We derive the chlorophyll a concentration (Chla) for three main phytoplankton functional types (PFTs) - diatoms, coccolithophores and cyanobacteria - by combining satellite multispectral-based information, being of a high spatial and temporal resolution, with retrievals based on high resolution of PFT absorption properties derived from hyperspectral measurements. The multispectral-based PFT Chla retrievals are based on a revised version of the empirical OC-PFT algorithm (Hirata et al. 2011) applied to the Ocean Colour Climate Change Initiative (OC-CCI) total Chla product. The PhytoDOAS analytical algorithm (Bracher et al. 2009, Sadeghi et al. 2012) is used with some modifications to derive PFT Chla from SCIAMACHY hyperspectral measurements. To combine synergistically these two PFT products (OC-PFT and PhytoDOAS), an optimal interpolation is performed for each PFT in every OC-PFT sub-pixel within a PhytoDOAS pixel, given its Chla and its a priori error statistics. The synergistic product (SynSenPFT) is presented for the period of August 2002 - March 2012 and evaluated against in situ HPLC pigment data and satellite information on phytoplankton size classes (PSC) (Brewin et al. 2010, Brewin et al. 2015) and the size fraction (Sf) by Ciotti and Bricaud (2006). The most challenging aspects of the SynSenPFT algorithm implementation are discussed. Perspectives on SynSenPFT product improvements and prolongation of the time series over the next decades by adaptation to Sentinel multi- and hyperspectral instruments are highlighted.

Samples for the pigment data set were collected during RV Maria S. Merian cruise MSM9/1 in July/August 2008 in the North Atlantic from Bremen, Germany to St. John's, Newfoundland, Canada (see cruise report Kucera et al. 2014). Sampling was conducted at various stations from a rosette coupled to a conductivity temperature density (CTD) profiler at three to four depths and otherwise sampling regularly every 6 to 8 hours the surface water (approx. 10 m depth) from the ship's moonpool. Water samples were filtered on GF/F filters. Filters were immediately shockfrozen in liquid nitrogen and stored at -80°C until further analysis at the laboratories of the Alfred-Wegener-Institute Helmholtz Centre for Polar and Marine Research (AWI). The composition of pigments that were soluble in organic solvents was analyzed by HPLC following the method by Barlow et al. (1997) and adjusted to our temperature controlled instruments (a Waters 600 controller combined with a Waters 2998 photodiode array detector, a Water717plus auto sampler and a LC Microsorb C8 HPLC column) as detailed in Taylor et al. (2011).