CHAPTER 1 . THE PHYSICAL ENVIRONMENT 



1.1 DEFINITION OF AN INTERTIDAL FLAT 



Intertidal flats represent one habitat among 

 several that comprise an estuarine system (Figure 

 1). Although the other extensive habitats (i.e., salt 

 marshes, seagrass beds, unvegetated subtidal bot- 

 toms, and the overlying water column) will not be 

 directly described in this community profile, it 

 will be necessary to compare the intertidal flats 

 with these other estuarine habitats. Science is a 

 comparative process such that data are only 

 meaningful relative to other contrasting data. The 

 intertidal flat habitat, like the estuarine eco- 

 system as a whole, is an open system physically 

 and biologically, with the consequence that nutri- 

 ents, organic particles, and living organisms move 

 readily in and out of the habitat. An understand- 

 ing of the ecology of an intertidal flat requires 

 some knowledge of processes occurring elsewhere 

 within the broader estuarine system. 



Intertidal flats are defined as those portions 

 of the unvegetated bottom of sounds, lagoons, 

 estuaries, and river mouths which lie between the 

 high and low tide marks, as defined by the ex- 

 tremes of spring tides. Intertidal flats occur along 

 the shorelines of islands and of the mainland, but 

 some emergent soft bottom also occurs in areas 

 unconnected to dry land. Such bars and flood 

 tidal deltas are not directly considered in this text, 

 although these habitats share many of the 

 characteristics of a true intertidal flat. An inter- 

 tidal flat is unvegetated only in the sense that it 

 lacks macroscopic plants such as grasses, shrubs, 

 and seagrasses. Benthic microalgae, such as dia- 

 toms and blue-green algae, are usually very abun- 

 dant. Intertidal flats are composed of sandy and 

 muddy sediments in a wide range of relative pro- 

 portions. Ocean beaches are specifically excluded 

 from this definition of an intertidal flat. 



1.2 SEDIMENTARY ENVIRONMENT 



Soft sediments bear a clear stamp of the phys- 

 ical environment in which they are found. Coarser 

 sediments (sands and even highly abraded shell 

 fragments and pebbles) dominate in relatively 

 high energy environments, whereas fine sediments 

 (silts and clays) are indicative of environments of 



lower physical energy (Sanders 1958, Warme 

 1971). Sediment texture reflects the physical en- 

 vironment because fine sediments are suspended 

 and ultimately transported away from areas of 

 high water turbulence and rapid velocities (high- 

 energy locales), whereas these same silts and clays 

 are deposited out of the water column in still 

 waters of low turbulence (low-energy locales). 



Tidal currents exhibit their highest velocities 

 within channels in the immediate vicinity of inlets 

 that connect the ocean with an estuary or lagoon 

 (Figure 2). Tidal current velocities within an estu- 

 ary or lagoon gradually decline with increasing 

 distance from the inlet and with decreasing water 

 depth. The inlet itself is an area of very high phys- 

 ical energy, characterized by relatively coarse 

 sands and abraded shell fragments. Inlet sediments 

 are extremely mobile; even coarse sands are 

 transported readily by the strong currents. New 

 sand flats are continually forming and old ones 

 eroding. Such high substrate instability renders 

 this environment inhospitable to nearly all species 

 of benthic macrofauna. Densities of macrofaunal 

 benthic invertebrates are so low on most intertidal 

 flats in inlets that one might justifiably consider 

 this environment almost devoid of benthos. 



At some distance from the inlet or on semi- 

 protected shallow flats not far from the inlet 

 where tidal currents have slowed sufficiently, the 

 intertidal flats are composed of finer, but still 

 sandy, sediments which are stable enough to sup- 

 port a benthic community (Figure 2). In general, 

 the silt and clay (i.e., mud) content of the sedi- 

 ments progressively increases on a gradient away 

 from the inlet. Technically, clean sand sediments 

 are considered to be those with a silt-clay fraction 

 (particles <62 microns in diameter) smaller than 

 5% by dry weight. Muddy sands contain between 

 5% and 50% silt-clay. True muds are composed of 

 greater than 90% silt-clay, while sandy muds pos- 

 sess a mud content of 50% to 90% (Folk 1974). 

 These distinctions should require that one dis- 

 tinguish between intertidal sand flats on the one 

 hand and mud flats on the other hand, although 

 in practice all intertidal flats are often inaccu- 

 rately termed mud flats. In this publication, this 

 sedimentological distinction will be retained 



