## 3 Results
### Cipigs from Satellite Remote Sensing
#### 3.2.2 Global Distribution of Cpigs  
As an example, MuPI was applied to MERIS L3 global annual (2007) average \(R_{\rm rs}(\lambda)\) to obtain the global distribution of the five different pigment concentrations (Figures 7 and 8) and their ratios to [Chl-a] (Figure 9). The global patterns of [Chl-a], [Chl-b], [Chl-c], [PFC], and [PSC] mimic the major gyre systems and other large-scale circulation features of the world ocean. High values of Cpigs are found within regions of persistent large-scale upwelling (e.g., subarctic gyres, equatorial divergences, eastern boundary currents, etc. ), while low values are observed where large-scale downwelling is observed (e.g., subtropical gyres). The basin-scale [Chl-a] distribution follows the pattern reported in the literature [4; 5]. In comparing the global map of [Chl-a] derived in this study with the standard product from NASA ocean color website (Figure 7), some differences were noticed. In the previous section, when using HPLC data from BATS for validation, the retrieved [Chl-a] showed higher accuracy via MuPI than that from the standard OC4E algorithm. To better understand the differences in [Chl-a] distribution at the global scale, a validation dataset from SeaBASS was used. This dataset was obtained by searching match-ups of in situ measured [Chl-a] with those from MERIS \(R_{\rm rs}(\lambda)\). A dataset containing 608 pairs of [Chl-a] and MERIS \(R_{\rm rs}(\lambda)\) were obtained in which [Chl-a] concentrations ranged from \(\sim\)0.017 to \(\sim\)40.3 mg\(\cdot\)m\({}^{-3}\) (locations shown in Figure 7A). In comparing the estimated [Chl-a] from two different methods with in situ measurements (Figure 7C), MuPI and OC4E showed comparable results with mean UAPD of 48.8% and RMSE of 4.51 mg\(\cdot\)m\({}^{-3}\) for OC4E and mean UAPD of 49.3% and RMSE of 4.05 mg\(\cdot\)m\({}^{-3}\) for MuPI. As shown in Figure 7C, the [Chl-a] estimated from OC4E is biased slightly high (\(\sim\)10%) in the range of 1-10 mg\(\cdot\)m\({}^{-3}\) compared with results from MuPI. This range of [Chl-a] (1-10 mg\(\cdot\)m\({}^{-3}\)) is mainly from coastal and inland waters for which the influences from colored dissolved and detrital matter result in lower accuracy in band-ratio estimated [Chl-a] [80]. For several samples, the [Chl-a] values from MuPI are biased low (\(\sim\)70%) as shown in Figure 7C. There are two possible reasons for this: (1) bad input MERIS \(R_{\rm rs}(\lambda)\), not only the values but also the spectral shape, especially at the blue bands that are susceptible to poor atmospheric correction, where negative values are often observed for coastal waters [81]; (2) the limitation of the algorithm as a result of the empirical parameters used to reduce the unknowns in MuPI, and the low contribution of \(a_{\rm ph}(\lambda)\) to the total absorption coefficients in the 400-750 nm range, which will be further discussed in Section 4. The pigment ratios to [Chl-a] showed complicated patterns (Figure 9). In high [Chl-a] regions, [Chl-b]/[Chl-a] and [PFC]/[Chl-a] are low. In some low [Chl-a] regions, the ratios of [Chl-c]/[Chl-a] and [PSC]/[Chl-a] are relatively high, such as in the East Pacific Ocean. These results agree with previous findings about the global distribution of phytoplankton groups and pigment ratios, as lower [PFC]/[Chl-a] ratios correspond to high [Chl-a] and larger particle size [9; 17; 82; 83; 84]. As recorded in  
Figure 6: **A**: Time series of phytoplankton pigment to chlorophyll \(a\) (Chl-a) ratios at BATS from HPLC measurements. **B**: Chl-b, Chl-c, PPC and PSC to Chl-a ratios from HPLC versus from MERIS measured \(R_{\rm rs}(\lambda)\) using MuPI. the literature, in different regions of the global ocean, the quality and quantity of light and nutrient, as well as temperature, is highly variable [85]. These highly unpredictable and rapid changes of the environment usually result in phytoplankton taxonomic composition variation (long-term adaptation) or physiological acclimation (short-term acclimation) [24].