FISHERY BULLETIN: VOL, 70, NO. 3 



the estuary obviously determines also the physi- 

 cal distribution of certain reservoirs within the 

 system, and this aspect of cycling compounds 

 the difficulties of developing a realistic model. 

 Application of the term "ecosystem" to an estu- 

 ary, however, implies that an estuary behaves 

 as a discrete, identifiable unit and, despite the 

 clumped distributional patterns exhibited by or- 

 ganisms in the environment, spatial distribution 

 of reservoirs within the system is assumed insig- 

 nificant; i.e., the system model is homogeneous. 



In addition to tidal flushing of planktonic or- 

 ganisms, biotic inputs and outputs include the 

 contribution of terrestrial primary productivity, 

 brought into estuaries as dissolved organic ma- 

 terial and particulate organic detritus in runoff, 

 and the seasonal migrations of larger inverte- 

 brates and fish into and from estuarine waters. 



A final input which must be considered con- 

 sists of human waste materials. Organic waste 

 materials affect productivity and biological spe- 

 cies structure of the ecosystem and probably also 

 aflfect the availability of metallic elements 

 through organic complexing. In addition, large 

 amounts of certain metallic elements are intro- 

 duced into many estuaries directly in industrial 

 and municipal effluents. 



FACTORS INFLUENCING 



DISTRIBUTIONS AND TRANSFERS OF 



ELEMENTS AMONG THE VARIOUS 



RESERVOIRS IN ESTUARIES 



The various groups of biota, the dissolved and 

 particulate components in the water column, and 

 sediment compartments, represent the major 

 reservoirs per se for metallic elements, but the 

 interaction of these compartments and regula- 

 tion of their sizes are influenced greatly by other 

 variable characteristics or components of estu- 

 arine ecosystems including inputs of energy and 

 auxiliary materials or factors which aflfect res- 

 ervoir sizes or transfer rates within the system. 

 These factors represent "nondynamic state vari- 

 ables" in the modeling terminology of Kowal 

 (1971). These characteristics may be categor- 

 ized as: (1) those that affect productivity in 

 the trophic web and (2) those that afl'ect the 



physical state or rates of exchange between com- 

 ponents for the specific elements of interest. The 

 influences of major nondynamic state variables 

 are summarized briefly in Table 3. 



Solar energy is perhaps the best example of 

 the first type of input. Solar energy drives the 

 primary productivity supporting all the biotic 

 reservoirs and establishes the basic temperature 

 regime for the entire system, which is a dominant 

 factor influencing the rate of material transfer 

 processes, biological and nonbiological. The sea- 

 sonal variability of temperature can readily be 

 incorporated into systems models according to 

 a sinusoidal function. This general approach has 

 been described and applied by Williams (1969). 

 In the first category also are dissolved gases, 

 e.g., CO2 and O2, and inorganic nutrients, espe- 

 cially the various forms of P and N. Certain 

 dissolved organics may also be included here, 

 although absolute requirements for these have 

 not been demonstrated in the nutrition of estu- 

 arine organisms, and dissolved organic-metal 

 complexes constitute a major reservoir already 

 considered. Acidity, or pH, is an important fact- 

 or in that it acts with temperature to control CO2 

 solubility and thereby aflTects primary produc- 

 tivity, but pH also falls into the second category 

 because it determines the equilibrium distribu- 

 tion of metals in adsorption reactions, i.e., ex- 

 change between water and sediments, or between 

 water and biological surfaces; and in chelation 

 reactions, including metal-organic complexes in 

 the dissolved phase. 



Wind is another significant energy input in 

 shallow estuaries. Wind increases turbulence 

 and suspension of particulate matter which pro- 

 motes exchange of elements between water and 

 sediments and affects the size of the suspended 

 particulate reservoir subject to flushing. 



Salinity is also a determinant in the distribu- 

 tion and cycling of elements. Salinity, as a mea- 

 sure of ionic strength, affects the adsorption 

 equilibria established between sediments and 

 water and biological surfaces and water, and 

 probably also influences the configuration of pro- 

 teins and other biological polymers which form 

 complexes with metals. This mechanism may be 

 the basis of salinity eflfects on active transport 

 processes in ionic and osmotic regulation in es- 



966 



