LIMNOLOGY 619
over a period of up to two weeks. This longer time period and
the location at the lake bottom greatly reduce the hazard from
toxins that might be released by the dying algal cells. Alum
can provide long-term reduction in the occurrence of algal
toxicity if internal phosphorus loading is reduced. Alum has
also been found to reduce the sediment-to-water migration of
blue-green algae in Green Lake in Seattle (KCM, 1994).
The use of alum salts may cause toxic conditions. Alum
causes zooplankton to flocculate and settle out of the water
column, along with sediment and phytoplankton, which can
stress the food chain of a lake. To date, alum treatments have
not resulted in adverse effects on fish and have not dam-
aged invertebrate populations in well-buffered lakes (Cooke
et al. , 1993a; Narf, 1990). Invertebrate populations may,
however, be more sensitive to alum application in softwater
lakes. For example, the alum/sodium aluminate treatment of
Lake Morey in Vermont (alkalinity 30 to 50 milligrams of
calcium carbonate per liter) resulted in a short-term decrease
in density and species diversity of benthic invertebrates
(Smelzer, 1990).
Although most case studies of alum treatments dem-
onstrate multiple-year success, failures have also occurred.
These have been attributed to insufficient dose, lake mixing,
inadequate reduction in external nutrient inputs, and a high
coverage of macrophytes.
Other nutrient inactivation techniques have been used
with less success than alum. Calcium hydroxide (lime) has
recently been used in hardwater Alberta, Canada lakes to
control nutrient supply and algal growth (Murphy et al. ,
1990; Kenefick et al. , 1992). However, lime would not offer
the same phosphorus-binding benefit in softwater lakes
(Cooke et al. , 1993a).
The release of nutrients from lake sediments can also be
controlled by removing the layer of the most highly enriched
materials. This may result in significantly lower in-lake nutri-
ent concentrations and less algal production. Several types of
dredging equipment can be used to remove sediments from
lakes; a hydraulic dredge equipped with a cutterhead is the
most common choice. The cutter loosens sediments that are
then transported as a slurry of 80 to 90 percent water through
a pipeline that traverses the lake from the dredging site to a
remote disposal area. In the United States, a permit from the
U.S. Army Corps of Engineers is normally required before
sediments can be dredged from a lake (Cooke et al. , 1993b;
U.S. EPA, 1990).
Sediment removal to retard nutrient release can be
highly effective. For example, in Lake Trummen (Sweden),
the upper, nutrient-rich layer of sediments was removed,
increasing the lake depth from 3.6 feet to 5.8 feet. The sedi-
ment was disposed of in diked-off bays and upland ponds.
Return flow from the ponds was treated with alum to remove
phosphorus. The total phosphorus concentration in the lake
dropped sharply (U.S. EPA, 1990).
However, sediment removal has high potential for seri-
ous negative impacts on the treated lake and its surround-
ing watershed. The disposal area must be sufficiently large
to handle the high volume of turbid, nutrient-rich water
that accompanies the sediments. Unless the sediment-waterslurry can be retained long enough for settling to occur. The
turbid, nutrient-rich runoff water will enter the lake outlet
and end up in a tributary stream or another lake downstream
of the treated lake. Turbidity, algal blooms, and dissolved
oxygen depletion may result in the receiving waters (Cooke
et al. , 1993b; U.S. EPA, 1990).
Prior to dredging, the lake sediments must be analyzed
for heavy metals (especially copper and arsenic, which have
been extensively used as herbicides), chlorinated hydrocar-
bons (which have been used in pesticides), and other poten-
tially toxic chemicals. Special precautions will be required
if these substances are present in high concentrations (Cooke
et al. , 1993b; U.S. EPA, 1990).
Another technique for preventing the release of phospho-
rus from lake sediments to the water column is hypolimnetic
aeration. This technique involves oxygenating the bottom
waters of a lake without causing destratification. Air is used
to raise cold hyplimnetic water in a tube to the surface of deep
lakes, where the water is aerated through contact with the
atmosphere, loses gases such as carbon dioxide and methane,
and is then returned to the hypolimnion. Phosphorus release
from the sediments is limited by hypolimnetic aeration if
there is sufficient iron in solution to bind phosphorus in the
re-oxygenated waters. Aeration oxidizes the soluble ferrous
phosphate to insoluble ferric phosphate, which would then
precipitate out into the sediments and remain there. In addi-
tion, hypolimnetic aeration increases habitat and food supply
for coldwater fish species. The technique has been used with
varying levels of success. Unsuccessful treatments have been
attributed to inadequate oxygen supplies, disruption of lake
stratification, or lack of sufficient iron (Cooke et al. , 1993b;
KCM, 1994; U.S. EPA, 1990).
It is important that hypolimnetic aeration not destratify
the water column. Premature destratification (e.g., before
fall turnover of the lake) can be stressful and become toxic
to aquatic life when bottom waters with little dissolved
oxygen, low pH, and high concentrations of toxic gases
mix with surface waters. Destratification can also stimulate
algal growth by supplying hypolimnetic nutrients to sur-
face waters and mixing algae throughout the water column.
In shallow lakes, destratification could occur due to wind
mixing (KCM, 1994).
An alternative to aerating the hypolimnion is to remove
this nutrient-rich, anoxic water layer either through a deep
outlet in a dam or by a siphon, thereby accelerating a lake’s
phosphorus loss and perhaps producing a decrease in phos-
phorus concentration in surface waters. There are few docu-
mented case histories of hypolimnetic withdrawal (Cooke
et al. , 1993b; Nurnberg, 1987; U.S. EPA, 1990).
There are major disadvantages to hypolimnetic with-
drawal. The hypolimnion water that is discharged may be of
poor quality and therefore may require aeration or other treat-
ment. Federal, state or local regulatory agencies may require a
permit to discharge this water. Hypolimnetic withdrawal could
destratify the water column, thereby introducing nutrient-rich,
oxygen-poor water to the surface of the lake and triggering on
algal bloom (Cooke et al. , 1993b; Nurnberg, 1987; U.S. EPA,
1990).C012_002_r03.indd 619C012_002_r03.indd 619 11/18/2005 10:36:48 AM11/18/2005 10:36:48 AM