estuarine habitats that the species which inhabit 

 them must be well-adapted to recovery following 

 disturbances. Such recovery capability implies 

 that the system is capable of withstanding the im- 

 pact of some carefully controlled, periodic 

 dredging without substantial long-term ecological 

 harm. This is probably a more accurate descrip- 

 tion of temperate-zone estuaries than it is of 

 tropical systems where natural environmental 

 variability is reduced (Copcland 1970). 



6.2 THE SIGNIFICANCE OF AN 

 INTERTIDAL FLAT 



When one is asked to place a value on a habi- 

 tat or upon an ecosystem, the answer requires 

 some sort of value judgement which is necessarily 

 subjective. Nevertheless, planners are often forced 

 to make decisions which require the ranking of 

 certain habitats or ecosystems on some basis of 

 worth. Such comparative rankings are certainly 

 easier to reach and more easily defended than 

 absolute valuations. 



The importance of intertidal flats to estuarine 

 systems is addressed either implicitly or explicitly 

 in almost every earlier section of this publication. 

 For instance, the significance of the tidal flats to 

 the shorebirds of a coastal wetland has been em- 

 phasized. It is clear that the majority of probing 

 and wading shc^rebirds do virtually all of their 

 feeding on intertidal flats. For those species and 

 for the avian segment of estuarine ecosystems in 

 general, the intertidal flat habitat is clearly of far 

 greater significance than the salt marshes, the sea- 

 grass beds, or any of the other estuarine habitats. 

 However, it is not clear whether shorebird popula- 

 tions are limited by the extent of the intertidal 

 flats available. Recent studies (Goss-Custard 1977) 

 have addressed this question without reaching a 

 definitive answer. Habitat availability is limiting 

 to the populations of many other types of birds, 

 so it would not be surprising to learn that shore- 

 birds are similarly dependent upon the areal 

 extent of intertidal flat habitat. 



For the other large consumers in an estuarine 

 ecosystem, the shrimp, crabs, and fishes, the rela- 

 tive importance of the intertidal flat habitat is not 

 as clear. The relative importance of various estu- 

 arine habitats to species within these categories 

 depends upon the relative contributions of the 

 various types of primary producers lo the food 



chains upon which these consumers depend. In 

 other words, more 6 '^C data such as those gener- 

 ated by Haines and Montague (1979) and Thayer 

 et al. (1978) are badly needed. These 6 "C ratios 

 permit one to make inferences concerning the 

 importance of marsh plants versus algae versus 

 seagrasses in the nutrition of consumers, as is de- 

 scribed in Chapter 2. 



As an approximate evaluation of the relative 

 importance of the various groups of primary pro- 

 ducers in an estuarine ecosystem, one might 

 simply total the annual production of each plant 

 type in the system. This was done by Bigelow 

 (1977) for the Newport River estuary in North 

 Carolina, and the results appear in Table 1. This 

 technique would imply that the salt marshes, for 

 example, contribute 42% of the energy ultimately 

 consumed by estuarine species. There are three 

 serious drawbacks in utilizing primary productivity 

 values in this fashion to represent the relative 

 importance of various types of plants in estuarine 

 food chains. First, it is not known how much of 

 the annual primary production is actually utilized 

 by consumers. Some of the plant material is 

 carried out of the estuarine system by tidal 

 currents and river flow. Some plant material is 

 decomposed before it can be used by consumers, 

 and some is buried too deeply in the sediments to 

 be available to consumers. 



The second problem with utilizing primary 

 production data to estimate the relative impor- 

 tance of each plant type lies in the realization 

 that consumers differ greatly from one another in 

 diet. For a given species, one type of plant will be 

 far more important and others far less important 

 in its diet than is suggested by the relative contri- 

 bution of that plant to the total primary produc- 

 tion of the estuarine system. For example, the 

 oyster feeds almost exclusively on algae, including 

 various phytoplankters and probably also sus- 

 pended benthic microalgae. One needs to know 

 much more about the fate of each source of 

 primary production before evaluation of the 

 significance of various types of plants and various 

 habitats can be appropriately made. 



The third major reason for avoiding the use of 

 simple productivity measures to estimate the rela- 

 tive importance of various types of primary pro- 

 ducers is implicit in the earlier discussion of the 

 role of bacteria in estuarine ecosystems. Because 



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