400 



450 



650 



500 550 600 



Wavelength (nm) 



Fig. 3. Data from the high and low 

 chlorophyll curves plotted as percentage 

 of the incident light and compared with 

 data taken on the same day from an area 

 with very low chlorophyll concentration 

 south of the Gulf Stream. 



400 450 500 550 600 650 



Wavelength (nm) 

 Fig. 4. Spectra of backscattered light mea- 

 sured from the aircraft at 305 m on 27 

 August 1968 at the following stations (Fig. 

 2) and times (all E.D.T.): Station A, 

 1238 hours; Station B, 1421 hours; Station 

 C, 1428.5 hours; Station D, 1445 hours; 

 Station E, 1315 hours. The spectrometer 

 with polarizing filter was mounted at 53° 

 tilt and directed away from the sun. Con- 

 centrations of chlorophyll a were mea- 

 sured from shipboard as follows: on 27 

 August, Station A, 1238 hours; on 28 

 August, Station B, 0600 hours; Station C, 

 0730 hours: Station D, 1230 hours. 



the incident light at five localities along 

 the flight path of 27 August is presented 

 in Fig. 4. Simultaneous measurements 

 from the aircraft and the ship were 

 made in the slope water at Station B. 

 The ship's observations at Stations C, 

 D, and E were not made until the fol- 

 lowing day, but the range of chloro- 

 phyll values was so great that the dif- 

 ferences among the stations can be 

 relied upon for the present comparison. 

 Time' did not permit the ship to reach 

 the locality of the aircraft's observation 

 at Station A in the Sargasso Sea south 

 of the Gulf Stream, but the chlorophyll 

 content of the water there was almost 

 certainly lower than in the slope water 

 north of the Stream. Along the entire 

 transect the shape of the spectral curves 

 changed progressively as chlorophyll 

 values increased from south to north. 

 The percentage of backscattered light 

 diminished markedly in the blue region 

 and increased relatively in the green 

 region, with an indication of an inflec- 

 tion point at about 515 nm and with 

 little change in the red region. This 

 result agrees satisfactorily with the cal- 

 culated values for the effect of increas- 

 ing amounts of chlorophyll on ocean 

 color presented by Ramsey (9). The 

 change in shape with increasing chloro- 

 phyll is reflected in decreased mean 

 slope of the spectra. Anomalies in the 

 shape and amplitude of these spectra, 

 and of some taken on other occasions, 

 make it evident that other factors play 

 a role that merits further investigation. 

 Our investigation shows that large dif- 

 ferences occur in the spectra of the light 

 backscattered from the ocean and that 

 they can be recorded from aircraft. In 

 the present instance, the slopes of the 

 spectra correlate quite closely with dif- 

 ferences in chlorophyll concentration. 

 The discrepancies are believed to be 

 due to difference in time within paired 

 observations, to differences in surface 

 reflection, to scattered air light, and to 

 the presence in the water of material 

 other than chlorophyll that affected the 

 light selectively. If such interference 

 can be eliminated, or identified and al- 

 lowed for. spectrometric procedures 

 from aircraft (and perhaps from satel- 

 lites) will be of great value in the rapid 

 investigation of oceanic conditions, in- 

 cluding conditions important for biolog- 

 ical productivity. 



George L. Cl-IiRKe 



GiFFORD C. EWING 

 C.\RL J. LORENZEN 



Woods Hole Oceanographic 

 fnsliiulion, Woods Hole, 

 Massachusetts 20543 



References and Notes 



1. G. L. Clarke and E. J. Denton, in The Sea, 

 M. N. Hill, Ed. (Interscience, London, 1962), 

 pp. 456-468; G. L. Clarke. Oceanol. Int. 2, 



38 (1967); , BioScience 18, 965 (1968); 



N. G. Jerlov, Optical Oceanograpliy (Elsevier, 

 Amsterdam, 1968). 



2. J. H. Ryther and C. S. Yentsch, Limnol. 

 Oceanogr. 2, 281 (1957); G. C. Ewing, D. L. 

 Inman, L. N. Liebermann, G. Newmann, D. 

 P. Petersen, W. S. Wooster, C. S. Yentsch 

 (Panel on Oceanography), Useful Apphca- 

 tions of EaTth'Oriemed Satellites (National 

 Academy of Sciences, Washington, D.C., 

 1969). vol. 5, pp. 44-54; J. H. Ryther, Science 

 166. 72 (1969). 



3. R. C. Smith and J. E. Tyler, /. Opt. Soc. 

 Anier. 57, 589 (1967); J. E. Tyler and R. C. 

 Smith, ibid., p. 595. 



4. J. D. H. Strickland, "The estimation of sus- 

 pended matter in sea w.^te^ from the air," in 

 Ms. Rep. Ser. (Oceanogr. Limnol.) No. 88 

 (Fisheiies Research Board of Canada, 1967); 

 G. L. Clarke, G. C. Ewing, A. Conrad, R. 

 M. Alexander. G. Mayer, in Sunwtary' of In- 

 vestigations Conducted in 1967 (Ref. No. 68- 

 32. Woods Hole Oceanographic Institution, 

 1968); G. L. Clarke, in Remote Sensing in 

 Ecology, P. L. Johnson. Ed. (Univ. of Georgia 

 Press. Athens, 1969). 



5. G. C. Ewing, Deep-Sea Res. 16 (Suppl.), 35 

 (1969). 



6. L. A. Gore, unpublished report (Electronic 

 Systems Division. TRW Systems, Inc., 1968). 



7. C. S. Yentsch and D. W. Menzel, Deep-Sea 

 Res. 10. 221 (1963); O. Holm-Hansen. C. J. 

 Lorenzen, R. W. Holmes, J. D. H. Strick- 

 land, /. Cons. Cons. Pertna. Int. Explor. Mer 

 30, 3 (1965); C. J. Lorenzen. Deep-Sea Res. 

 13, 223 (1966). 



8. C. J. Lorenzen, unpublished manuscript (1969). 



9. R. C. Ramsey, unpublished final report (Elec- 

 tronic Systems Dept., TRW Systems. Inc., 1968). 



10. Supported by NASA contract NASA 12-631 

 and by Office of Naval Research contract 

 ONR 241. We thank the crews of our research 

 ship and aircraft; we also thank Miss Anne 

 Bowen, Mr. Carl Fontneau, and Mr. Robert 

 E. Frazel, research assistants. Contribution 

 No. 2394 of the Woods Hole Oceanographic 

 Institution. 



12 December 1969 



20 FEBRUARY 1970 



5-4 



