Light plays an important role in aquatic ecosystems, both marine and freshwater. Penetration of light underwater influences various biogeochemical processes and also influences activities and behavioral patterns of marine organisms. In addition, dissolved and particulate water constituents present in the water column absorb and scatter light, giving water its characteristic color. The concentration or abundance of these constituents, referred to as optically active constituents (OACs) also determine light availability underwater. Thus color being an indicator of water column content, serves as a water quality parameter. Monitoring of the ocean color variables, such as the OAC concentrations and their optical properties, therefore, allows assessment of the health of an ecosystem. Advances in optical methodologies have improved the understanding of our ecosystems through multispectral and hyperspectral in situ measurements and observations. However, the ocean environment is vast and dynamic and so limitations of spatial and temporal coverage have been overcome with satellite remote sensing that provides oceanographers with repeated synoptic coverage. Being recognized as an essential climate variable (ECV) ocean color is monitored as part of the climate change initiative (CCI) of the European Space Agency (ESA). This chapter aims to provide the reader with an overview of the science of ocean color, introducing involved common terminologies and concepts and its global coverage using satellite remote sensing.

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The Total Algae Peak Integration Retrieval TAPIR relates the chlorophyll-a absorption coefficient at 440 nm (a440) to the reflectance peak near 683 nm induced by chlorophyll-a properties. The two-step retrieval provides both the hyperspectral quantification of the phytoplankton fluorescence and scattering and the estimation of a440 from reflectance signals. Integrating the peak, the Total Algae Peak (TAP) accounts for the variance in the peak's magnitude, shape, and central peak wavelength. TAPIR is a solely optical approach estimating a440 and supports the application of retrieval-independent individual regional bio-optical models afterwards to retrieve the chlorophyll-a concentration. Simulations reveal the major sensitivity on the considered model chlorophyll-a absorption spectrum and its single scattering albedo. Additional water and atmosphere constituents have a low impact. An uncertainty assessment reveals uncertainties of less than 30% for TAPIR a440 greater than 0.8 m-1 and less than 38% for lower a440. In optically complex waters, first validation efforts promise the applicability of TAPIR for high chlorophyll-a concentration estimations in the presence of additional water constituents. The technique is generic and considers external conditions (sun zenith angle, number of measurement bands, surface or satellite measurements, and radiometric quantity). TAPIR applies to all kind of waters including optically complex waters, arctic to tropical regions, and inland, coastal, and open ocean waters. Among other hyperspectral satellite sensors, the Environmental Mapping and Analysis Program (EnMAP) provides sufficient sampling bands for the application of TAPIR.

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