salt marsh plants are composed largely of struc- 

 tural proteins (lignins, etc.), they are relatively 

 poor in nitrogen and are not a nutritious food 

 source (Tenore 1977). Many workers feel that 

 only after the plant material has been colonized 

 by bacteria is the plant's energy truly available to 

 detritivores and higher level consumers in the 

 food web. In other words, salt marsh detritus 

 must ordinarily be transformed into bacteria be- 

 fore any consumer can derive benefit from con- 

 suming it. Such a conversion, by the second law 

 of thermodynamics, is associated with a loss in 

 energy available to the next trophic link. In con- 

 trast, algae are highly nutritious and are ordinarily 

 consumed directly by various species in the estu- 

 arine system. Exactly how much one must depre- 

 ciate the amount of marsh plant productivity to 

 account for the energy loss involved in passing it 

 through bacterial intermediates before it is con- 

 sumed is not clear and needs further study. 



Answers to these ecological questions are 

 necessary before estuarine habitats can be accu- 

 rately compared. In any event, is seems clear that 

 intertidal sand and mud flats represent a very im- 

 portant habitat in an estuarine ecosystem. Since 

 the lack of hard facts makes it virtually impossi- 

 ble to evaluate the worth of one acre of mud flat 

 compared with one acre of salt marsh, it would 

 seem prudent to protect and treasure both types 

 of estuarine acreage. Disposal of dredge spoil 

 should certainly not be accomplished by dumping 

 it on unvegetated flats simply because they 

 appear unproductive. They are indeed productive 

 and their output is directly usable by consumers. 

 Probably, the use of dry land habitat above the 

 high tide ■ line for disposal of spoil should be 

 favored. There the spoil is rapidly colonized by 

 land plants, which undergo a normal process of 

 succession (Soots and Parnell 1975). Such spoil 

 dumps on land are immediately useful as nesting 

 habitat for many shorebirds, including several 

 gulls and terns which prefer to nest on relatively 

 unvegetated areas (Soots and Parnell 1975). 

 Creation of spoil areas thereby helps to take the 

 place of ocean beaches, many of which are now 

 too disturbed to act as breeding sites for these 

 ground-nesting birds. 



6.3 THE ROLES OF INTERTIDAL FLATS 

 IN ESTUARINE SYSTEMS 



Because information on the function of inter- 

 tidal flats is scattered throughout the earlier chap- 



ters, a concise summary is presented here. The 

 proportion of an estuary's total primary produc- 

 tion that occurs in the sediments of intertidal 

 flats is small relative to the contributions from 

 salt marshes and from the water column. Water 

 column production is enhanced by the presence 

 of intertidal flats through increasing the euphotic 

 zone and through rapid recycling of mineral nu- 

 trients. This enhancement is substantial in any 

 estuary where intertidal flats cover a large propor- 

 tion of the estuarine area. Despite the relatively 

 low total primary production from intertidal 

 flats, the algae produced there are directly utiliz- 

 able by consumers. 



The production of utilizable plant material is 

 only one function of the intertidal flat habitat. A 

 major role of the intertidal flat habitat is to serve 

 as the substrate where primary production is con- 

 sumed and thereby transformed into animal bio- 

 mass. Studies of salt marshes and of seagrass beds 

 emphasize a common characteristic: most of the 

 primary production of both of those habitats is 

 not utilized in situ but is instead carried away by 

 water currents. This is not true of intertidal flat 

 algae, which are directly consumed by deposit- 

 feeding and suspension-feeding benthic inverte- 

 brates on the flat. More importantly, the inter- 

 tidal flat habitat serves as the site where much of 

 the exported production from salt marshes and 

 seagrass beds is deposited and subsequently con- 

 sumed by and transformed into benthic inverte- 

 brates. Intertidal sand and mud flats thus func- 

 tion not only as important producers of plant 

 matter, but even more significantly as a major site 

 of conversion of plant matter from all esuarine 

 habitats into animal tissue. 



The benthic invertebrates living on intertidal 

 flats and utilizing both the locally produced and 

 the imported plant material are themselves a 

 major food source for higher level consumers. 

 These higher level consumers include blue crabs, 

 shorebirds, some shrimp, and larger bottom- 

 feeding fishes. Predation from these sources is 

 usually sufficient to keep the population densities 

 of benthic invertebrates at low levels on intertidal 

 flats. Such low densities belie the importance of 

 the benthos in the transfer of energy to higher 

 level consumers. Most shorebirds are totally 

 dependent upon the intertidal flat as a feeding 

 ground. 



Intertidal flats serve not only as a primary site 



62 



