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wet lands, they are part of a dynamic system and in fact have an ac- 
tive role as well as passive in the biological community. 
Shallow parts of the wet lands owe their existence to the balance 
between erosion and deposition. Thus in the steady state, tidal flats, 
even those with no protective vegetation, are maintained at a fairly 
constant level, the sediment eroded during storms, and carried seaward 
is replaced by new sediment from the estuary. There is constant 
deposition over the shallow wet lands (since some material must 
always settle out of suspension), and there is likewise constant erosion, 
the two processes acting in turn. The balance between these processes 
is largely dependent upon the amount of material suspended in the 
water. Erosion is largely independent of the concentration of suspen- 
soids, so that given constant physical conditions (range of tides, 
current velocities, wave height), erosion will remain unchanged, even 
if the amount of suspended sediment is altered. Deposition, on the 
other hand, is highly dependent upon the concentration of suspended 
sediments, both because the number of particles which settle to the 
bottom is proportional to the concentration of particles, and because 
various physical and chemical processes increase deposition as particle 
concentration is increased. Since sea level is rising slowly (it has been 
rising since the last glaciation), deposition must slightly exceed 
erosion for balance to be achieved. The factors controlling the balance 
of sedimentation and erosion are in fact much more complex than this, 
but for our purposes, we may assume that undisturbed, the wet lands 
will reach some sort of equilibrium, in which the various parts of the 
environment, the marsh, tidal flats, and open estuary achieve a stable 
configuration, more or less. In the next section the impact of man 
upon this equilibrium will be considered. 
Considerable attention has been given in the literature to the 
effects of the biota upon the processes of sedimentation, but much 
less work has been done on the converse problem. Erosion of the salt 
marsh surface must affect productivity; burial of the benthic micro- 
flora must affect their growth. The solutions to these problems await 
further research, but some speculations are in order. Ranwell’ has 
measured rates of sediment accretion in British salt marshes, and 
found rates of up to 10-12 cm./yr., supporting similar reports of 
much earlier workers.’ There seemed to be no adverse effect of such 
high rates of sediment accumulation upon Spartina, indicating that 
higher plants may not be limited by high rates of deposition. The 
effect of this rapid deposition upon the microflora was not studied. 
Richards ° arrives at somewhat different conclusions from his studies 
of the Dovey Estuary, which indicate that in areas of rapid accretion, 
most of the energy of higher plants is required for upward growth of 
the main stem, with little development of branches. Faster accretion 
could presumably overwhelm the plants, leaving a barren surface. It 
is clear, however, that this would not represent an equilibrium con- 
dition: the factors controlling deposition will not allow continuous 
rapid accretion in the wet lands for long periods, especially if plants 
are absent. Thus we may conclude that rapid deposition may curtail 
the net production of higher plants, but will probably not eliminate 
their growth. i ; ceil 
It would seem that rapid accretion might affect the benthic micro- 
flora more markedly than the higher plants. The ability of diatoms to 
Footnotes at end of article. 
