458 CLARKE AND DENTON [OHAP. 10 



regions and to values of less than 3 ni (A; =1.5) in very turbid harbours or 

 estuaries. 



The clearest ocean waters, particularly those in the warmer areas of the 

 world, have their maximum transparency in the blue portion of the spectrum 

 (400-500 mfi.). Measurements with the bathyphotometer in the Atlantic Ocean 

 and in the Mediterranean Sea have revealed the presence of optically uniform 

 water of high transparency {k = 0.03-0.04) at depths below the first few hundred 

 metres and extending at least to 700 m in some instances. Equally transparent 

 water probably occurs at still greater depths and, perhaps, to the very bottom 

 of the ocean, but exact determination of transparency has not been possible in 

 the deeper strata due to the fact that below about 700 m the presence of 

 luminescence from marine organisms has interfered. However, some detectable 

 light from the surface was recorded down to a depth of 800 m in the Mediter- 

 ranean (Clarke and Breslau, 1959) and to 950 m in Caribbean waters (Clarke, 

 unpublished data). (See Fig. 1.) 



Measurements with the bathyphotometer have shown that at times and 

 depths at which only weak illumination is received from the surface (that is, 

 at night or at deeper levels during the day) bioluminescence contributes a 

 significant portion of the total light present. In all localities thus far studied in 

 the Atlantic (Clarke and Hubbard, 1959), Pacific (Kampa and Boden, 1957), 

 and Mediterranean (Boden and Kampa, 1958; Clarke and Breslau, 1959), at 

 least some bioluminescence was recorded at every depth investigated at night, 

 and during the day, at all depths below the level at which light from the surface 

 interfered. Measurements in the slope water south-east of New York showed 

 that the frequency of luminescent flashing at night reached a maximum of 

 over 160 per minute at 100 m, with a secondary maximum of about 90 flashes 

 per minute at 900 m, and dropped to a minimum of about 1 flash per minute 

 at 3750 m. During the day flashes appeared above the ambient light penetrating 

 from the surface at about 400 m and increased in frequency with depth to 

 over 1 10 per minute at 900 m. The flux received from the brightest flashes was 

 above 10~3 ptW/cm^ or about 10^'^ x surface light with bright sun. Flashes 

 ranged from less than 0.2 sec to more than 1 sec in duration, with light from 

 overlapping flashes lasting sometimes for more than 10 sec. Measurements in 

 the Mediterranean indicated the occurrence of a larger number of the brightest 

 flashes (some over 10~2 fj,W/cm2) but the frequency of flashing was generafly 

 less than in the Atlantic. In Phosphorescent Bay, Puerto Rico, sustained light 

 levels of above 10^^ j^W/cm^ were produced by continued agitation of the 

 water which contained large populations of dinoflagellates (Clarke and Breslau, 

 1960). 



A great many kinds of marine organisms are known to emit luminescent 

 light, either spontaneously or when stimulated, and it is difficult to determine 

 which types are chiefly responsible in any given situation. Some macroscopic 

 forms have been photographed by means of the luminescence camera which is 

 triggered by the animal's own flash (Breslau and Edgerton, 1958). However, in 

 the Mediterranean study of Clarke and Breslau (1959), since only a few 



