Alafia River Minimum Flows and Levels - Southwest Florida Water ...

versus flow. The third plot typically presented is a plot of the residuals obtained
from a LOWESS regression of the parameter versus flow. The last plot is used
to evaluate if a parameter loading has increased or decreased over time
irrespective of flow. The results of a Kendall’s tau analysis on the residuals was
used to help determine if apparent increasing or decreasing trends in a
parameter were statistically significant.
2.4.1 Macronutrients: Phosphorus and Nitrogen
Concentrations of the two major macronutrients, phosphorus and nitrogen, have
been monitored for some time at the Lithia gage site and somewhat less
frequently at the North and South Prong sites. The exact chemical form of the
nutrient monitored has changed over time (e.g., total nitrate, dissolved nitrate,
nitrite+nitrate, etc.), however, for purposes of the discussion that follows and for
trend analysis, values for some constituents were combined to provide a
sufficient number of data points for analysis.
2.4.1.1 Phosphorus
Phosphorus concentrations have been reported by the USGS as total
phosphorus, dissolved phosphate, and as ortho-phosphate. For purposes of this
discussion, it was assumed that dissolved phosphate and ortho-phosphate are
essentially equivalent. Although some of the older data were reported as mg/l
phosphate, all values were converted and expressed as mg/l phosphorus (P). As
similarly described for the Peace River (SWFWMD 2002), historic P
concentrations in the Alafia River and its major tributaries (North and South
Prongs) were impressive (see Figures 2-32, 2-33). A record high of 105 mg/l P
was reported for a sample collected on the North Prong on April 28, 1969.
Considering that background concentrations for a tributary in the Bone Valley
area should probably be between 0.1 and 0.5 mg/l P, this high value is 200 to
1000 times higher than should be expected. Although 105 mg/l P is the extreme,
values in excess of 20 mg/l were frequently found before 1975 in the North
Prong. While considerably improved over past conditions, P concentrations in
the neighborhood of 5 mg/l still occur in the North Prong of the Alafia. Fewer
measurements have been taken from the South Prong, but it appears that
concentrations have never been as high as those on the North Prong. However,
concentrations above 1.5 mg/l should not be considered natural for this system.
Because the North and South Prongs contribute approximately 81% of the flow
(and a combined 72% of the watershed above the gage) as measured at the
Lithia gage, it should be expected that concentrations of most constituents
should be reflected in concentrations of water samples taken at Lithia. While
concentrations greater than 4 mg/l P have not been recorded since 1990, P
concentrations have not yet reached levels that would be considered
2-66