64 MARINE FISHERIES OF NORTH CAROLINA 



Productivity 



It now seems opportune to discuss, as best we can, the potential produc- 

 tivity of these North Carolina waters, reviewing where desirable hydro- 

 graphic features of special significance to this subject. 



With sunlight as their source of energy, the chlorophyll-bearing micro- 

 scopic algae that are so abundant as drifters (plankton) in aquatic habitats 

 synthesize inorganic ingredients into food. As a result they grow and multiply 

 and thus photosynthesis provides the basic food supply of these areas just as 

 it does through the growth of grass in pastures. 



Attempts to compare this basic productivity for different types of environ- 

 ments indicate that coastal marine regions exceed cultivated land and other 

 terrestrial associations. The comparisons may be stated in terms of photo- 

 synthetic efficiency which is the ratio of energy fixed by photosynthesis to 

 the solar energy reaching the surface of the environment. In a discussion of 

 plankton studies on Long Island Sound, Riley (1941) rates the mean 

 efficiency of the sea at 0.31 per cent. For forests this photosynthetic efficiency 

 is 0.16 per cent and for cultivated lands 0.13 per cent, according to figures 

 quoted by Lindeman (1942) from an unpublished manuscript by G. E. 

 Hutchinson. It is difficult to generalize on fresh waters because of the extreme 

 variability between different lakes, ponds, rivers, etc.; however, many lakes 

 and ponds that have been studied equal and even surpass marine waters in 

 productivity per unit of area. Lindeman (1942) credits Lake Mendota with 

 a photosynthetic efficiency of at least 0.27 per cent and possibly 0.40 per cent 

 and reports only o.io per cent for senescent Cedar Bog Lake. 



Certain characteristics help account for the comparatively high produc- 

 tivity of marine habitats. The microscopic size of the basic producers, the 

 algae, offers a high surface-volume ratio for the utilization of radiant energy 

 and the absorption of nutrients. The penetration of light to considerable 

 depths results in photosynthetic activity over an appreciable vertical range, 

 a feature only slightly approached on land even in forest regions. The char- 

 acteristic minerals of sea water plus those contributed from land drainage 

 provide a rich supply of important nutrients. (With the exception of the 

 minerals of sea water, the above characteristics apply to many lakes, which 

 explains why, with a favorable nutrient supply, they may equal or excel 

 marine waters in production.) 



From the marine fisheries standpoint it is a matter of record that relatively 

 shallow waters, as in estuaries and over the continental shelf, are the great 

 producers. Such waters are usually so located that they may benefit con- 

 siderably from land-drained nutrients and in many places they are enriched 

 by up welling currents from the ocean depths. Also, as living things die 'and 



