178 
tion. In most estuaries the bulk of sedimentation takes place in the 
peripheral, quiet areas, producing shallow flat sand banks. These, in 
turn, offer support for rooted marsh plants, which act as sediment 
stabilizers and accumulators, leading eventually to the development 
of the salt marsh. in areas of the United States where mangroves 
thrive, this process may be enhanced by the seeding of mangroves 
on sandy bottoms, followed by sediment accumulation and the 
buildup of a community. 
PRIMARY PRODUCTION 
The primary producers of the estuaries and associated salt marshes 
are of three types, including higher plants, algae, both on and in the 
sediments, and phytoplankton. Because of the basic instability of the 
environment, caused in part by the constantly shifting balance of 
deposition and erosion, the number of species in any wetland area is 
small. The competition between species is characteristically not 
intense, but rather the ability to withstand high salinity, strong winds, 
erosion or burial, salt spray, and periodic inundation is essential to the 
survival of higher plants. Likewise, algae must be able to withstand 
high turbidity, low salinities, tidal, and other environmental fluctua- 
tions. Many plants found in the wet lands have worldwide distribu- 
tions within their physical tolerances. Widespread dispersal is due in 
part to oceanic circulation, which distributes resistant seeds and plant 
parts, and in part to transplantation by man!. The uniformity of 
wetland communities offers considerable encouragement, since this 
would indicate that implications from detailed studies may be general- 
ized with more certainty than is generally possible in nonmarine 
ecosystems. 
Among the higher plants, the distribution of species in a given area 
is controlled largely by the range of tides, and a succession of plant 
groups generally can be discerned proceeding from the deeper estuary, 
through the tidal flats and up onto the meadowy surface of the higher 
marsh. Below low tide levels, only those plants capable of withstanding 
continued submergence are found. Eel grass (for example Zostera 
marina) is a typical representative of this group. It flourishes on the 
muddy substrate found in protected estuaries, and grows in depths 
from slightly above mean low tide level downward to considerable 
depths, perhaps 75 feet in some areas ?. 
In the region extending from low tide level upward to about mean 
sea level, including the areas of mud flats, seed plants are often 
absent except for local colonies of eel grass or salt marsh grass. Tidal 
flats present an unusually harsh environment for plants, with charac- 
terestic extreme temperature fluctuations and rapid water movements. 
Salt marshes are typically developed on mud or muddy sand, 
between mean half tide and high tide levels, where perennial grasses 
or grass-like plants predominate. Lower levels within the salt marsh 
are exposed to the greatest influence of tidal influx, and coarse, reedy 
grasses predominate, while at higher levels, finer, lower grasses are 
found. Various varieties of Spartina are characteristic of the salt 
marsh vegetation ”. 
Mangrove communities constitute a separate class in this discussion, 
which to this point has dealt with plants typical of temperate wetlands, 
similar to those found on the eastern coast of the United States. In 
Footnotes at end of article. 
