Eutrophication potential of Payette Lake, Idaho

Payette Lake was studied during water years
1995-96 to determine the 20.5-square-kilometer
lake's assimilative capacity for nutrients and, thus,
its eutrophication potential. The study included
quantification of hydrologic and nutrient budgets,
characterization of water quality in the limnetic
and littoral zones, development of an empirical nutrient load/lake response model, and estimation of
the limnological effects of a large-scale forest fire
in the lake's 373-square-kilometer watershed during the autumn of 1994.

Streamflow from the North Fork Payette
River, the lake's primary tributary, delivered about
73 percent of the lake's inflow over the 2 years.
Outflow from the lake, measured since 1908, was
128 and 148 percent of the long-term average in
1995 and 1996, respectively. The larger volumes
of outflow reduced the long-term average water-
residence time of 2.35 years to 1.84 and 1.42 years
for 1995 and 1996, respectively. The lake retained
54 percent of its 1995-96 influent load of nitrogen
and 79 percent of its influent load of phosphorus.
The North Fork Payette River contributed an average of 69.4 percent of the lake's nitrogen load and
28.2 percent of its phosphorus load. The 1994 forest fires substantially increased the loads of nitrogen and phosphorus delivered to the lake; however, only nitrogen concentrations were noticeably increased in the lake.

Payette Lake was classified as oligotrophic
on the basis of annual geometric mean concentrations, in micrograms per liter, of total phosphorus
(4.7), total nitrogen (225), and chlorophyll-a (1.3)
during 1995-96. Secchi-disc transparencies ranged
from 2.3 to 8.0 meters, indicative of mesotrophic
conditions. Median ratios of dissolved inorganic
nitrogen to dissolved orthophosphorus ranged from
38 to 254, thereby indicating phosphorus limitation
of phytoplankton growth. Phytoplankton populations were taxonomically dominated by diatoms;
blue-green algae were rare. One diatom, Tabellaria
fenestrata, contributed 52 percent of the biovolume. Within the littoral zone, median periphyton
production, normalized to photosynthetically active
radiation input, ranged from 0.0007 to 0.02 milligrams of chlorophyll-a per square meter per Einstein, a difference of 28.6 times. Multiple linear regression analysis failed to detect any significant relation between periphyton production and various
indices of nearshore development. Nine genera of
aquatic macrophytes were identified, including
Eurasian milfoil (Myriophyllum spicatum var. spicatum), which is considered a nuisance aquatic
plant.

Despite its oligotrophy, Payette Lake had
substantial dissolved-oxygen deficits in 1995-96,
which led to 4-month periods of anoxia in the near-bottom waters of its southwest basin. The hypolimnetic dissolved-oxygen deficit was much larger than
that predicted by the nutrient load/lake response
model. The southwest basin's propensity for developing anoxia was related to the lengthy water-residence time and incomplete water-column circulation and reaeration during the spring and autumn,
coupled with a long-term accumulation of oxygen-
demanding organic matter produced within the
lake or delivered by its watershed.