graph by assuming that all larval settling 

 occurs on October 1 (from a late summer 

 spawn) and by connecting successively longer 

 modal groups by the shortest possible time, 

 based on calendar months. Although this ap- 

 proach is arbitrary, the closeness of the fit 

 would suggest that our modal groupings repre- 

 sent size classes from individual spawnings of 

 the clam. 



Predictions of growth from the curve in 

 figure 10 correspond well to previous size and 

 age estimates for Rangia . The predicted maxi- 



mum length, a = 75.62 mm., is also compatible 

 with our observations on Rangia in the field. 

 Clams over 70 mm. long are rare. Of the 

 6,287 clams measured for this study, only 7 

 exceeded 70 mm. and the maximum length was 

 73 mm. The largest specimen (81 mm.) seen 

 in our radioecological study was from Lewes 

 Ferry on the Neuse River. Thus, despite the 

 limited length-frequency data upon which this 

 study was based, the von Bertalanffy equations 

 derived from these data appear valid for 

 estimating growth of Rangia . 



POLLUTION STUDIES PROGRAM 



Thomas W. Duke, Chief 



Sediments have the capacity to act as reser- 

 voirs for significant amounts of certain radio- 

 nuclides in shallow estuaries. A knowledge of 

 this capacity, in conjunction with estimates of 

 rates of exchange of elements among sediment, 

 water, and biota will allow us to describe the 

 total movement of trace elements in an estua- 

 rine ecosystem. 



During fiscal year 1968, we have studied the 

 exchange of elements between sediments and 

 water in both the laboratory and field, A tech- 

 nique was developed to measure the rate of 

 exchange of elements between water and sedi- 

 ments in the laboratory. This research was 

 carried out in conjunction with the Biogeo- 

 chemistry Program. A field project has been 

 underway since October 1966 to measure the 

 variation in concentrations of iron, nnanganese, 

 and zinc in surface sediments collected monthly 

 in the Newport River estuary. This project is 

 scheduled for completion in September 1968 

 and only preliminary results will be discussed 

 in this report. To further our knowledge of 

 influences which the biota may have on the 

 cycling of trace elements, concentrations of 

 iron, manganese, and zinc were determined 

 for seven species of polychaetous worms 

 collected from the Newport River estuary and 

 adjacent Bogue Sound. In a separate research 

 project, respiration measurements were made 

 on five species of estuarine fish to determine 

 if the relation between oxygen consumption and 

 weight can be described by a single equation 

 for all species. 



A TECHNIQUE FOR STUDYING THE 



EXCHANGE OF TRACE ELEMENTS 



BETWEEN ESTUARINE SEDIMENTS 



AND WATER 



Thomas W. Duke, James N. Willis, 

 and Douglas A, Wolfe 



Trace elements are continuously exchanged 

 between sediments and water in the estuarine 

 environment. This exchange is an important 



part of the biogeochemical cycling of elements 

 and often determines the availability of the 

 elements to the biota. Sediments usually con- 

 tain large concentrations of trace elements in 

 relation to the water and serve as a "reser- 

 voir" for the elements. In this report we 

 describe a technique and specialized equip- 

 ment for studying the exchange of elements 

 between sediments and water. Exchange rates 

 of zinc are determined for a series of sediment 

 samples from the Newport River estuary at 

 Beaufort, N.C. 



Materials and Methods 



Natural cores of estuarine sediments are 

 collected with a coring device constructed with 

 a polyethylene cylinder which is plugged with 

 a vinyl stopper and detached after a core is 

 taken (fig. 11). The cylinders are then trans- 

 ported to the laboratory and submerged in 

 water from the sampling station in a 125-1. 

 plastic container. The water, circulated by 

 aeration, is maintained at 22° ± 2° C. and the 

 pH and salinity are recorded. The sediment 

 is left in contact with the water until the con- 

 centration of the trace element of interest be- 

 comes constant in the water. In the present 

 experiments, zinc was analyzed periodically 

 until the difference between two consecutive 

 measurements was less than 5 percent of the 

 total concentration. After this equilibrium is 

 reached, the cylinders containing the cores and 

 about 800 ml. of water are removed from the 

 plastic can. The water is pumped from the 

 cylinder and filtered through a 0.45// mem- 

 brane filter. The inside of the cylinder is wiped 

 clean, about 600 ml. of the filtered water are 

 returned to the cylinder, and the remainder 

 used for chemical analysis. These operations 

 must be connpleted carefully to avoid disturbing 

 the sediment surface. 



After the filtered water is in place over the 

 core, the cylinders are slipped into a specially 

 constructed lead shield (fig. 12). The shield 

 consists of three parts (fig. 12) arranged so that 



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