Systematic, comprehensive studies of the area began after the 1940s. Studies of the soils 
indicated that they could only have accumulated in a perennially wet, marshy environment 
beginning about 5,500 yrs ago. The organic materials accumulated slowly to a depth of 
approximately 14 feet in the deeper and upper parts of the Everglades. The climate did not 
change significantly during the period of soil accumulation. 
The reduction of the freshwater flow to the Everglades and consequently to Florida Bay has 
resulted in increased salinities in the Bay in more locations and for longer periods than during 
pre-drainage (Mclvor et al., 1994). The filling of passes and shallow banks between several of 
the Keys during construction of the Overseas Railroad reduced circulation in the Bay, 
worsening the high salinity conditions. Effects on the biota have been noted including reduced 
recruitment of pink shrimp, snook, and redfish; lowered reproductive success of ospreys and 
great white herons; and shifts in distribution of West Indian manatees, American crocodiles, 
and others. 
Natural fluorescence of river water, caused by dissolved humic acids, has been used as a 
tracer of freshwater input to nearshore environments (Smith et al., 1989). It has been shown 
that massive hermatypic corals such as Solenastrea bournoni possess fluorescent bands within 
their skeletons, and the frequency and intensity of the bands have a high correlation with 
terrestrial runoff. A core taken from the S. bournoni specimen from the Petersen Key Basin 
showed clear fluorescent banding under ultraviolet light. The relationship between flow in the 
Shark River and Taylor Sloughs and the fluorescent banding from 1940 to the present were 
used to hindcast flow for the period of 1881 - 1939. From the fluorescence pattern, a 
sustained, marked decline in freshwater flow, which began in 1912 and ended around 1931, 
was noted. Fluorescence was significantly higher earlier in the record (prior to 1932) than 
later on, and Smith et al. (1989) interpreted this as indicating decreased freshwater flow from 
the Everglades into Florida Bay of perhaps as much as 59% in the later period. This onset of 
decreased freshwater flow coincided with the construction of drainage canals to the east and 
south of Lake Okeechobee. Periods of reduced growth observed in growth patterns of a 1-m- 
high specimen of the coral S. bournoni from the Petersen Key Basin, Florida Bay, appeared to 
correlate with major anthropogenic environmental perturbations (Hudson et al., 1989). This 
coral species is resistant to sedimentation and water temperature extremes, and no correlation 
was apparent between growth rates and major meteorological events such as hurricanes and 
freezes. 
7.3. Soil subsidence 
A review of subsidence of organic soils in the Everglades can be found in Stephens (1984) and 
is abstracted in this section. The Everglades contains the largest single tract of organic soils in 
the world, over 3,100 square miles. These soils formed under marshy conditions and subsided 
when drained. The subsidence was caused by compaction due to: dissecation, consolidation, and 
tillage; biochemical oxidation; wind erosion; and/or burning. Biochemical oxidation accounted 
for approximately two thirds of the total loss of arable soils in the region. Subsidence has had 
serious environmental effects on agriculture, water supplies and wildlife. The sequence of 
observed subsidence of organic soils at three sites in the Everglades is shown in Figure 13. The 
sites are the North River Canal, just below old South Bay Lock; the Bolles Canal, a major drain 
at Okeelanta; and the Everglades Experiment Station. Ground surface elevation has decreased 
by approximately 9 ft at all three sites. Soil losses have been greatest near the original 
drainage canals and "subsidence valleys' several miles wide were formed along either side. 
Valley depths were greatest where drainage was best. 
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