| Summary: | <p>Understanding sources of variability in the optical properties of phytoplankton and how the properties vary over the global ocean is essential in the development of ocean-colour algorithms to map the distribution of phytoplankton functional groups. Using data from oceanographic cruises covering different biogeochemical biomes and provinces over a broad latitudinal gradient of the Atlantic Basin, this study examined the spatial distribution of phytoplankton diagnostic pigments and absorption coefficients and how they varied with ocean observables (temperature and chlorophyll-a concentration) that can be detected from space. It was found that variation in the chlorophyll a-specific absorption coefficient of phytoplankton at 440 nm, 𝑎<sup>&ast;</sup><sub>𝑝ℎ</sub>(440), was influenced significantly by changes in the proportion of the non-photosynthetic pigment (NPP), whereas pigment indices of cell size showed only a weak correlation with 𝑎<sup>&ast;</sup><sub>𝑝ℎ</sub>(440). There were province- and biome-specific changes in the relationships between both temperature and chlorophyll 𝑎 and 𝑎<sup>&ast;</sup><sub>𝑝ℎ</sub>(440). This suggests that when developing remote-sensing algorithms that use temperature or chlorophyll as a predictor of phytoplankton size structure, the underlying physical oceanographic processes that control the biogeochemistry and ecology of the region should be taken into consideration.</p>
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