44 



MARINE FISHERIES OF NORTH CAROLINA 



demonstrated for Chesapeake Bay waters by Beaven (1946) who further 

 noted that periods of high stream flow have a cumulative effect such that 

 ''when saHnity is depressed it does not recover fully for a period of weeks 

 or months." 



Rain falling directly on the sounds (see the precipitation data of Figure 

 19) must also be considered as a major factor reducing salinity. As with 

 river discharge, its effects are minimized by evaporation and transpiration. 

 The Corps of Engineers (1935), using rates determined at Lake Michie near 

 the inland city of Durham, set the average evaporation from the sounds at 

 25,392 acre-feet per day but there are no figures for transpiration. Though 

 seasonal or even annual evaporation-transpiration totals are not available, 

 it is obvious that these are very significant considering the 1,648,000 acres of 

 water surface in the sounds, their lengthy coastlines with vast areas of 

 emergent vegetation, and the fact that the evaporation figure alone is more 

 than half the river discharge estimated by the Corps of Engineers. Both the 

 evaporation and transpiration are probably greatest in the late spring and 

 summer; so their role in offsetting rainfall and river discharge is probably at 

 a minimum in winter and in early spring when the river flow is often at a 

 peak. Figure 2 1 showing monthly changes in salinities at Beaufort shows this 

 seasonal effect. 



Winds, currents, and diffusion tend to mix the fresh water flowing in a 

 general seaward direction from the river mouths with the ocean water enter- 

 ing the sounds through the inlets. Because of differences in density, however, 

 some stratification of fresher water over that from the sea is to be expected 

 even in such shallow estuaries as Pamlico Sound. In numerous papers Nelson 

 (1928 and 1931, for examples) has described the existence of salinity strata 

 in Barnegat Bay and vicinity in New Jersey, where the depth is considerably 

 less than in much of PamHco Sound. ^' Also, Figures 16 and 17 suggest slight 



17. There is some evidence of this for North Carolina waters in the published records of 

 Grave (1904) as offered in the following two tables. This is relegated to the status of a footnote 

 here, however, because accompanying temperature records are not given, and the consistently 

 higher densities toward the bottom may be, in part at least, a result of temperature and not 

 salinity. 



Average densities at 3 stations over 

 an oyster bed in North River 



Average densities at 4 stations over 

 an oyster bed in Newport River 



Average depth 3^ to 4 feet at low tide. 

 From Grave (1904) 



Depth 3 to 9 feet at low tide. From Grave 

 (1904) • 



