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Thalaisa to waters of 2 meters or less depth is most probably due to 
this effect. Thalasia is not inhibited by strong illumination as is much 
of the wetlands population, so that it grows best in very clear, shallow 
water A small average increase in the turbidity of the water in which 
Thalasia is growing could therefore be expected to inhibit net produc- 
tion by this group, and perhaps reduce the area over which it is viable. 
The turbidity of the water might be least important to those 
microscopic algae which migrate downward into the sediments during 
periods of inundation, and at the other end of the spectrum, the 
turbidity of the water has a salutory effect on those plants for which 
the incident radiation is great enough to be inhibiting during summer 
months. Pomeroy * has presented photometric measurements which 
indicate that incident radiation may exceed the optimum value 
(cf. Ryther, 1956) by 0800 hours, and remain above optimum until 
1600 hours during the spring and early summer in Georgia salt marhes. 
He postulated that when the marsh is under water, much of the 
sediment surface is within the optimum illumination range. 
For those plants which live on or in the bottom, the effect of 
suspended sediment may vary according to the plants’ light require- 
ments. In some cases, increased turbidity may be inhibiting, for other 
plants it is not. This would suggest that long-term variations in sus- 
pended sediment concentrations might cause changes in the com- 
munity of primary producers, giving an advantage to certain groups. 
Thus a decrease in average turbidity would favor the spread of plants 
such as Thalasia, since the increased illumination would have an 
inhibitory effect upon competing species. 
The effect of suspended sediment upon phytoplankton growth is 
similar, since like most of the benthic microflora, attached algae, and 
higher plants, phytoplankton productivity is mhibited by excess as 
well as insufficient illumination. Verduin,® for example, has shown that 
during early spring, before the phytoplankton pulse develops, growth 
is low regardless of turbidity; but as the season progresses, phytoplank- 
ton crops develop most rapidly in waters of intermediate turbidity. 
The crops remain low in the most turbid waters, and show only slow 
growth in the clearest waters. In considering the effect of turbidity 
upon phytoplankton growth, the entire water column must be taken 
into account, since in most estuaries mixing insures that the phyto- 
plankton will be distributed more or less uniformly in the vertical 
direction. Thus if the light is essentially extinguished at half the depth 
of the estuary, the average time per day spent in light is reduced to 
some 6 hours. This type of argument may be extrapolated for any 
turbidity, and it is evident that increased suspended sediment not 
only diminishes the light available to plants in the upper waters, but 
also decreases the average time spent by plants in lighted water. It is 
well to note at this point that though the control of photosynthesis by 
ilumination is obvious, there are other factors which are at least as 
important, any one of which may inhibit growth. Verduin ° reports 
low phytoplankton crops in Lake Erie which were obviously not 
limited by high turbidities. 
To this point we have treated the suspended matter in the water 
as a passive element, affecting primary production indirectly through 
the reduction of light transmission in the water. While it may seem 
strange to visualize the sediment particles as very active agents in the 
Footnotes at end of article. 
