Description: The relationship between phytoplankton assemblages and the associated optical properties of the water body is important for the further development of algorithms for large-scale remote sensing of phytoplankton biomass and the identification of phytoplankton functional types (PFTs), which are often representative for different biogeochemical export scenarios. Optical in-situ measurements aid in the identification of phytoplankton groups with differing pigment compositions and are widely used to validate remote sensing data. In this study we present results from an interdisciplinary cruise aboard the RV Polarstern along a north-to-south transect in the eastern Atlantic Ocean in November 2008. Phytoplankton community composition was identified using a broad set of in-situ measurements. Water samples from the surface and the depth of maximum chlorophyll concentration were analyzed by high performance liquid chromatography (HPLC), flow cytometry, spectrophotometry and microscopy. Simultaneously, the above- and underwater light field was measured by a set of high spectral resolution (hyperspectral) radiometers. An unsupervised cluster algorithm applied to the measured parameters allowed us to define bio-optical provinces, which we compared to ecological provinces proposed elsewhere in the literature. As could be expected, picophytoplankton was responsible for most of the variability of PFTs in the eastern Atlantic Ocean. Our bio-optical clusters agreed well with established provinces and thus can be used to classify areas of similar biogeography. This method has the potential to become an automated approach where satellite data could be used to identify shifting boundaries of established ecological provinces or to track exceptions from the rule to improve our understanding of the biogeochemical cycles in the ocean.

Description: The PhytoDOAS algorithm by Bracher et al. (2009), modified by and Sadeghi et al. (2011), enables the concurrent retrieval of global chl-a of phytoplankton groups (diatoms, cyanobacteria, coccolithophores, dinoflagellates) from hyperspectral satellite data, such as measured by SCIAMACHY onboard ENVISAT. For applying the Differential Optical Absorption Spectroscopy (DOAS) fit from 430-530nm the following absorbers are considered in the analysis: atmosphere: O3, O4, NO2, H2Og, Glyoxal, Ring; ocean: inelastic scattering, water, PFTs; The non-differentisl absorption and scattering is approximated with low order polynomial. The global data set of the four phytoplankton groups is avaiable on a monthly resolution for July 2002 until today.

Description: The composition and abundance of algal pigments provide information on phytoplankton community characteristics such as photoacclimation, overall biomass, and taxonomic composition. In particular, pigments play a major role in photoprotection and in the light-driven part of photosynthesis. Most phytoplankton pigments can be measured by High Performance Liquid Chromatography (HPLC) techniques applied to filtered water samples. This method, as well as other laboratory analyses, is time consuming and therefore limits the number of samples that can be processed in a given time. In order to receive information on phytoplankton pigment composition with a higher temporal and spatial resolution, we have developed a method to assess pigment concentrations from continuous optical measurements. The method applies an Empirical Orthogonal Function (EOF) analysis to remote sensing reflectance data derived from ship-based hyper-spectral underwater radiometry and from multispectral satellite data (using the MERIS Polymer product developed by Steinmetz et al. 2011) measured in the Atlantic Ocean. Subsequently we developed multiple linear regression models with measured (collocated) pigment concentrations as the response variable and EOF loadings as predictor variables. The model results show that surface concentrations of a suite of pigments and pigment groups can be well predicted from the ship-based reflectance measurements, even when only a multi-spectral resolution is chosen (i.e. eight bands similar to those used by MERIS). Based on the MERIS reflectance data, concentrations of total and monovinyl chlorophyll a and the groups of photoprotective and photosynthetic carotenoids can be predicted with high quality. As a demonstration of the utility of the approach, the fitted model based on satellite reflectance data as input was applied to one month of MERIS Polymer data to predict the concentration of those pigment groups for the whole Eastern Tropical Atlantic area. Bootstrapping explorations of cross-validation error indicate that the method can produce reliable predictions with relatively small data sets (e.g., 〈 50 collocated values of reflectance and pigment concentration). The method allows for the derivation of time series from continuous reflectance data of various pigment groups at various regions, which can be used to study variability and change of phytoplankton composition and photo-physiology.

Description: Quantitative distributions of major functional PFTs of the world ocean improve the understanding of the role of marine phytoplankton in the global marine ecosystem and biogeochemical cycles. Chl-a fluorescence gives insight on the health of phytoplankton and is related to phytoplankton biomass. In this study, global ocean color satellite products of different dominant phytoplankton functional types' (PFTs') biomass and chlorophyll fluorescence retrieved from hyperspectral satellite data using Differential Optical Absorption Spectroscopy (DOAS) are presented. Global biomass distributions from 2002 -2010 of different dominant PFTs (diatoms, cyanobacteria, coccolithophores, dinoflagellates) are derived with PhytoDOAS, the currently specialized method of DOAS for deriving chl-a of PFTs from satellite data of SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric Cartography) on ENVISAT (details in Bracher et al. BG 2009, Sadeghi et al. OSD 2011). Results of the global maps of PFT distribution are validated with global PFT data derived from using Hirata et al. (2011) approach of parameterizing satellite chl-a from Globcolour with collocated HPLC pigment data. In addition, SCIAMACHY monthly mean data of coccolithophore biomass in three selected oceanic regions were compared to related satellite products, including the total surface phytoplankton, i.e., total chlorophyll-a (from GlobColour merged data) and the particulate inorganic carbon (from MODIS-Aqua). The DOAS method was also adapted to detect successfully globally the filling-in of Fraunhofer Lines caused by chlorophyll fluorescence in the backscattered SCIAMACHY spectra and compared to data of fluorescence-line-height from MODIS and MERIS. The results suggest that DOAS is a valid method for retrieving PFTs and chlorophyll fluorescence from hyper-spectral measurements. Overall, DOAS products are much less dependent on a priori information than common multi-spectal ocean colour products which result from empirical and semi-analytical methods. In addition, the DOAS technique has the advantage to overcome problems with an accurate atmospheric correction encountered for the traditional multi-spectral ocean color products because only the differential signals of both atmospheric and oceanic absorbers are fitted while all other (broad-band) absorption and scattering processes are successfully approximated with an low order polynomial. Although current hyperspectral sensors have poor spatial resolution (〉30kmx30km), they are useful for the verification and improvement of the high spatially resolved multi-spectral ocean color products. Future applications of PhytoDOAS retrieval to other hyperspectral sensors and its synergistic use with information gained from multispectral ocean color sensors are proposed.

Description: The extent of light absorption in the near-infrared spectral region (NIR; 700­900 nm) of natural suspended particles was investigated by determining the absorption and mass-specific absorption coefficients of samples from different environments: river, coastal waters, tropical lagoon, and oceanic waters. Large amounts of sample were collected onto glass-fiber filters and measured inside the integrating sphere of a spectrophotometer. The absorption coefficient of particle suspension was also determined for visible wavelengths with a point-source integrating cavity absorption meter. Measurable nonzero particulate absorption in the NIR was determined in all samples, even in algal cultures. It was highest in the river samples (e.g., 1.7 m21 at 850 nm), reaching values similar to the NIR absorption of pure water--a strong NIR absorber. Lowest values were in oligotrophic waters and in algal cultures. Ratios of absorption at 750 nm to absorption at 442 and 672 nm varied between 2% to 30% and 3% to 80%, respectively. Mass-specific absorption in the NIR at 850 nm was also highest in the river (0.012 m2 g21) and lowest in oligotrophic waters (0.002­0.003 m2 g21). The observed NIR absorption can partly be explained by absorption of minerogenic particles, whereas the contribution of organic detritus to the NIR absorption is still mostly unknown.

Description: Gaussian decomposition and Singular Value Decomposition combined with Non-Negative Least Squares (SVD-NNLS) method are compared and combined to estimate the concentration of 18 phytoplankton pigments (names and abbreviations are displayed in Table 2) from phytoplankton absorption spectra. Results show that both methods tend to overestimate pigment concentration. Gaussian decomposition method provides robust estimation of TChl-a, TChl-b, Chl-c1/2, PSC and PPC. The estimates of TChl-a, Fuco, Diato, $\beta$-Caro, Prasino, TChl-b, Zea, Viola and Lut from SVD-NNLS show reasonable estimation accuracy, while the other pigments are subjected to relatively large prediction errors. The estimated pigments concentrations are further exploited based on Diagnostic Pigment Analysis to derive four phytoplankton functional types, i.e. diatoms, prymnesiophytes, green algae and prokaryotes. By the application of these two methods to the particulate absorption spectra collected by underway spectrophotometry during three summer cruises in 2015 - 2017 in the Fram Strait, continuous surface phytoplankton functional types are estimated along the cruise course.