changes in Transparency 197 



able light is passing in every direction. In the upper strata the hght 

 passing in the direction of the refracted rays from the sun is the 

 strongest. With increasing depth the direction of strongest light in- 

 tensity tends to move toward the vertical, and a limiting pattern of 

 diffusion is established (Jerlov, 1951). Thus, although a variable de- 

 gree of diffusion occurs, the directional character of the light in the 

 water is never entiiely lost. 



Changes in Transparency. Parts of the ocean and of lakes that 

 are well stirred exhibit a uniform transparency from the surface down- 

 ward, but, when strong density gradients exist, considerable changes 

 in transparency may occur with depth. Furthermore, the transpar- 

 ency of coastal marine areas and of inland water bodies often changes 

 profoundly from season to season, owing to differences in amount of 

 stirring, in the discharge of muddy rivers, and in the growth of plank- 

 ton. Such variations in transparency are added to the seasonal 

 changes in the intensity of the light received at the surface and in the 

 length of the day. The combined effect of these influences results 

 in surprisingly great fluctuations in the light factor at subsurface 

 levels. i 



Observations made off the coast of Massachusetts illustrate the mag- 

 nitude of the foregoing seasonal influences (Clarke, 1938). In this 

 region the maximum hourly intensity of light received at the surface 

 during the summer averaged about twice that received during the 

 winter period. The average total solar radiation received per day 

 in summer was about four times as great as that received during the 

 winter. Significant departures from the average for periods up to 

 several weeks occurred, and such variations are undoubtedly im- 

 portant in causing changes in the response to the light factor from 

 year to year. Values for individual days may also depart markedly 

 from the average. During the investigation refen'ed to above the 

 maximum light received on the brightest day of summer was forty 

 times greater than that received during the dullest day in winter. 

 In the course of the same year the extinction coefficient of the water 

 varied fourfold. Since the effect of extinction is cumulative, the il- 

 lumination in the subsurface layers varies more because of changes 

 in transparency than because of seasonal changes in radiation reach- 

 ing the surface. By combining the two effects it was found that for 

 an organism living at a depth of only 30 m the minimum hourly 

 light intensity was 7000 times greater and total daily radiation was 

 10,000 times greater in May, when high incident light was combined 

 with high transparency, than in December, when both incident il- 

 lumination and transparency were low. The magnitude of seasonal 



