Trophic State and Eutrophication - Freshwater Ecology: Concepts and Environmental Applications - page 361

Lake Okeechobee
Lake Okeechobee in southern Florida is one of the largest lakes in the
United States in terms of surface area (1840 km 2 ). However, the mean
depth is 3 m, which is less than the maximum wave height possible given
the fetch. The shallow lake is characterized by turbid conditions from
wind-induced sediment resuspension. The lake has become more eutrophic
because agricultural runoff in the watershed has led to a doubling of total
P (Havens et al., 1996a). This eutrophication threatens a recreational fish-
ery (valued at more than $1 million per year) and the lake's value as a do-
mestic and agricultural water supply.
Nutrient enrichment of the lake has caused increases in cyanobacteria
known to produce toxins and increases in algal biomass. Phosphorus in-
puts have apparently stimulated N 2 fixing species. Predicting when and
where cyanobacteria are going to bloom is difficult. Wind induces mixing
of the lake and leads to lower probabilities of bloom formation by in-
creasing light limitation (Bierman and James, 1995; James and Havens,
1996). Nutrient concentrations are related more directly to bloom forma-
tion in the shallower western regions (James and Havens, 1996).
Historically, Lake Okeechobee was P limited, but increases in P load-
ing related to agriculture increased the degree of N limitation (Havens,
1995). Improvements in agricultural management practices have lowered P
inputs into the lake by 40% compared to those in the 1980s. Nitrogen in-
puts from water pumped from nearby agricultural areas have also been de-
creased, lowering N inputs by about 50%. However, these reductions have
not led to improvements in water quality parameters, probably because of
the large amounts of P stored in the well-mixed sediments of the lake
(Havens et al., 1996b).
To further complicate the management scenario, the Everglades receive
water from Lake Okeechobee. The complex hydrological management of
the Everglades system was discussed in Sidebar 4.2. Eutrophication prob-
lems in the Everglades are discussed later.

The Clark Fork River
I discussed the problems associated with metal contamination from
mine runoff and this river previously (Sidebar 14.1). An additional prob-
lem is dense algal growth related to point source inputs of nutrients dur-
ing times of low discharge in the summer (mainly municipal sewage out-
fall). Benthic chlorophyll values frequently exceeded 100 mg m 2 (Watson,
1989) and this high algal biomass was perceived as a nuisance.
Two approaches were used to calculate total N and P in the water col-
umn that would lead to acceptable periphyton biomass (Dodds et al.,
1997). The first was to identify reaches where chlorophyll levels were gen-
erally acceptable and analyze the total N and P. The second was to use
equations generated from the general relationship between TN, TP, and
benthic chlorophyll similar to those used in lakes to predict what level of
nutrient should lead to generally acceptable values of benthic chlorophyll.
Both methods suggested about 350 and 30

g liter 1 total N and total P,
respectively. A simple model of N and P inputs was then used to estimate
the influence of sewage effluent controls on water column total N and P.