Chapter II — 13 — The Marine Environment 



The amount of photosynthetic activity which occurs at different 

 depths as indicated by oxygen production and carbon dioxide consump- 

 tion, or other means, is one criterion of the penetration of light. At 

 depths wliere there is much photosynthetic activity during the hours of 

 dayhght, more oxygen is hberated than the amount which is consumed 

 by the respiration of organisms. With decreasing depth a point is reached 

 where the amount of oxygen hberated is just equal to the amount con- 

 sumed. It is termed the compensation point. 



Clarke (1936) found the compensation point in the Sargasso Sea at 

 a depth of 80 meters. In the Gulf of Maine the compensation point was 

 at a depth of 24 to 30 meters and in Woods Hole Harbor it was only 7 

 meters. He observed measurable amounts of photosynthesis at 18 meters 

 in Woods Hole Harbor, 40 meters in the Gulf of Maine, and 140 meters in 

 the Sargasso Sea. 



Besides the effect of light on photosynthetic organisms which are the 

 primary producers of the sea (Clarke and Oster, 1934), light is the most 

 important factor controlling the diurnal migrations of plankton (Clarke, 

 1936). Not only does light influence the migrations, vertical distribution, 

 production, and physiology of zooplankton, but the attachment and rate 

 of growth of sedentary animals are also influenced directly or indirectly by 

 light (Russell, 1936). The specific effect of radiations on the activities 

 and distribution of marine microorganisms is outlined in Chapter V. 



The literature on the penetration of light through water has been re- 

 viewed by Atkins (1932). The reader is also referred to the symposium 

 volume on the measurement of submarine light and its relation to biolog- 

 ical phenomena by Clarke (1936), Russell (1936), Utterback (1936), 

 and others. 



Temperature of the marine environment: — Closely related to the 

 penetration of light is the temperature of sea water, which is primarily a 

 function of the intensity of solar radiation. The effects of volcanic dis- 

 turbances and adiabatic heating or cooling are far less than the direct or 

 indirect effects of solar radiation. 



The surface temperatures of sea water vary with season and latitude. 

 Tropical waters in the open sea may have surface temperatures as high as 

 28° to 30° C, or 38° to 40° C. in localized regions near shore, while in polar 

 seas water temperatures approximate the freezing point of the water. The 

 freezing point is a function of the salinity. Water having a salinity of 

 35°/oo begins to freeze at — 1.91° C. Because the salinity is never suffi- 

 ciently high, except in localized regions, to depress the freezing point lower 

 than — 2.0° C, this approximates the lowest temperature found in the 

 sea. The temperature range of the marine environment, — 2° to 40° C. is 

 small contrasted with the range of air temperatures, — 65° to 65° C. 



The temperature of surface water usually fluctuates less than 1° C. 

 throughout the day. Diurnal changes are barely perceptible below a 

 depth of 10 meters and are probably too small to be of biological impor- 

 tance. Annual changes may affect the temperature of sea water to a 

 depth of 10 to 200 meters. Below this depth the water temperatures are 

 fairly constant throughout the year, although fluctuations may be caused 

 by the movements of water masses. The temperature of sea water usu- 

 ally decreases with depth except in shallow turbulent seas or during the 

 winter at high latitudes. 



The temperature of the water in the uppermost layers, which are 



