curve" for determination of these elements in 

 the presence of calcium. High concentrations 

 of calcium in the sannples probably affect the 

 analysis for any element by two antagonistic 

 phenonnena: the calcium itself causes light 

 losses at the analytical line, thereby stimu- 

 lating high concentrations of the metal being 

 analyzed; and the sensitivity of analysis is 

 decreased because of lowered efficiency of 

 atomization. 



The origin of the interferences by calcium 

 in atomic absorption has been attributed to 

 light scattering and to molecular absorption. 

 The relation of response from calcium nitrate 

 solutions to the analytical wavelengths for 

 copper, manganese, and zinc in the present 

 study is shown in figure 8. This inverse rela- 

 tion suggests that light scattering does indeed 

 contribute to the interference by calcium. The 

 effects of light scattering may become more 

 pronounced as the concentration of the inter- 

 fering element is increased, since the 

 response- wavelength plot (fig. 8) becomes 

 more nearly linear as the concentration of 

 calcium is increased. 



If interferences from matrix absorption 

 were superimposed upon light scattering by 

 calcium, the method of correction whereby 

 response from a nearby nonabsorbing line is 

 subtracted from the response on the absorption 

 line would be invalid because spectral structure 

 may produce significant differences inabsorp- 

 tion over a small range of wavelength. When 

 scattering or absorption effects are suspected, 

 therefore, it is necessary that standards nnatch 

 the samples not only in density or flow rate 

 but also in the contents of interfering elements. 

 Since absorption effects are produced by mo- 

 lecular species of the matrix, the concentra- 

 tions of both cations and anions must be dupli- 

 cated. Thus, the concentrations of copper, 

 manganese, and zinc in nitric acid digests of 

 oyster shells were determined through the 

 use of standard solutions of metals containing 

 25 percent Ca(N03)2 (6,1 percent Ca). Samples 

 of oyster shell were diluted to the same con- 

 centration of calcium (equivalent flow rates), 

 and absorption of samples and standards was 

 compared directly. The concentrations in fresh 

 oyster shells (average and standard deviation 

 for 18 samples) were 0.34 + 0.27 p. p.m. 

 copper, 25.9 ± 4.3 p,p.m. manganese, and 

 2.1 ± 1.1 p. p.m. zinc. 



GROWTH OF Rangia cuneata (GRAY) 



Douglas A. Wolfe and Ernest N. Petteway 



Rangia occurs in brackish waters from the 

 Chesapeake Bay to Texas and is harvested 

 commercially for human consumption to a 

 limited extent from estuarine areas along the 

 North Carolina coast. Although Rangia is of 

 potential commercial importance throughout 



its range, very little is known about the growth 

 or general life history of the species. In 

 Louisiana, Rangia are reported to spawn during 

 spring (March- May), and then again, but less 

 intensively, from late summer into November , 

 Mature gametes were also found in Rangia 

 from the Potomac River, Md,, in late August, 

 thereby confirming autumnal spawning. From 

 analysis of growth lines, average annual size 

 increments through the first 3 years of growth 

 have been inferred for the Louisiana popula- 

 tion. The average shell length of 3-year-old 

 clams was 24 to 34 mm. In the Potomac popu- 

 lation, other investigators thought clams 35 to 

 45 mm, long to be 4 years old. Additional in- 

 formation on growth of Rangia in natural 

 populations is unavailable in the literature. 

 The large samples of Rangia orginally collected 

 for our radioecological study of fallout pro- 

 vided an opportunity to determine the growth 

 of this species. 



Sampling Methods and Ecology of the 

 Sampling Station 



Rangia were collected by raking near the 

 shore of the Trent River at the junction of 

 Wilson Creek, The prongs on the clam rakes 

 were 20 to 25 mm. apart; clams shorter than 

 30 mm. were therefore not sampled reliably. 

 Collections were made at irregular intervals 

 (usually once every 4 to 6 weeks) from Novem- 

 ber 9. 1965, to July 6, 1967, Although the total 

 bottonn area involved was only about 500 m,"^ 

 (10 by 50 m,), the population in it was not 

 depleted by removal of one-half to three- 

 quarters of a bushel of clams per sample. The 

 12 samples had from 129 to l,297animals each 

 (mean 524), 



At each sampling, salinity and water temper- 

 ature were measured with an electrodeless 

 induction salinometer. Salinity measurements, 

 which ranged from 0.01 p.p.t, (March29, 1966) 

 to 6,0 p,p,t, (December 20, 1965), showed a 

 seasonal variation from fresh water in the 

 spring and summer to brackish water in the 

 late fall and early winter. The lower salinities 

 in this range apparently constituted a barrier 

 to Rangia because the clams were absent some 

 3,2 km, farther upstream. The water tempera- 

 tures measured at Wilson Creek ranged from 

 4,8° C. (February 28, 1966) to 34,7° C, (July 5, 

 1966), Other mollusks encountered with Rangia 

 at Wilson Creek included marsh clams, Poly- 

 mesoda caroliniana ; platform mussel, Congeria 

 leucophaeta ; and occasionally, freshwater mus- 

 sels, Anodonta imbecilis . 



Analysis of Growth in Rangia 



The left valve from each clam was measured 

 with a ruler to the nearest millimeter, and the 

 percentage frequency by length was calculated 

 for each sample of clams. The percentage 

 frequencies were then graphed for each sample. 



19 



