Watercolors in the Coastal Zone: What Can We See?

Hydrological optics has a rich history, play­ ing a significant role in physical, chemical, and biological oceanography. The success over the last 30 years has provided oceanog­ raphers with a non-invasive means to study regional and global scale physical, chemical, and biological processes (Figure 1). The abil­ ity to map the color of the world’s oceans has been used to estimate global ocean pro­ ductivity (Longhurst et al., 1995; Platt and Sathyendranath, 1988; Sathyendranath et al., 1989; Behrenfeld and Falkowski, 1997), aid in understanding radiant heating processes (Ohlman et al., 2000), assist in delineating oceanic biotic provinces (Longhurst, 1998), and document regional shelf break frontal processes (Ryan et al., 1999a, 1999b). The scientific utility of mapping ocean color led to wide community support that has result­ ed in three generations of satellites launched by the United States, complemented by an international constellation of ocean color satellites from Europe, Japan, China, and India. The utility of remote sensing results from algorithms that use satellite-measured re flectance to estimate the concentration of biogeochemically signifi cant constituents. These algorithms were developed for opti­ cally simple waters where the optical prop­ erties of the ocean are largely defi ned by phytoplankton and water molecules (Fig­ ure 2; see article by Mobley et al., this issue). The spectral properties of water (Figure 2B) and phytoplankton are distinct. Increasing the concentration of phytoplankton (Figure 2C) in a volume of water selectively absorbs blue wavelengths of light, effectively green­ ing the water reflectance in a predictable fashion. This greening allows empirical relationships to be derived that estimate chlorophyll a concentrations from the re flectance ratio of blue-to-green wavelengths of light. Many times, however, the optical signature of the ocean reflects the presence of materials other than phytoplankton and water molecules. The resulting complexity can directly influence the interpretation of what you see using satellite refl ectance sig­ nals. A good example is the usually optically simple, high nutrient-low chlorophyll zones (HNLC). It has been proposed that deposi­ tion of atmospheric dust is a signifi cant fac­ tor regulating overall productivity in HNLC zones (Martin, 1990; Prospero and Nees, 1986). Yet, if present in signifi cant concen­ trations, the optical signature of the dust can compromise the empirical satellite algo­ rithms (Moulin et al., 2001). The presence of significant submicron dust particles, which