The absorption of light by algal pigments determines the cellular absorption of phytoplankton and thus contributes to the in situ optical signatures of coastal and offshore waters. This is the basis of a range of bio-optical approaches used for monitoring phytoplankton distribution (taxa and biomass) and is the focus of this chapter. Details regarding bio-optical signatures of phytoplankton and their spectral absorption, scattering and fluorescence characteristics are covered in Chapter 13, this volume. Details of how phytoplankton adjust their pigments in response to variation in light climate are reviewed in Chapter 11, this volume.
Phytoplankton blooms cover spatial scales that vary from patches of 1 m2 to large blooms covering more than 1 × 106 km2 (Franks, 1997; Smyth et al., 2004; Schofield et al., 2008). Related to the spatial scale is the temporal variability of these blooms (from minutes to years), depending on the physical and biological processes at a given location. The development of techniques for monitoring phytoplankton blooms at different geographical and temporal scales has evolved rapidly in recent years (Kahru and Brown, 1997; Schofield et al., 1999; Babin et al., 2008). Because of the wide range in scale, different methods and approaches (different sensors and corresponding platforms) are needed for monitoring water masses and associated phytoplankton blooms as a function of environmental change (i.e. temperature, salinity, circulation and light regime), biogeochemical cycling, eutrophication, ocean acidification and pollution. Globally, monitoring of phytoplankton bloom dynamics is important to estimate changes in primary productivity affecting carbon and nutrient cycling in the world oceans.