CHAPTER 4. OYSTER REEF DEVELOPMENT, DISTRIBUTION, 

 PHYSICAL EFFECTS, AND AREAL EXTENT 



4.1 REEF DEVELOPMENT 



From a physical standpoint, a reef is 

 a biologically constructed, wave-resistant 

 or potentially wave-resistant structure. 

 Worldwide, reefs range from mounds less 

 than 1 m in height and diameter to massive 

 structures 1,000 m across and 100 m thick 

 (Pettijohn 1975). In general, reef mor- 

 phology is a function of the constituent 

 organisms and organism byproducts of which 

 it is built, whether these organisms are 

 corals, encrusting or sediment-binding 

 algae, tube-building polychaetes, or oys- 

 ters. 



The thesis presented here is that the 

 location of oyster reefs in the salt 

 marsh-estuarine ecosystem is not acciden- 

 tal; rather, it is the result of interact- 

 ing physical and biological processes 

 that, if fully understood, would explain 

 the natural distribution of reefs in a 

 given area. Marshall (1954) concluded from 

 a study of the distribution of oyster bars 

 in Alligator Harbor, Florida, that physio- 

 graphic conditions and predation were the 

 most important factors. 



In terms of physical conditions, a 

 minimum stability is undoubtedly required; 

 that is, a water current or wave energy 

 regime above a certain threshold level 

 will prevent the development of an inter- 

 tidal oyster reef. At the same time, the 

 development of a reef presumably affects 

 the physical stability of an area by damp- 

 ening current velocity and wave energy. To 

 be viable, a reef also needs a minimum 

 current velocity for the input of food and 

 the export of waste products. The local 

 reef area could thus be self-limited by 

 its dampening influence on the current 

 regime. 



The following general model of oyster 

 reef initiation, "ontogeny," and decline 

 has four stages: (1) initial colonization, 

 (2) clustering phase, (3) accretionary 

 phase, and (4) maturation and senescence. 



Initial Colonization 



Initial reef formation begins with 

 the settlement and growth of single oys- 

 ters and small scattered oyster clusters 

 within the lower intertidal zone. A suit- 

 able substrate must be present for the 

 settlement of oyster spat and initial oys- 

 ter growth in an area where water flow is 

 sufficient to prevent stagnation (Galtsoff 

 and Luce 1930). Suitable substrates may 

 consist of either sand, firm mud, or clay. 

 Shifting sand and extremely soft mud are 

 the only bottom types totally unsuitable 

 for oyster communities (Galtsoff 1964). 

 Oyster larvae will attach to any hard 

 object, such as fallen trees, driftwood, 

 bushes, branches, old shell material, or 

 discarded solid waste (bottles, cans, 

 plastic, etc.) exposed in the intertidal 

 zone. It is important that the areas be 

 subject to little sediment deposition. 



Clustering Phase 



With time, additional generations of 

 oyster larvae will settle in the area of 

 the new reef and attach themselves to 

 other live oysters and dead shell sur- 

 faces. This process results in the forma- 

 tion of distinct oyster clusters. A clus- 

 ter is a small colony of three to seven 

 generations of oysters, the majority of 

 which are dead (Grave 1905). The oldest 

 and lowest oysters in the cluster die from 

 overcrowding and suffocation, but their 

 shells remain to support the upward and 

 outward growth of the cluster. This sup- 

 port is aided by the relatively flat shape 

 and low specific gravity of oyster shells. 



Accretionary Stage 



Small oyster clusters increase in 

 size through the settlement of additional 

 spat and eventually coalesce, forming 

 larger, massed oyster clusters (Grinnell 

 1971) that comprise the true construc- 

 tional nucleus of the intertidal oyster 

 reef. If environmental conditions remain 



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