Global Marine Biomass from Ocean Color Remote Sensing
The ability to derive global maps of chlorophyll a concentration (milligrams per cubic meter) in the upper ocean from ocean color sensors was a groundbreaking achievement for the oceanographic community (Figure 9.2). This biomass estimate can then be related to primary productivity and the marine carbon cycle. Although clouds prevent ocean color sensors to see the entire ocean surface on each orbital pass, a global picture of the distribution of photosynthetic plant biomass emerges from averaging data over several consecutive days or weeks.
The first ocean color sensor was the Coastal Zone Color Scanner (CZCS), an experimental proof-of-concept mission operating on the Nimbus 7 satellite between 1978 and 1986. The CZCS demonstrated that it is possible to detect subtle changes in the color of the ocean and relate these to the concentration of chlorophyll a, the light-harvesting pigment found in all plants. In particular, chlorophyll a concentrations are quantified by empirical algorithms relating spectral band ratios (blue to green) to the concentration of chlorophyll in the ocean (Clark 1981, Gordon and Morel 1983, O’Reilly et al. 2000). A major requirement is that the spectral radiance measurements made by the satellite be corrected to remove the effect of the atmosphere, which comprises more than 90 percent of the top-of-atmosphere signal. This was a major technological breakthrough after the launch of the CZCS (Gordon et al. 1980). Contrary to its name, the sensor was better at estimating biomass in the open ocean than in the coastal zone. Phytoplankton and dissolved organic matter are the primary sources of optical variability in the open ocean (so-called Case 1 waters [Morel and Prieur 1977, Gordon and Morel 1983, Siegel et al. 2002]), whereas in coastal regions, mixtures of organic and inorganic materials affect the ocean color. The problem of differentiating and quantifying individual constituent concentrations in the coastal ocean remains a challenge today.
The ocean color technology pioneered by the CZCS has since been improved and incorporated into modern space instruments. The first modern global ocean color sensor was Japan’s Ocean Color and Temperature Sensor (OCTS) launched in August 1996 aboard the Advanced Earth Observing Satellite (ADEOS). The U.S. Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) followed in August 1997, shortly after the ADEOS experienced structural damage after only 9 months in orbit. SeaWiFS is owned by Orbital Sciences Corporation, with a guarantee from NASA to buy data for the scientific research community. Ocean color data continue to be acquired by the Moderate Resolution Imaging Spectroradiometers (MODIS) aboard the Terra and Aqua satellites launched in 1999 and 2002, respectively, and by a number of other ocean color instruments operated by other countries (Table 9.1).