CARBON IN ESTUARIES 237 



an appraisal is important to a clear resolution of the role of estuaries in the 

 world carbon budget. Despite the possibility of a large influx of fixed carbon 

 from outside estuaries, the NEP of estuaries probably does not exceed over large 

 areas the mean net primary production of the estuary. 



Total respiration is the most direct measure of biotic function and of the 

 division of the carbon flux between metabolism and sedimentation. A limited 

 number of partial appraisals confirm that metabolism is high. 6,8 ' The most 

 convincing evidence, however, remains data from samplings of fish, which seem 

 to confirm that a major segment of the net production goes to support living 

 systems. Our review suggests that 0.1 to 1.0% of net production of estuaries may 

 be harvestable as fish. This efficiency of transfer of net primary production to 

 fish production is similar to the 0.25% estimated by Ricker for the North Sea. 

 Thus the higher productivity of fish in estuaries seems to be due not to higher 

 efficiency of energy transfer to fish but to higher primary production. 



CONCLUSION 



The simpler aspects of the morphology and hydrodynamics of the world's 

 estuaries have been described in detail in other works; this review shows that 

 what we know of the broadest biotic functions of estuaries is still largely 

 guesswork. Simple questions are still useful; What do estuaries do as units of the 

 biosphere? The most broadly useful answers would come through an evaluation 

 of the production equations for the world's estuaries, including an appraisal of 

 inputs, outputs, rates of sedimentation, net production, and total respiration. 

 These data are currently being acquired and should, with new data on the area of 

 estuaries available through satellite mapping, bring major improvements in the 

 precision of analyses, such as this one. 



ACKNOWLEDGMENT 



Research carried out at Brookhaven National Laboratory under the auspices 

 of the U. S. Atomic Energy Commission with supplementary support from 

 National Science Foundation Grant No. AG-375 to the authors. 



REFERENCES 



1. J. L. McHugh, in Estuaries, pp. 581-620, G. H. Lauff (Ed.), American Association for 

 the Advancement of Science, Washington, 1967. 



2. National Estuary Inventory, U. S. Department of the Interior, Fish and Wildlife Service, 

 Bureau of Sport Fisheries and Wildlife and Bureau of Commercial Fisheries, Superin- 

 tendent of Documents, U. S. Government Printing Office, Washington, 1970. 



3. National Estuarine Pollution Study, Report of the Secretary of the Interior to the U. S. 

 Congress, Superintendent of Documents, U. S. Government Printing Office, Washington, 

 1970. 



