Abstract
Lake Okeechobee in south-central Florida is the second
largest freshwater lake in the contiguous United States.
Excessive phosphorus loading, harmful high and low water
levels, and rapid expansion of non-native vegetation have
threatened the health of the lake in recent decades. A study
was conducted to monitor discharge and nutrient concentrations
from selected tributaries into Lake Okeechobee and to
evaluate nutrient loads. The data analysis was performed at
16 monitoring stations from December 2003 to September
2008.
Annual and seasonal discharge measured at monitoring
stations is affected by rainfall. Hurricanes affected three wet
years (2004, 2005, and the latter part of 2008) and resulted
in substantially greater discharge than the drought years of
2006, 2007, and the early part of 2008. Rainfall supplies
about 50 percent of the water to Lake Okeechobee, discharge
from the Kissimmee River supplies about 25 percent, and
discharge from tributaries and groundwater seepage along the
lake perimeter collectively provide the remaining 25 percent.
Annually, tributary discharge from basins located on the west
side of the Kissimmee River is about 5 to 6 times greater than
that from basins located on the east side. For the purposes of
this study, the basins on the east side of the Kissimmee River
are called "priority basins" because of elevated phosphorus
concentrations, while those on the west side are called "nonpriority"
basins. Total annual discharge in the non-priority
basins ranged from 245,000 acre-feet (acre-ft) in 2007 to
1,322,000 acre-ft in 2005, while annual discharge from the
priority basins ranged from 41,000 acre-ft in 2007 to 219,000
acre-ft in 2005.
Mean total phosphorus concentrations ranged from 0.10
to 0.54 milligrams per liter (mg/L) at the 16 tributaries during
2004–2008. Mean concentrations were significantly higher at
priority basin sites than at non-priority basin sites, particularly
at Arbuckle Creek and C 41A Canal. Concentrations
of organic nitrogen plus ammonia ranged from 1.27 to 2.96
mg/L at the 16 tributaries during 2004–2008. Mean concentrations
were highest at Fisheating Creek at Lake Placid (a
non-priority site), and lowest at Wolff Creek, Taylor Creek
near Grassy Island, and Otter Creek (three priority basin
sites), and at Arbuckle Creek (a non-priority basin site). Mean
concentrations of nitrite plus nitrate ranged from 0.01 to 0.55
mg/L at the 16 tributaries during 2004–2008. Mean concentrations
measured in priority basins were significantly higher than
those measured in non-priority basins.
Nutrient concentrations were substantially lower in
the non-priority basins; however, total loads were substantially
higher due to discharge that was 5 to 6 times greater
than from the priority basins. Total phosphorus, organic
nitrogen plus ammonia, and nitrite plus nitrate loads from
the non-priority basins were 1.5, 4.5, and 3.5 times greater,
respectively, than were loads from the priority basins.
In the non-priority basins, total phosphorus loads ranged
from 35 metric tons (MT) in 2007 to 247 MT in 2005. In
the priority basins, the loads ranged from 18 MT in 2007 to
136 MT in 2005. In the non-priority basins, organic nitrogen
plus ammonia loads ranged from 337 MT in 2007 to 2,817 MT
in 2005. In the priority basins, organic nitrogen plus ammonia
loads ranged from 85 MT in 2007 to 503 MT in 2005. In
the non-priority basins, nitrite plus nitrate loads ranged from
34 MT in 2007 to 143 MT in 2005. In the priority basins,
nitrite plus nitrate loads ranged from 4 MT in 2007 to 27 MT
in 2005.
