General Considerations 



17 



brates and in the shells and other hard parts of 

 invertebrates. 



The length of time an organism retains the 

 average atom of a given element varies greatly 

 from one element to another. This is some- 

 times measured as the biological half-life, al- 

 though the relative rate of loss is not a simple 

 linear function of time as is the case with radio- 

 active decay. Much is known about the reten- 

 tion times of different elements in man (see, 

 for example. Handbook 52 of the National 

 Bureau of Standards, 1953), but there are few 

 data for most marine organisms. The rate of 

 excretion of an element and the amount ulti- 

 mately retained, will be quite different if the 

 element is taken up quickly from a single dose 

 or is taken up slowly over a long time. 



The processes of uptake, accumulation, and 

 loss of elements by marine and other aquatic 

 organisms, are discussed in more detail by 

 Boroughs, Chipman and Rice (Chapter 8 of this 

 report), Krumholz, Goldberg, and Boroughs 

 (Chapter 7), and by Krumholz and Foster 

 (Chapter 9). 



Effects of organisms on spatial distributions of 

 elements in the sea 



Those elements of which a large proportion 

 is cycled through organisms are modified pro- 

 foundly in their spatial distributions by the ef- 

 fects of the biosphere, so that they are quite 

 differently distributed in the sea than elements 

 in which the distribution is determined only by 

 physical and inorganic chemical processes. We 

 have already mentioned phosphorus as a notable 

 example. Ketchum (Chapter 5 of this report) 

 has written a detailed discussion of the general 

 effects of the ecological system on the distribu- 

 ticfti of elements in the sea. 



The marine biosphere acts as a reservoir for 

 those elements that are removed selectively 

 from sea water by organisms. This reservoir is 

 not stationary in space, however, because many 

 of the living organisms make both vertical and 

 horizontal migrations of large extent, while 

 their dead bodies and fecal materials continu- 

 ally fall toward the bottom under the influence 

 of gravitation. The effects of the living reser- 

 voir in the distribution of elements vary not 

 only from one part of the sea to another, but 

 also seasonally in the same area. 



Because organisms in the sea are more abun- 

 dant in the upper layers than deeper down, 



those elements in scarce supply that are essen- 

 tial to life tend to be retained by the biosphere 

 in the upper layers and to be returned to solu- 

 tion in the deeper layers. Stationary popula- 

 tions, such as attached benthic organisms, act 

 as a fixed reservoir. 



Where there are currents at different levels 

 in opposite directions, the accumulation of ele- 

 ments by pelagic organisms, together with grav- 

 ity effects on their dead bodies and fecal ma- 

 terials, can result in local concentrations of ele- 

 ments at intermediate depths greater than the 

 concentrations in either the overlying or the 

 deeper waters. This pattern, as noted by 

 Ketchum, is common in estuaries, continental 

 shelves, and in the vicinity of coastal upwelling. 



Migration of organisms may result in a net 

 transport of elements from areas of high con- 

 centration to areas of lower concentration. Thus, 

 for example, the vertical migrations of the or- 

 ganisms of the deep scattering layer can result 

 in a transport from the deeper layers into the 

 upper mixed layer. Salmon which spawn and 

 die in fresh waters after accumulating elements 

 in the sea can transport significant quantities of 

 some elements from the sea to fresh waters. 



Finally, the remains of organisms, falling out 

 as particulate matter, are an important com- 

 ponent of the sedimentation process in the deep 

 sea, and are thus important in the geochemical 

 cycle, as noted by Carritt and Harley (Chapter 

 6) and others. 



Although we have some understanding of 

 the various processes involved, data for making 

 useful quantitative assessments are almost en- 

 tirely lacking. 



Effects of introduction of radioactive elements 



Since the isotopes of most chemical elements 

 are similar in chemical behavior, it can be as- 

 sumed that organisms do not appreciably dis- 

 tinguish between the radioactive and non-radio- 

 active isotopes, and that, to a good degree of 

 approximation, the path of a radioactive element 

 through the biological system is the same as 

 that of its non-radioactive isotopes. 



The accumulation of radio isotopes in organ- 

 isms will, therefore, depend on the same factors 

 as the accumulation of normal isotopes (their 

 concentration in the water where the organisms 

 are located, the concentrations of other elements 

 by which uptake is influenced, the size of the 

 population of organisms concerned, the concen- 



