to account for high estuarine produc- 

 tivity without calling for a fertili- 

 zation of the estuary by river in- 

 puts. In general, there are four 

 major themes other than freshwater 

 input which appear in the litera- 

 ture, and I think the quotes given 

 below will give a good feel for the 

 diversity of opinion they represent. 



1. Fertilization by advection of 

 deeper offshore waters 



General conclusions are that 

 the usual pattern of exchange 

 between inshore and offshore 

 waters tends to enrich the 

 coastal zone irrespective of 

 enrichment by freshwater 

 drainage. .. (Riley 1967). 



2. Fertilization by marshes 



Apparently, large rivers do 

 not have as great a local effect 

 on the productivity of estua- 

 ries and coastal waters as 

 was once assumed. I think the 

 most important discovery we 

 have made in our 15 years 

 study of production dynamics 

 on the Georgia coast is that 

 the high fertility of this 

 region is self-produced with- 

 in the salt-marsh estuary, and 

 is not due to nutrients wash- 

 ed down the rivers (Odum 1968). 



3. Fertilization by concentration 

 - the nutrient trap 



In estuaries fresh water de- 

 rived from the land... mixes 

 with sea water and is carried 

 seaward in the upper layer of 

 the embayment. A counter-cur- 

 rent of sea water moves in 

 from the outer sea to replace 

 that entrained in the surface 

 outflow. .. consequently , the 

 redistribution of nonconserva- 

 tive elements by the sinking 



of organized matter will tend 

 to cause the concentration of 

 N to increase upstream rela- 

 tive to the motion of the sur- 

 face layer. The estuarine 

 circulation creates a trap in 

 which nutrients tend to accum- 

 ulate. (Redfield et al 1963). 



4. Fertilization by rapid recycling 



. . .For the Georgia and South 

 Carolina shelf, nutrient in- 

 flux to the coastal zone via 

 outwelling is of minor im- 

 portance, mixing of deep water 

 across the edge of the shelf 

 is of minor importance, and 

 in situ regeneration is the 

 most important process in 

 maintaining high rates of nu- 

 trient flux and hence high 

 rates of biological produc- 

 tivity in the shelf waters. 

 (Haines 1975). 



Each of these mechanisms deserves 

 a serious consideration that is 

 beyond the scope of this paper. 

 But as a start, the various possi- 

 bilities can be brought together 

 in a conceptual model (Figure 2) , and 

 we can begin to focus on fresh 

 water input as one of at least five 

 alternative explanations for estu- 

 arine production. It may be, of 

 course, that different explanations 

 apply to different estuaries or 

 that estuarine productivity is a 

 consequence of all of these things 

 happening together, a conclusion 

 reached by Correll (1978) in his 

 recent consideration of the problem: 



Thus, estuaries maintain high 

 production by maintaining high 

 nutrient levels in bottom sed- 

 iments and water column. This 

 is done by nutrient/plankton 

 trapping via the "salt wedge" 



35 



