“…Fulfilling this accuracy requirement is however challenged by uncertainties affecting the absolute and vicarious calibration of the space sensors, the atmospheric correction process and the bio-optical characteristics of the ocean (Gregg and Casey, 2004). Furthermore, global empirical algorithms, such as those used to operationally retrieve CHL, are derived from regression analyses of large in situ databases collected from waters around the world (O'Reilly et al, 1998;O'Reilly et al, 2000;Werdell and Bailey, 2005) and therefore have a tendency to perform well only at global scale (Bailey and Werdell, 2006;Bailey et al, 2000;Gregg and Casey, 2004;Hooker and McClain, 2000;O'Reilly et al, 1998). The accuracy limit for chlorophyll has been shown to be unrealistic for many open ocean regions, such as the Baltic Sea (Darecki and Stramski, 2004), the Southern Ocean (Kahru and Mitchell, 2010) and the Mediterranean Sea (Volpe et al, 2007).…”