupled with phenological tools. Through a matchup exercise, three CPR assemblages were associated \textcolor{blue}{with} specific radiance anomalies. The maps of detection of these specific radiances anomalies are in close agreement with current \textit{in-situ} ecological knowledge.
Highlights
Phytoplankton play a key-role in oceanic biogeochemical cycles (Falkowski, 1994; Beaugrand, 2015)
PHYSAT (Alvain et al, 2005, 2008) is an ocean-color algorithm that has allowed us to identify five phytoplankton groups based on ocean-color radiance anomalies analysis in terms of shape and amplitude
This empirical algorithm was limited to dominant cases due to the use of biomarker pigment ratios in the empirical calibration step
Summary
Phytoplankton play a key-role in oceanic biogeochemical cycles (Falkowski, 1994; Beaugrand, 2015). Phytoplankton species scatter and absorb light differently according to their concentration, pigments composition, size, morphology, intracellular structure, cells arrangement, and associated dissolved organic matter (Morel and Bricaud, 1981; Bricaud and Morel, 1986; Dubelaar et al, 1987; Stramski and Kiefer, 1991; Siegel et al, 2005; Clavano et al, 2007; Boss et al, 2009; Whitmire et al, 2010). Using particulate organic carbon, or chlorophyll-Chromophoric Dissolved Organic Matter (CDOM), remote-sensing products have been released (e.g., Stramski et al, 2008; Duforêt-Gaurier et al, 2010). These oceancolor products provide medium-resolution and synoptic scale data that are required to study the main characteristics of phytoplankton distribution
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