LIMNOLOGY 617
drains and thus in lakes. Failing septic systems can discharge
nutrients from raw sewage to lakes.
The result of all these inputs of phosphorus, nitrogen,
sediment, and organic matter in large algal blooms which
are unsightly and can severely restrict lake beneficial uses
including swimming, fishing, boating, and aesthetic appre-
ciation. Beneficial uses of a lake may also be degraded by
other water quality problems related to eutrophic conditions,
including low dissolved oxygen levels, fish kills, algal toxic-
ity, and excessive aquatic macrophyte growth.
The level of dissolved oxygen in lakes is one determi-
nant of the habitat available to aquatic organisms. Oxygen is
added to a lake from exposure to the air, and by the contri-
bution or aquatic plants through photosynthesis. Oxygen is
removed from a lake by the respiration of aquatic organisms
and plants, and the bacterial decomposition of organic matter
in the water and sediments. Eutrophic lakes with large algal
blooms are characterized by high phosphorus concentrations
and low dissolved oxygen concentrations in the lake hypo-
limnion in the summer. This happens because decaying algae
and other plants fall to the bottom of the lake where they
contribute phosphorus and remove oxygen. Photosynthesis
does not take place in the hypolimnion of a eutrophic lake
because light does not penetrate to that depth; hence, the
oxygen that is being depleted is not replaced. Anoxic (lack
of oxygen) conditions at the water—sediment interface on
the lake bottom usually increase the potential for nutrient
release by converting iron phosphate in the sediments from a
water-insoluble to a water-soluble form.
The very low dissolved oxygen levels in the hypolim-
nion of eutrophic lakes during the summer months may be
too low to support coldwater fish such as salmon and trout.
The salmon and trout would then move to the lake epilim-
nion, but the water temperatures may be too high for them in
the surface waters. Eutrophic conditions in lakes often lead
to decreased quantity and quality of fish habitat and stressed
fish populations.RESTORING BALANCE TO LAKE ECOSYSTEMSManagement of Eutrophic LakesWhen a lake is eutrophic with unsightly algal blooms, water
quality problems, and impaired beneficial uses, its ecosys-
tem is out of balance. Lake restoration involves reducing
the impact of human activities on lake water quality, with
the goal of decreasing biological productivity and improv-
ing water quality and associated beneficial uses of the lake.
Several methods are available to accomplish this goal. Each
method has its advantages and drawbacks.
In order to determine the most effective method(s) to
use in a given lake, it is first necessary to be knowledgeable
about the physical, chemical, and biological components of
the lake’s ecosystem. This can be accomplished through one
or two years of monitoring parameters such as transparency,
lake temperature, acidity, alkalinity, dissolved oxygen, lake
level, amount of precipitation, nutrient levels, chlorophyll a,fecal coliform bacteria (a group of bacteria associated with
human, other mammal, and bird wastes), algae, zooplankton,
benthic infauna, an fish. Once monitoring data are obtained,
they need to be summarized and pollution sources priori-
tized for control.
Most lake water quality problems are associated with an
overabundance of nutrients, which results in excessive plant
growth. In managing such water quality problems, it is impor-
tant to assess what nutrient limits plant growth. In eutrophic
lakes, phosphorus is often the limiting nutrient; this means
that the amount of phosphorus in the lake will determine the
amount of plant growth. Therefore, most lake management
strategies focus on reducing phosphorus loading.
If the lake’s phosphorus budget shows that most of the
phosphorus is coming from within the lake, in-lake restora-
tion techniques should help to reduce phosphorus levels and
make the lake less eutrophic. On the other hand, if most of
the phosphorus is coming from the watershed (this is often
true of small lakes with very large watersheds), then the focus
should be on watershed best management practices (BMPs)
to control sources of nutrients. In some lakes, phosphorus
comes from within the lake and the watershed, so both types
of actions are needed.Watershed Best Management Practices Implementation
of watershed best management practices (BMPs) improves
water quality by reducing the quantity of pollutants enter-
ing the lake. Most pollutants within a watershed result from
human activities. Pollutants originating on each parcel of
land within a watershed can collectively become a serious
threat to the receiving water quality. BMPs are structural and
nonstructural methods, including common sense “house-
keeping measures,” used to prevent or reduce pollution by
controlling erosion, surface water runoff, sources of nutri-
ents, and sources of toxic chemicals. Watershed BMPs can
be basin-wide or can target management of developed prop-
erty. These measures can include native plant revegetation of
lake shorelines, retention/detention ponds and biofiltration
swales for stormwater treatment, and homeowner/business
owner BMPs to enhance water quality through better land-
scaping methods, alternative household and gardening prac-
tices, better animal-keeping practices, drainage controls,
and septic system maintenance and repairs. Local and state
agencies can work in partnership with lake associations and
other citizen groups in a watershed to educate residents and
business owners about BMPs that are inexpensive, easy to
implement, and make a difference in protecting lake water
quality and aquatic biota. Ideally, this environmental educa-
tion should include hands-on water quality activities (e.g.,
storm drain stenciling, lakeshore revegetation) and habitat
monitoring activities for community volunteers including
schoolchildren because these activities impart a sense of
lake stewardship to people who live on or upstream of a lake.
Following is a discussion of each type of BMP.
Many lakes have no native plants growing on the shore-
line; houses may have manicured lawns leading to the water’s
edge. Where shoreline vegetation is absent, surface water
runoff enters the lake directly, degrading lake water quality.C012_002_r03.indd 617C012_002_r03.indd 617 11/18/2005 10:36:48 AM11/18/2005 10:36:48 AM