WOLFE and RICE: CYCLING OF ELEMENTS IN ESTUARIES 



isotopes into the ecosystem (Wolfe and Schelske, 

 1969; Wolfe and Jennings, in press). A high 

 rate of instantaneous uptake has also been dem- 

 onstrated for many organisms and many ele- 

 ments in experiments on radioisotope accumula- 

 tion, but, in most cases, concentrations of stable 

 element counterparts for the radioisotopes were 

 undetermined so that rates of flux for the stable 

 element could not be computed from the observed 

 flux of radioactivity. Although the accumulation 

 and turnover of a radioisotope in a single com- 

 ponent can be modeled mathematically inde- 

 pendent of the stable element chemistry (Reichle, 

 Dunaway, and Nelson, 1970), it is the flux of 

 stable elements which determines the movement 

 of radionuclides among the various components 

 of an ecosystem and investigators should rou- 

 tinely collect data on the stable element compo- 

 sition of the compartments involved in their ac- 

 cumulation studies. In this way, experimentally 

 observed radioisotopic accumulation rates can be 

 used in conjunction with the specific activity, i.e., 

 the concentration ratio of radioisotope to total 

 element, to determine rates of elemental turn- 

 over. It seems probable that instantaneous up- 

 take of an element is a direct function of avail- 

 able environmental levels whereas instantaneous 

 loss is a direct function of accumulated amounts. 

 The instantaneous uptake rate is also a function 

 of other environmental variables. For example, 

 in the estuarine clam Rangia cuneata, the instan- 

 taneous uptake rate and the equilibrium concen- 

 tration of ^"Cs increase with temperature and 

 decrease with salinity (Wolfe and Coburn, 

 1970). Salinity, temperature, pH, and total Zn 

 also influenced the accumulation of ^^Zn by var- 

 ious estuarine organisms under experimental 

 conditions (Duke et al., 1969). 



Net accumulation (or net loss) results when 

 instantaneous uptake exceeds (or is less than) 

 instantaneous loss, and the physiology and me- 

 tabolism of the organism determine the residence 

 times required for passage through its many al- 

 ternate internal compartments and pathways. 

 Retention times are usually discussed in terms 

 of biological half-life. ( See for example Baptist, 

 Hoss, and Lewis, 1970.) Since organisms have 

 several compartments simultaneously interact- 

 ing with the environment, retention typically 



consists of several components with diff'erent 

 rates. One might expect the faster rates to be 

 associated with surface adsorption reactions, in- 

 termediate rates with excretion of unassimilated 

 material as feces, and slow rates with the turn- 

 over of the assimilated and metabolized fraction 

 of the elemental content. Although the relative 

 amounts of an accumulated radioisotope involved 

 with different retention components can be de- 

 termined for the particular conditions and time 

 period of accumulation and loss used in the ex- 

 periment, these amounts will not be represent- 

 ative of stable element pools unless all of the in- 

 ternal compartments are equally labeled, i.e., to 

 a uniform specific activity. Thus, long-term ac- 

 cumulation experiments under conditions of 

 constant specific activity are required (Cross, 

 Willis, and Baptist, 1971). The individual re- 

 tention components for an element probably will 

 have to be considered for each important reser- 

 voir in modeling the overall flux of that element 

 in the ecosystem. 



We have discussed several aspects of elemental 

 cycling in estuaries and have demonstrated the 

 incompleteness of man's knowledge of how an 

 estuary operates as a system with many integral, 

 smoothly functioning components. We believe, 

 however, that many of the unsolved problems 

 which have presented themselves will be realist- 

 ically resolved only by a holistic approach to eco- 

 logical research. The foregoing discussion rep- 

 resents an eff"ort to conceptualize the elemental 

 cycling system that operates in our southeastern 

 coastal zone estuaries. Prior recognition of the 

 complexity and integrity of the system as a whole 

 provides an improved basis for planning mean- 

 ingful research on the transfer processes be- 

 tween individual components of an estuary. Con- 

 siderable research is required before we can act- 

 ually quantify the reservoirs, routes, and rates 

 of elemental flux involved in this preliminary 

 model of these ecosystems. The complexity of 

 the ecosystem defies precise quantification of all 

 the reservoirs and all the transfer rates under 

 any set of environmental conditions. In such a 

 system, however, the predictability of the mag- 

 nitude and variability of any elemental reservoir 

 depends upon recognition of all the interactions 

 impinging upon that reservoir. Research now 



969 



