CHAPTER 6 

 TIDAL FLATS: THEIR IMPORTANCE AND PERSISTENCE 



6.1 INTRODUCTION 



It has been recognized since the late 

 1950's that nearshore marine habitats, 

 particularly estuaries and coastal embay- 

 ments, are vitally important as nursery 

 and spawning grounds for fishes and as 

 habitats for shellfish. Tidal flats func- 

 tion in many of the same ways as deeper- 

 water, coastal habitats in addition to 

 providing resting and feeding sites for 

 coastal birds. Because the coastal zone 

 is heavily used for other land- and 

 marine-based recreational and commerical 

 purposes, tidal flats frequently are sub- 

 jected to reversible and irreversible man- 

 induced environmental impacts. Conflicting 

 demands on the use of tidal flats necessi- 

 tate legislative participation in the man- 

 agement of these areas and it is important 

 to address questions such as: How valu- 

 able are tidal flats relative to other 

 coastal habitats and how resistent or 

 resilient are tidal flat organisms to 

 environmental perturbation? In other 

 words, can we afford to lose tidal flat 

 habitats without experiencing unacceptable 

 alterations in the productivity of marine 

 biota? 



6.2 RESPONSE OF TIDAL FLATS TO ENVIRON- 

 MENTAL PERTURBATIONS 



The majority of man-induced impacts 

 on tidal flats can be categorized as 

 follows: (1) dredging and channelization 

 to maintain navigable waterways and the 

 construction and maintenance of water- 

 dependent industries or businesses (e.g., 

 marinas), (2) discharge of pollutants from 

 waste disposal and industrial outfalls or 

 non-point sources (e.g., sewage, chemi- 

 cals, oil), (3) building of dams and jet- 

 ties resulting in altered inorganic depo- 

 sition, (4) spoil disposal for the crea- 

 tion of salt marshes, or landfill for 

 residential and/or commercial purposes. 



and (5) overexploitation of commercially 

 important tidal flat shellfish. 



The response of tidal flat organisms 

 and their ability to recover from man's 

 activities depends upon the type, magni- 

 tude, and frequency of the impact. Envi- 

 ronmental impacts can be classified as 

 those which are (1) destructive (e.g., 

 dredging and spoil disposal) and result in 

 changes in habitat quantity or (2) those 

 that alter habitat quality (e.g., exces- 

 sive organic pollution) and result in the 

 degradation of the habitat. 



The most easily detected effects upon 

 tidal flats are those that lead to habitat 

 destruction. Generally these impacts are 

 incremental and vary widely. Dredging 

 and spoil disposal, for instance, can 

 result in dramatic changes in the physi- 

 cal, chemical, and biological nature of a 

 tidal flat. When these perturbations are 

 taken to extremes, the result is irrevers- 

 ible habitat loss or modification. Dredg- 

 ing eliminates feeding sites for shore- 

 birds and spoil deposition destroys ben- 

 thic invertebrates and feeding sites for 

 vertebrates. 



The response of tidal flat popula- 

 tions to severe habitat alteration has 

 usually been studied by examining change 

 in species ^ composition and abundance 

 following perturbation. Field studies may 

 involve monitoring the patterns of repopu- 

 lation by benthic organism.s following 

 spoil disposal (e.g., Rhoads et al. 1978) 

 or after experimental elimination of the 

 fauna in relatively small areas (e.g., 

 Grassle and Grassle 1974; McCall 1977; 

 Zajac 1981). Despite differences in the 

 type of disturbance, environmental charac- 

 teristics, and species composition consid- 

 ered, there are common trends in benthic 

 community re-establishment and develop- 

 ment. Early colonizers of a disturbed 

 habitat are small species, predominately 



66 



