received as reflection from the water 

 surface. We placed a polarizing filter, 

 oriented at right angles to the major 

 axis of polarization, over the receiving 

 aperture of the spectrometer and tilted 

 the instrument at Brewster's angle (di- 

 rected away from the sun). 



When we operated the spectrometer 

 from our C-54-Q research aircraft, the 

 signal that we wished to measure, 

 namely, the spectrum of the light back- 

 scattered from beneath the sea surface, 

 was sometimes difficult to detect be- 

 cause of interference from "noise" 

 caused not only by surface reflection 

 but also by "air light." Air light is light 

 that has been scattered to the instru- 

 ment by the air and by material in the 

 air between the sea surface and the 

 aircraft. As the altitude of observation 

 increases, the area of the sea from 

 which light can enter the instrument 

 enlarges, reaching the dimensions of 

 about 52 by 9 feet (16 by 3 m) at 1000 

 feet (305 m). Smaller irregularities in 

 surface reflection or in the nature of the 

 seawater will be averaged out. At the 

 same time interference from air light 

 will increase with altitude because of 

 the greater path length through the 

 atmosphere. The curves shown in Fig. 



I were taken at altitudes ranging from 

 600 to 10,000 feet (183 to 3048 m) over 

 an area east of Cape Cod (Station S, 

 Fig. 2), where the water was 200 m 

 deep and the estimated chlorophyll 

 content, although not measured at the 

 time, was probably about 0.6 mg/m''. 

 As altitude increased, the values for 

 upwelling light received increased 

 markedly and regularly in all parts of 

 the spectrum. The remainder of the 

 measurements reported here were made 

 at an altitude of 1000 feet (305 m). 



Representative spectral measure- 

 ments obtained over water with high 

 chlorophyll content (about 4 mg/m''. 

 Buzzards Bay), with low chlorophyll 

 content (about 0.3 mg/m^, north of 

 the Gulf Stream), and with very low 

 chlorophyll content (less than 0.1 mg/ 

 m^, Sargasso Sea) are presented in Fig. 

 3. The values for the backscattered 

 light from these areas have been cal- 

 culated as percentages of the incident 

 light. The curves display characteristic 

 differences in shape. For the water 

 with high chlorophyll content the 

 backscattered light rose from values 

 mostly about 2.2 percent of the inci- 

 dent light in the blue region of the 

 spectrum to about 2.5 percent in the 



Fig. 2. The flight of the aircraft after leaving Nantucket on 27 August 1968 and 

 the location of Stations A to E. Station S was occupied on 26 August. Representative 

 temperatures measured from the aircraft flying at 305 m are shown to the left or below 

 the flight path; representative chlorophyll concentrations in milligrams per cubic meter 

 measured from the surface ship are shown to the right or above the flight path. 



1120 



green, and then dropped to about 0.3 

 percent in the red. For water with low 

 chlorophyll content the values were 

 higher in the blue, dropped rapidly to 

 much lower values in the green, and 

 continued to drop in the red. Where 

 chlorophyll content was very low, the 

 backscattering was higher at all wave- 

 lengths shorter than 500 nm and 

 reached a maximum of 7 percent at 

 400 nm. 



On 27 August 1968 -a more extensive 

 survey of the changes in backscattered 

 light from contrasting bodies of water 

 was conducted during a flight from 

 Buzzards Bay and Nantucket Sound to 

 a point in the Sargasso Sea south of the 

 Gulf Stream, then north on a 556-km 

 transect that crossed successively the 

 Gulf Stream, the slope water, a transi- 

 tion zone, Georges Bank, Georges 

 Shoals, and the southern part of the 

 Gulf of Maine, and returned via Cape 

 Cod Bay (Fig. 2). Records of the spec- 

 trum were taken at frequent intervals 

 with the TRW spectromet,er, and a con- 

 tinuous trace of surface temperature 

 was obtained by P. M. Saunders by 

 means of a Barnes infrared radiometer. 

 A continuous record of the temperature 

 and the chlorophyll concentration of the 

 surface water was obtained from the 

 R.V. Crawford by means of a thermistor 

 and a continuous-flow Turner fluorom- 

 eter (7). Water for this purpose was 

 drawn from an intake valve through 

 the hull of the vessel 2 m below the 

 surface. Analysis of these data shows 

 that the surface temperature and the 

 surface chlorophyll of the slope water, 

 the Bank water, and the Gulf of Maine 

 are statistically differentiated to a highly 

 significant degree. We also have evi- 

 dence from a previous study (S) that 

 surface chlorophyll values may be use- 

 ful as an index of biological productiv- 

 ity. During four cruises in the Atlantic 

 and Pacific, one of us (C.J.L.) collected 

 91 samples, which covered a range of 

 surface chlorophyll concentrations from 

 0.04 to 28.3 mg/m''. Analysis showed 

 highly significant correlations with mea- 

 surements of the total chlorophyll in the 

 euphotic zone and with Xhe primary 

 productivity of the phytoplankton in 

 the waters studied. Temperature values 

 obtained from the aircraft agreed close- 

 ly with values obtained from the ship 

 (see Fig. 2). Owing to the relative 

 sterility of warm Gulf Stream water, the 

 lower chlorophyll measurements tend 

 to be associated with higher sea tem- 

 peratures. 



A comparison of the spectra of the 

 backscattered light as a percentage of 



SCIENCE. VOL. 167 



5-3 



