supported by the relatively static distri- 

 bution of reefs within the Duplin River 

 basin over time, shown in Figure 21. 



Specific interactions shown in Figure 

 23 are described below: 



(1) The local tidal regime is the 

 primary forcing function for 

 oyster reef distribution (and 

 relative area) in a given salt 

 marsh drainage unit. The tidal 

 effect is shown interacting si- 

 multaneously with water area and 

 wetland area. These respective 

 components (water and wetlands) 

 have a 1 to 2 ratio in the Geor- 

 gia marsh-estuarine ecosystem 

 (Pomeroy and Wiegert 1980). The 

 pattern of distribution of oys- 

 ter reefs in the Duplin River, 

 as shown in Figure 21, is prob- 

 ably not a chance distribution. 

 For example, oyster reefs are 

 absent from the upper one-fourth 

 of the basin, probably because 

 of ecosystem level processes 

 (e.g., a function of reduced 

 current velocities in the upper 

 reaches of the river). 



(2) Oyster reef area in a given lo- 

 cale can affect local turbidity 

 levels by filtration and bi ode- 

 position. By stabilizing and 

 elevating sediment, wetland de- 

 velopment can be enhanced. Marsh 

 grass and oyster reefs have a 

 reciprocal functional relation- 

 ship in that reefs develop al- 

 most exclusively at the inter- 

 face between wetland and water. 

 There they subsequently grow and 

 trap sediment, eventually becom- 

 ing colonized by Spartina . The 

 marsh invades formerly subtidal 

 areas in this leapfrog fashion. 

 For example, subsurface (fossil) 

 oyster reefs occur in a pattern 

 of increasing depth extending 

 from an existing reef into the 

 marsh. (S. Stevens, University 

 of Georgia Marine Institute, 

 Sapelo Island; pers. comm. ). 



(3) Suspended materials in water 

 column inhibit the primary pro- 

 duction by phytoplankton as a 



result of shading. Therefore, 

 oyster reefs theoretically aug- 

 ment phytoplankton productivity 

 by actively filtering these 

 materials and thereby reducing 

 turbidity. 



(4) Oyster reefs in local areas also 

 contribute to primary production 

 (especially of phytoplankton and 

 benthic algae) by rapidly miner- 

 alizing ingested organic matter 

 into usable plant nutrients. 

 Kuenzler (1961) showed that the 

 regeneration of phosphorus by 

 mussels in the salt marsh was 

 more important than their role 

 in energy transformation. Kit- 

 chell et al. (1979) discussed 

 the roles of consumers in nutri- 

 ent cycling. Oyster reefs by 

 Interactions (3) and (4) can 

 increase food availability, pro- 

 viding feedback in keeping with 

 ecosystem theory, (e.g., Odum 

 1971). 



(5) Tidal currents maintain extreme- 

 ly high suspended sediment loads 

 in some study area estuaries, 

 like the Duplin River (Hanson 

 and Snyder 1979). The conse- 

 quences of this siltation relate 

 to Interactions (2) and (3). 



5.4 REEF LEVEL CONCEPTUAL MODEL 



The third conceptual model is shown 

 in Figure 24, where reef development is 

 expressed as growth in three dimensions: 

 (1) upward toward the high intertidal 

 zone, (2) downward toward the subtidal 

 zone, and (3) lateral accretion. 



The interactions involved in such 

 changes are described below: 



(1) Ingestion by oysters and other 

 suspension-feeding members of 

 the reef community is affected 

 negatively by increased water 

 turbidity (Section 2.3). 



(2) Turbidity of estuaries in the 

 study area is usually high and 

 closely related to the high 

 tidal current regime. Thus, 



74 



