WOLFE and RICE: CYCLING OF ELEMENTS IN ESTUARIES 



Table 1. — Metallic elements of biological significance. 



1 Goldberg, 1963. 

 3 Mason, 1958. 



zirconium, niobium, ruthenium, scandium, and 

 Plutonium — all of which occur in only trace 

 amounts in the biosphere, but have radioisotopes 

 which are produced in significant quantities by 

 man and accumulated by animals and plants. In 

 the following discussions "metallic elements" 

 will be used to refer generally to all of the ele- 

 ments mentioned above and in Table 1. Space 

 will not permit separate discussion of each ele- 

 ment; instead we will attempt to discuss those 

 general aspects of ecosystem structure and func- 

 tion likely to be most significant in governing the 

 flux of elements within and through estuaries 

 and to identify those elements known to be valid 

 examples or exceptions. 



Although an estuarine ecosystem is very com- 

 plex and difficult to model mathematically, we 

 will take the approach of defining the variables 

 which should be considered in a mathematical 

 model of the dynamic cycling of metals in estu- 



aries, with particular emphasis on the coastal 

 plain estuaries of the southeastern United States. 



MAJOR RESERVOIRS IN THE 

 CYCLING OF ELEMENTS 



In marine and estuarine environments, nearly 

 all of any given metallic element occurs in the 

 sediments and water (Duke, Willis, and Price, 

 1966; Duke et al, 1969; Pomeroy et al, 1969; 

 Lowman, Rice, and Richards, 1971). Both sedi- 

 ment and water contain several individual reser- 

 voirs for elements, with some components much 

 more important than others to cycling in the 

 system. The "water components" include dis- 

 solved ionic forms, dissolved organic complexes, 

 and suspended particulate matter — all of which 

 interact with sediment compartments via ad- 

 sorption, ion-exchange or sedimentation-resus- 

 pension processes. In tracer studies with ^^Zn, 

 the rapid instantaneous uptake of radioisotope 

 by estuarine sediments indicated a high rate of 

 continuous exchange of zinc across the sediment- 

 water interface (Duke, Willis, and Wolfe, 1968). 

 In sedimentary deposits, elements may become 

 "trapped" in deeper sediments and be effectively 

 removed from further exchange with the system. 

 In salt marsh, however, Spartina recycles phos- 

 phorus and zinc from deep subsurface sediments 

 (Pomeroy et al., 1969), and burrowing animals 

 such as crabs, polychaetes, and clams rework 

 shallow and surface deposits, thereby prolonging 

 contact with overlying waters (Gordon, 1966; 

 Rhoads, 1967; Rhoads and Young, 1971). In 

 preliminary studies on the exchange of zinc be- 

 tween estuarine sediments and water, the resu- 

 spension of sediments by the burrowing and feed- 

 ing activity of polychaetes markedly increased 

 the rate of loss of tracer *^^Zn from the water 

 overlying the experimental cores (Duke et al., 

 1968, and unpublished observations). Surface 

 silt layers of high organic content may be a 

 major source of elements to deposit feeders in 

 detritus-based food webs (Cross, Duke, and Wil- 

 lis, 1970; Lowman et al, 1966). In turbulent 

 estuaries, these surface deposits may become 

 suspended and be ingested by filter-feeding or- 

 ganisms such as menhaden, bivalve molluscs, and 



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