Trophic State and Eutrophication - Freshwater Ecology: Concepts and Environmental Applications - page 360

Dredging lowered P concentrations in the sediments from 2.4 to 0.1
mg liter 1 , leading to a 90% decrease of P in the water column, an increase
in Secchi depth from 0.23 to 0.75 m, and decreases in nuisance cyanobac-
terial blooms. The lowered phosphorus and increased mixing depth were
responsible for lower phytoplankton biomass. The dredging and lowered
nutrient inputs to the lake resulted in better water quality during the past
25 years, and fishing, swimming, and wind surfing continue to be popular
activities on the lake (Cooke et al., 1993).
Lake Tahoe
Lake Tahoe is one of the most visited large oligotrophic lakes in the
United States. It is naturally oligotrophic, with a small watershed (812
km 2 ) to surface area (501 km 2 ) ratio and great depth (505 m maximum).
As development threatens the lake because of associated nutrient input,
considerable study on the primary production has occurred. Separating
natural variation in processes controlling production of phytoplankton
from the effects of nutrient pollution has been important. Dr. Charles
Goldman (Biography 16.1) and his research group documented a threefold
increase in primary production from 1968 to 1987 (Goldman et al., 1989).
Tahoe is a large tectonic lake on the border between California and
Nevada. This lake is ultraoligotrophic, with Secchi depths historically
reaching about 40 m and a retention time of 700 years. Tahoe has gone
from a regionally popular vacation spot with numerous homes and cabins
in the watershed to an international tourist destination. There are gambling
casinos on the Nevada side of the lake and several ski areas around
the lake.
Part of the tremendous attraction of Lake Tahoe is the steely blue color
associated with its ultraoligotrophic nature. Over time, a decrease in clar-
ity occurred that was linked directly to increased nutrient input into the
lake. The initial problem was that septic system drain fields were leaking
nutrient-rich waters into the lake. The solution to this problem involved
installing sewage systems and pumping the treated sewage out of the lake
basin. The installation of sewage systems had the unforeseen negative im-
pact of encouraging further construction and development. The associated
removal of trees, increases in area of paved surfaces, and road building in-
stigated more nutrient runoff into the lake (non-point source runoff).
The nutrient limitation in the lake has switched with pollution input.
When septic systems were polluting the lake, an excess of phosphorus was
present (primarily from detergents containing phosphate), and nitrogen
was limiting. As watershed disturbance and atmospheric deposition be-
came the dominant sources of nutrient pollution, phosphorus pollution be-
came less important, nitrogen additions increased, and the lake passed
through a stage of colimitation by N and P to a state of P limitation (Gold-
man et al., 1993; Jassby et al., 1995).
The future of Lake Tahoe is uncertain. The economic pressures for real
estate development along the shore are immense. If development continues,
the biomass of the phytoplankton will continue to increase. This increased
amount of algae will lead to decreased clarity, and the lake will become a
more typical oligotrophic lake instead of one of the purest lakes in the
world.