CARBON IN ESTUARIES 227 



net carbon balance of a unit of landscape. It was defined simply enough as the 

 difference between gross production (GP) and total respiration (Re): 



NEP=GP-R E (1) 



There appears to be advantage in extending the concept of net ecosystem 

 production to apply to ecosystems where exchanges of fixed carbon with other 

 systems are important by accounting for both inputs and losses. The original 

 concept of net ecosystem production appears best preserved by the following 

 modification of the equation: 



NEP = (GP + NP,) - (R E + NP ) 



= rate of accumulation or loss of carbon from 

 sediments or the biota (2) 



where NPj is net production in and NPq is net production out. 



A principal objection to this formulation might be based on the possibility 

 that NEP could be positive through import of fixed carbon into an area that had 

 no gross production. We consider this a minor fault outweighed by the advantage 

 that simple summing of NEPs for separate systems provides the NEP for the 

 larger unit and ultimately for the biosphere as a whole. 



The importance of exchanges in estuaries with uplands and with coastal 

 oceans has been emphasized recently by recognition of the possibility that large 

 amounts of fixed carbon may be transported as dissolved organic matter in 

 water. 15 A daily net export of 1 mg C/liter in tidal water 1 m deep would be 

 equivalent to 1200 to 1500 g (dry organic matter) m 2 year ' or the net 

 production of many vegetations. This observation simply emphasizes the 

 possibility that NEP for estuaries could be large and either positive or negative. 

 The assumption seems questionable, however, that NEP drops to zero in 

 estuaries at successional maturity as it seems to do in terrestrial systems. 

 According to this analysis a positive NEP would indicate a net storage within the 

 estuary, either as sediment or as increased biota; a negative NEP would indicate a 

 net loss, which would appear as an input to some other system elsewhere. The 

 possibility exists of course that, in estuaries where imports of fixed carbon may 

 be large, total respiration is substantially higher than gross production. No clear 

 indication of whether this is commonly or even rarely true is available at present. 

 Data are, however, available on net production of estuaries and associated 

 marshes, and limited inferences are possible. 



A summary of data on the productivity of estuarine marshes is presented in 

 Tables 3 to 6. Four different types of vegetation contribute to the productivity 

 of estuaries: salt marshes and mangroves, submerged angiosperms, epiphytic and 

 benthic algae, and phytoplankton. In each instance the number of separate 

 estimates is small. The estimates of net production are difficult to compare, one 

 with another and with studies of terrestrial ecosystems, because of differences in 



