FISHERY BULLETIN: VOL. 70, NO. 3 



the mechanisms and pathways of elemental 

 transformations and rates of elemental turnover 

 among reservoirs, and determining the response 

 of these processes and reservoirs to changes in 

 the environment under study. 



In aquatic ecosystems, nearly all of the metal- 

 lic elemental content of the system resides in the 

 sediments and in the water. The fraction resid- 

 ing in the biota is small — with phytoplankton and 

 zooplankton constituting the most important liv- 

 ing elemental reservoirs in terms of turnover and 

 total physical transport and redistribution pro- 

 cesses. The major commercial species harvested 

 by man from estuaries have little effect on the 

 overall distribution of metallic elements in the 

 estuary, simply because most of these species are 

 high in the trophic scheme and have relatively 

 low biomass. Yet, in contaminated environ- 

 ments these same species may concentrate cer- 

 tain elements to levels potentially harmful to 

 people consuming large quantities of seafood. 

 Our consideration of elemental cycling must 

 therefore be biased toward biological species of 

 little importance to the overall elemental mass 

 balance because these species form the major 

 interface between man and the remainder of the 

 ecosystem. 



METALLIC ELEMENTS OF 

 PRIMARY CONCERN 



In the preceding and ensuing paragraphs we 

 discuss the cycling of "metallic elements." Such 

 a generalization is not entirely practical because 

 chemical and physical characteristics of the dif- 

 ferent metals determine to some extent their be- 

 havior in the natural environment and their rel- 

 ative toxicity to biological systems influences the 

 extent of our interest in the various metals. In 

 addition, the relative abundance of various ele- 

 ments affects both their ecological behavior and 

 human interest. In terms of chemical and physi- 

 cal properties, we can categorize the metals into 

 five groups: alkali metals, alkaline earths, tran- 

 sition elements, rare earths, and actinide ele- 

 ments. The transition elements may be further 

 subdivided into "family groups" on the basis of 

 the electronic configurations of the atoms. The 

 term "heavy metal" usually refers to all those 



metals with atomic numbers higher than 20 (Ca) , 

 i.e., beginning with the first transition series: 

 Sc, Ti, V, Cr, etc. In the popular literature 

 "heavy metal" frequently also carries the conno- 

 tations of pollution and/or toxicity. Table 1 cat- 

 egorizes metallic elements according to biological 

 function and abundance and shows elemental 

 abundances for seawater and the crust of the 

 earth. Not all the elements listed in Table 1 as 

 "essential" are required by every animal and 

 plant — only a few higher plants may require alu- 

 minum or vanadium, silicon is an essential bulk 

 component of diatoms not required by other 

 forms, and molybdenum appears to be essential 

 only to organisms that derive their nitrogen by 

 nitrate reduction or fixation of free nitrogen 

 (Underwood, 1962; Epstein, 1965). Similarly, 

 the designation of toxicity is arbitrary, since 

 most of the essential elements are toxic at unu- 

 sually high concentrations. The nonessential 

 elements include a few (cesium, rubidium, stron- 

 tium) which consistently occur in organisms and 

 can replace to a large degree the biological func- 

 tions of essential elements (Rb and Cs for K, Sr 

 for Ca) . To a much lesser extent, the metals Cr, 

 Ni, and Cd may fulfill the biological functions of 

 Zn, Mn, or Cu. It should be noted that the metals 

 which are essential or relatively nontoxic to bi- 

 ological systems generally have a low atomic 

 number (only Rb, Sr, Mo, and Cs exceed 30) and 

 are relatively abundant in the biosphere. The 

 toxic metals generally have high atomic numbers 

 (an outstanding exception is Be with an atomic 

 number of 5) and low abundances (notable ex- 

 ceptions include As, Ba, and Pb, all of which oc- 

 cur consistently in living organisms, and Be and 

 Th, which are extremely variable in their bio- 

 logical distribution). 



In addition to the elements in Table 1, certain 

 other metals are of concern because of radiation 

 from radioisotopes, whether naturally occurring 

 or introduced from man's nuclear activities. The 

 abundances and composition of radionuclides in 

 the marine environment are summarized else- 

 where (Rice and Wolfe, 1971; Joseph et al,, 

 1971 ) . Most of the significant radionuclides are 

 isotopes of certain elements in Table 1 such as 

 chromium, iron, manganese, zinc, cesium, and 

 strontium. To this list we should add cerium, 



960 



