4. a space-borne LIght Detection And Ranging (LIDAR) for improved atmospheric correction and oceanographic measurements, as is also planned for ACE (NASA, 2007).
Both the NASA Ocean Biology and Biochemistry (OBB) program and the National Research Council’s (NRC) decadal survey plans call for a mix of ocean color satellite mission types that would help ocean scientists answer the high-level science questions they now face.
Global Hyperspectral Imaging Radiometer
Answering the first two priority science questions above will require the development of advanced global remote sensing capabilities. The planned NASA missions PACE/ACE will provide some of these new capabilities, including:
• Ultraviolet (UV) bands to improve the separation of chlorophyll and color dissolved organic matter (CDOM) absorption and thus significantly improved accuracy of both products. This capability is especially important because of projected changes in the ocean due to rising temperatures and ocean acidification.
• Short wave infrared (SWIR) bands (1,200-1,700 nm) demonstrated by Moderate Resolution Imaging Spectroradiometer (MODIS), which in that case led to improved atmospheric correction over turbid coastal waters, in comparison to what was achieved with Sea-viewing Wide Field-of-view Sensor (SeaWiFS).
• Additional bands in the UV that would help correct for absorbing aerosols, a major source of uncertainty for the present generation of ocean color sensors particularly in coastal waters, and a specific UV band at 317.5 nm that would provide simultaneous ozone corrections.
• Improved atmospheric correction by determining aerosol altitude and type using a profiling LIDAR, advanced polarimeter, or both as envisioned for ACE.
With these capabilities, it will be possible to separate phytoplankton functional groups such as carbon exporters (diatoms), nitrogen fixers (Trichodesmium sp.), calcium carbonate producers (coccolithophores), and the microbial loop organisms (Prochlorococcus sp.). It also will be possible to enable derivation and optimization of fluorescence retrievals, which are particularly beneficial in quantifying phytoplankton chlorophyll biomass during phytoplankton blooms and in coastal waters.
Conclusion: Advanced ocean color remote sensing capabilities are central to answering questions related to changing conditions in the marine ecosystem and biogeochemical cycles due to climate change.
Multi-Spectral High Spatial Resolution Imaging
Many coastal applications—such as monitoring for Harmful Algal Blooms (HABs), ecosystem-based fisheries management, and research on benthic habitats including coral reefs and coastal wetlands—require greater spatial resolution and additional spectral bands than are currently available from most satellites to resolve the complex optical signals that coastal waters produce. These measurements historically have been made from airborne sensors, usually flown by airplanes over a particular region. Airborne hyperspectral observations are well suited for routine studies of localized areas (e.g., coral reefs, seagrass beds) and for episodic events (e.g., HABs, oil spills) that require high spatial or spectral resolution, or on-demand repeat times. The technology is well proven for mapping shallow-water bathymetry and bottom type (e.g., Mobley et al., 2005; Dekker et al., in press), mapping and monitoring coral reefs (Hochberg and Atkinson, 2003; Lesser and Mobley, 2007), and detection of oil spills (Lennon et al., 2006). For example, the Airborne Visible and InfraRed Imaging Spectrometer1 (AVIRIS), developed by the Jet Propulsion Laboratory (JPL), made many flights over the Deepwater Horizon BP oil spill site.2 The hyperspectral information enabled researchers to map out the oil spill location and thickness. In addition, JPL is supporting the construction of a portable hyperspectral imager (i.e., the Portable Remote Imaging SpectroMeter [PRISM]3).
Although the capability has been built and demonstrated, past applications of this hyperspectral technology have been limited to short surveys yielding single snapshots of a given coastal region. Routine and sustained surveys of the U.S. coastal waters are not undertaken because it is difficult to find the necessary funding to routinely fly these airborne systems. Other countries, Australia and the People’s Republic of China in particular, have invested heavily in airborne hyperspectral imaging systems and routinely employ them in studies of their coastal and inland waters. The United States also would benefit greatly from dedicated and adequate support for airborne hyperspectral imaging systems that could be used for routine observations of coastal waters or to respond to episodic events as needed.
High spatial resolution, hyperspectral measurements also can be made from satellite missions and were recommended as part of the Decadal Survey (NRC, 2007). Airborne missions that gather such measurements provide them on an intermittent basis. Satellite hyperspectral remote sensing would make these observations routine and allow sustained application of the data for HAB detection, oil spill monitoring, shallow benthic habitat characterization, and other research and research management applications.
2 See http://www.jpl.nasa.gov/news/news.cfm?release=2010-184; accessed February 8, 2011.