The world's oceans (and its freshwater bodies, as well) are replete with life. Central to the marine foodchain is phytoplankton - microscopic plants that photosynthesize chemicals in sea water. This process depends on the plankton content of chlorophyll-a, a pigment that strongly absorbs red and blue light. As plankton concentrations increase, there is a corresponding rise in spectral radiances, peaking in the green. Upwelling masses of water (usually associated with thermal convection) containing phytoplankton take on green hues in contrast to the deep blues of ocean water with few nutrients. Plankton occur over the entire ocean expanse but tend to concentrate off continental shores because of the abundance of nutrients coming from the land. Here is a map of plankton distribution off the California coast:
Although often subtle in appearance, phytoplankton blooms can be seen in natural color images. This MODIS image of the Arabian Gulf shows a bloom consisting mainly of Noctiluca miliaris, which forms there mainly during winter months.
The Coastal Zone Color Scanner (CZCS) is a sensor specifically developed to study ocean color properties. These properties can be related to organic content, such as plankton, as well as sediment. CZCS launched in October 1978, as part of Nimbus-7's instrument complement and continued to operate until late 1986. It sensed colors in the visible region in four bands, each 0.02 µm in bandwidth, centered at 0.44 (1), 0.52 (2), 0.57 (3), and 0.67 (4) µm. A fifth band between 0.7 and 0.8 µm monitored surface vegetation and band six, at 10.5-12.5 µm sensed sea surface temperatures. Here is the first CZCS before fitting onto the Nimbus spacecraft:
In monitoring ocean color, band 1 (blue) measured chlorophyll absorption; band 2 (green), tracked chlorophyll concentration; band 3 (yellow), was sensitive to yellow pigments ("gelbstoff"); and band 4 (red), reacted to aerosol absorption. With data from those bands, we calculate the chlorophyll variation, which correlates closely with relative abundance of marine phytoplankton, as the ratio of band 1 to band 2 (for phytoplankton concentrations less than 1.5 mg/m3) and band 2 to band 3 (>1.5 mg/m3)A good primer for how to use CZCS and similar data sets is found in this (somewhat outdated) Goddard User's Guide. Additional information is available by clicking on Ocean Color. Some of the more informative CZCS scenes are found in this Goddard review.
The next pair of images are CZCS color composites: the left one emphasizes chlorophyll-enrichment (in reds) in the Georges Bank off the New England coast (BGR = bands 3,2,1) whereas the right one simulates natural ocean color (BGR = bands 1,2,3)
Using data from all relevant bands, here are chlorophyll concentrations spread over the Fall to Spring season in 1978-79 in the Atlantic Ocean:
The distribution of chlorophyll on a global scale averaged between 1978 and 1986 appears here:
We can correlate thermal data from CZCS with chlorophyll, as demonstrated in this plot:
In the color coding, blues correspond to the lowest levels of phytoplankton and reds to the highest. Thermal data as such are not directly contributing to the color composite of this image but notations about warm and cool areas in the water result from thermal data from another satellite. Note the eddies or rings. Phytoplankton tends to concentrate along the edges of warm core rings (which rotate clockwise) but concentrate centrally in cold core rings (counterclockwise motion). The motions in these rings are analogous to circulation around atmospheric high- and low-pressure systems. Warm core rings can extend over several hundred kilometers, as seen in this HCMM Night-IR thermal image (top), which shows such "gyres" in varying stages of development and coherence in Atlantic waters near the Canary Islands off the African coast. Cold core rings are usually less well shaped but are probably displayed in this June 19, 1976, Landsat-1 band 4 (green) image (bottom) of waters off the southwest coast of Iceland; concentrations of phytoplankton are associated in part with the lighter tones that may also be tied into sediment "murkiness".
These observations types - ocean color and thermal patterns - aid in locating conditions where large schools of fish are likely to live, so commercial and sport fishermen actively apply them to locate the best current fisheries.
The follow-on to the CZCS is NASA Goddard's SeaWiFS (Sea-viewing Wide Field-of-View Sensor), launched successfully on August 1, 1997. It is the prime sensor on a commercial satellite, Orbview-2 (originally named SeaStar) operated by the Orbital Imaging (OrbImage) Corporation. Once again, the sensor system monitors ocean color variations, especially those caused by concentrations of plankton and other sealife that strongly moderates chlorophyll response, detectable spectrally. Thus, the prime objectives are: 1) to quantify ocean plankton production; 2) to determine observable couplings of physical/biological processes; and 3) to characterize estuarine and coastal ecosystems. We show the prime sensor here:
The SeaWiFS sensor consists of eight channels at: 412, 443, 490, 510, 555, 670, 765, and 865 nm (nanometers: 1µm = 1,000 nm), each with bandwidths of 20 or 40 nm. The instrument can swing ±58° off nadir. From an orbital altitude of 705 km (438 mi), spatial resolution in the Local Area Coverage (LAC) mode is about 1.1 km (0.68 mi) (the optimal resolution is 0.6 km at nadir), and in the Global Area Coverage (GAC) mode, it is 4 km (2.5 mi). Swath widths are: LAC = 2,801 km, and GAC = 1,502 km.
Like CZCS, SeaWiFS produces regional scale images in which eddies and circulation patterns are evident. In this view of the western North American continent, marine eddies have formed off the British Columbia coast around Queen Charlotte; to the west is still another eddy-like pattern made by clouds.
One job that SeaWiFS does well is to monitor sedimentation patterns. Below is an image of the western Gulf of Mexico, showing sedimentation either in brown or in green (much lower concentrations of sediments, but enough to modify water color to lighter tones). (Tie this image with the one four illustrations down.)
But the main job of SeaWiFS is to monitor organic activity in the oceans. Here is a characteristic regional map, showing SeaWiFS-derived chlorophyll content for the globe using data recovered from September 4 through November 20, 1997, using the same color coding as CZCS:
The next two images show (top) chlorophyll mapped off the eastern U.S. coast on September 30, 1997, and (bottom) ocean color off southern Florida on September 25, with greens denoting high phytoplankton concentrations.
Chlorophyll response is also strong in floating algae. The Red Tide is a much dreaded form of algae that secrets and expels a poison that can be fatal to fish and shellfish such as oysters. This SeaWiFS image of the northeastern Gulf of Mexico shows (in light blue-green) a nearshore concentration of this algal bloom on March 1, 1999:
Another bout with the Red Tide occurred north of the Florida Keys in 2002, as seen in this SeaWiFs image that renders the red algae in almost natural color;
In Winter of 2002, another algal bloom of a different nature affected the eastern Gulf of Mexico north of the Florida Keys. The seawater became black for a while, then reverted to a brownish-green. The bloom extended over a larger area than customary. From a boat, the water indeed appeared black, which occurs when the concentration of both bloom and dead organics becomes high. Fish within the bloom were not killed but nevertheless left the area until the algae died off. Here is a SeaWifS image of the bloom at its height:
The Black Tide condition was bad in late 2004 extending into 2005, Both SeaWiFS and MODIS were effective in monitoring this situation:
Over large areas, patterns of chlorophyll concentrations can be quite eye-catching. The next two SeaWiFS images are a natural color and a chlorophyll map of the southern Atlantic Ocean of the Brazilian and Uruguayan coasts; the Malvinas current forms a narrow line running N-S offshore.
Plankton concentration (as revealed by its chlorophyll signature) is very sensitive to water temperatures. Thus, significant temperature changes in tropical waters associated with the El Niño event during the late 1990s can be the controlling factor in plankton distribution, as indicated by these two SeaWIFS images in May of 1998, in which plankton appear in profusion (expanded right inset) as temperatures drop in the area it represents:
The MODIS instrument on Aqua picked out a large phytoplankton bloom off the northern coast of Norway on July 19, 2003. Good news for Norwegian fishermen! Most of the blue-green color in the (near true color) image below was found to be due to Coccolithophore plankton whose carapaces (shells) are composed of chalky CaCO3.
We can also use the sensor data from SeaWiFS to map land surfaces on a local to global scale. We showed a global, true-color map of Earth near the end of the Introduction Section. This next SeaWIFS image depicts still another use for the meteorological/oceanographic class of satellites by monitoring a dust storm coming off the northwest Sahara desert of Africa as it blows out to sea:
ADEOS, that ill-fated but most promising multisensor satellite, had two instruments that could perform ocean color and phytoplankton assessments. First below is a January 1997 global image made by the OCTS (Ocean Color and Temperature Scanner) and beneath it is a April 1997 view of the Mediterranean made by POLDER.
As seen in several images elsewhere in the Tutorial, sensors operating mainly in the 0.4 to 0.7 µm spectral interval are capable of monitoring the distribution of sediments (current-driven) in circulating or even in standing water. The density of sediment in the waters very near the surface can be quantitatively assessed. In practice, on-site measurements can provide calibration points to determine densities. Here is an example of the determination of variations in sediment density in the San Francisco Bay and off the coast of the Peninsula there, as measured by Terra's ASTER (see also page 16-10 for complementary images).
The higher densities in the northern S.F. Bay and adjacent San Pablo Bay are due to sediment being carried into these waters from the Sacramento River.