for expressing the functional connections 

 in many different kinds of systems to com- 

 pare these systems in thermodynamic 

 (energy flow) terms. Odum calls the short- 

 hand "energese," and it is becoming more 

 popular, as evidenced by its increasing 

 use in published reports. This shorthand 

 "language" is flexible and information- 

 rich, and it can be used in both qualita- 

 tive conceptual models and in quantitative 

 "working" models. The symbols are defined 

 in Figure 22, taken from Odum (1971). 



5.2 REGIONAL LEVEL CONCEPTUAL MODEL 



The regional level model of oyster 

 reef function in the study area is broad 

 in its coverage and necessarily quite 

 simple. At this level of resolution, oys- 

 ter reefs were probably not a major factor 

 in the geomorphological development of the 

 area, although their wide surface distri- 

 bution and largely unknown subsurface 

 (fossil) distribution indicate that they 

 indeed may have played a geological role. 

 No one has as yet quantified the physical 

 importance of oyster reefs to long-term 

 coastal processes. 



In Figure 22 we illustrate the theo- 

 retical role of oyster reefs at this broad 

 regional scale. As indicated in Table 8, 

 the time-scale of change at the regional 

 level is in the geological range, outside 

 the realm of control of environmental 

 managers (although not immune to cultur- 

 ally induced alteration). 



The major process symbolized in the 

 regional scale conceptual model is the 

 dynamic tradeoff in area between inter- 

 tidal and subtidal zones. Oyster reefs 

 primarily are distributed at the interface 

 between these two zones, and thus the reef 

 "fringe" partially reflects the outline of 

 the marsh-water interface throughout the 

 study area. Changes in the position of 

 this outline are a function of such long- 

 term processes as subsidence, sea level 

 rise, and sedimentation regimes. For all 

 practical purposes, reef distribution at 

 the regional level can be considered spa- 

 tially homogeneous. 



Interactions between the intertidal 

 and subtidal zones are described in the 



order of the work gates (1-4) shown in 

 Figure 22. 



(1) A gradual and persistent rise in 

 sea level (about 4 mm/yr) has 

 occurred since the relative sta- 

 bilization of mean water level 

 (MWL) following the last ice 

 age. This has resulted in a con- 

 stant encroachment upon the in- 

 tertidal zone by open water. In 

 the absence of other processes, 

 the intertidal zone would even- 

 tually become open water. 



(2) The loss of intertidal area is 

 accelerated by erosion from 

 strong tidal currents and storm 

 surges. 



(3) Losses of intertidal habitat are 

 offset in most undisturbed por- 

 tions of the study area by in- 

 puts of sediment from rivers 

 and/or from the marine system. 

 This sedimentation process is 

 augmented by increases in the 

 volume of estuarine basins as a 

 function of sea level rise. Mean 

 water current velocities decline 

 as volume increases, and sedi- 

 mentation is enhanced. 



(4) Latitude determines tidal ampli- 

 tude in the study area, which, 

 in conjunction with sediment 

 sources, regulates the deposi- 

 tional patterns. 



5.3 DRAINAGE UNIT LEVEL CONCEPTUAL MODEL 



The components and interrelationships 

 of a marsh estuary drainage unit including 

 and affected by oyster reefs are shown in 

 Figure 23. A major assumption at this lev- 

 el of resolution is that there is an opti- 

 mum ratio of wetlands and open water 

 which, in conjunction with tides, support 

 the oyster reef area in a given drainage 

 basin. One implication of this assumption 

 is that relative reef area in a given 

 drainage unit is limited by ecosystem lev- 

 el processes, (e.g., the relationship be- 

 tween the velocity of tidal currents, the 

 cross-sectional area of tidal creeks, and 

 the distribution of reefs). This thesis is 



71 



