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areas favorable for mangrove growth, seedlings become established on 
shoals, banks, bars, and in bays and estuaries, on sandy soils which 
are commonly below sea level, but are moderately protected from 
waves. The young plants send out tiers of prop roots which act as 
traps for sediment and organic debris, and given proper conditions, 
the soil level may be built up into the mature swamp profile. After 
some 20 to 30 years growth, the mangroves become so thickly tangled 
that their roots act as highly effective sediment traps, and the com- 
munity becomes extremely resistant to the erosive forces of storms. 
Plants invade the swamp, taking advantage of the accumulating soil, 
and in the mangrove regions mature communities consist of salt marsh 
associations grading into mixed salt marsh and mangrove types.’ 
The second major group of wetlands primary producers are the 
algae. These may occur as attached macroscopic varieties, microflora 
living on or in the substrate, or as free living phytoplankton. The 
distribution of these types is dependant upon a variety of facters. For 
example, the attached forms may be lmited by tides, substrate, 
turbidity, temperature, salinity, pH, illumination, and wave height, 
among other factors. Red and brown algae are, on the whole, less able 
to adapt to reductions in salinity than are green algae, and thus those 
red and brown varieties which occur in the wetlands tend to pre- 
dominate in subtidal and lower zones near the mouths of estuaries 
and lagoons. Green algae, on the other hand, tend to predominate in 
the higher zones and near river mouths.* Some algae live on the 
soil, forming dense carpets in shallow water when conditions are 
favorable, and tend to stabilize marsh sand and mud; others live 
attached to stones or shells, and some are epiphytic on other algae or 
higher plants. Some types break loose from their substrate in rather 
large masses, and accumulate in deeper water. 
Microscopic algae living on and in the sediment of the intertidal 
zone constitute an important part of the wetland flora. Pennate 
diatoms, green flagellates, dinoflagellates, and blue-green algae are 
common varieties. Diatoms migrate vertically each tidal cycle, moving 
downward a few microns into soft, wet sediments before inundation 
by the rising tide. Shortly after the sediments are exposed by the 
falling tide, they migrate to the surface, forming a layer one to five 
cells thick. The distribution of the benthic microflora is deter- 
mined by a complex interaction of factors similar to those cited above 
for attached forms. 
The phytoplankton in estuaries is predominantly marine in origin, 
adapted to withstand greater or lesser dilution by fresh water. These 
forms are closely dependent on nutrients supplied from land by the 
inflowing rivers, and great increases in phytoplankton concentrations 
often occur following the flooding of streams, bringing a large influx 
of nutrients, or following artificial fertilization by sewage containing 
nitrates and phosphates. Offsetting the effect of increased nutrient 
supply accompanying periods of great river discharge, is the inhibiting 
effects of increased turbidity, which reduces the light penetration. 
Thus the phytoplankotn in estuaries is generally more abundant in 
the lower estuaries or immediately outside, than at the head. The 
reduced salinities at the heads of most estuaries is probably a con- 
tributing factor in this respect. arow gai 
This brief review of the primary producers should give an indication 
of the variety of plants which live in the wet lands. While it is by no 
Footnotes at end of article. 
