36 MARINE FISHERIES OF NORTH CAROLINA 



There are, of course, pronounced short-term fluctuations within any given 

 month and thus the averages tell but part of the story. Figure 13 and Table 7 

 indicate such fluctuations in Beaufort Harbor, and shallow flats probably 

 show even greater extremes. One extreme condition for these waters is 

 described by the following statement from the Supplement to the U. S. Coast 

 Pilot (1945): "Ice may be expected as far south as the headwaters of the 

 North River and Pasquotank River, and Albemarle Sound, and in very severe 

 winters, farther south." 



Since the North Carolina inland waters are generally shallow in proportion 

 to area, continuous mixing is likely to offset tendencies toward temperature 

 stratification. It is possible, however, that the latter may develop for short 

 periods, especially as a secondary effect of salinity stratification (see the 

 following section on Salinity and Nutrient Salts). 



The seasonal relationships between temperatures in sounds, etc., and those 

 offshore are of considerable ecological significance. Figure 11 indicates the 

 typical comparison, the inland water temperatures being higher in summer 

 and lower in winter. Warm-water fishes that are in the sounds during the 

 summer always have a retreat, unobstructed as far as temperature is con- 

 cerned, to the more temperate offshore waters when the inland waters cool. 



Salinity and Nutrient Salts 



The dissolved salts ^^ in the ocean average about 35 parts per thousand. 

 The percentage composition of these as found dissociated or ionized is: ^* 



Sodium (Na + ) 30.4% Chlorine (C1-) 55.2% 



Magnesium (Mg+ + ) 3.7% Sulphate (SO^--) 7.7% 



Calcium (Ca + + ) 1.16% Bromine (Br-) 0.19% 



Potassium (K+) 1.1% Boric acid (H3BO3) 0.07% 



Strontium (Sr+ + ) 0.0470 Bicarbonate & carbonate (HCO3-- & CO3--) 0.35% 



Minor constituents 0.02 — 0.03% 



This well buffered, slightly alkaline solution contains all elements known 

 to be required by living things but there are limitations in quantity and avail- 

 ability which in turn may limit productivity. So limited are the supplies of 

 phosphates and nitrogen salts and so great is the demand by living things 

 that the quantities of these nutrients in aquatic habitats vary considerably as 

 they are consumed and later returned to solution. Numerous factors may 

 affect the availability of such important dissolved materials. Dead organisms, 

 which sink to great depths, drain nutrients from the productive photic zone. 



13. In referring to the concentration of these salts we use the expression salinity which is 

 defined, on the basis of a standardized chemical procedure, as: The total amount of solid materia] 

 in grams contained in one kilogram of sea water when all the carbonate has been converted to 

 oxide, the bromine and iodine replaced by chlorine, and all organic matter completely oxidized. 



14. From Harvey (1945). 



