Trophic State and Eutrophication - Freshwater Ecology: Concepts and Environmental Applications - page 353

buoyant, they concentrate on the surface. They are more apparent as “scum”
and visually indicate the high level of nutrients in the lake, especially P. This
example illustrates the importance of limiting factors and indicates how P
limitation can be important in regulating algal communities.
Treatment in the Lake
When a lake becomes eutrophic, several methods can be used to treat
the symptoms. Treatment becomes more difficult when O 2 has disappeared
from the hypolimnion because phosphate that would bind with Fe 3 in an
oxic hypolimnion is released from the sediments, drastically increasing
rates of internal loading. The FePO 4 locked in sediments dissociates into
Fe 2 and PO 4 3 in the anoxic hypolimnion and diffuses into the water col-
umn. The phosphate released from the sediments becomes available to phy-
toplankton when the lake mixes. Phytoplankton utilize luxury uptake to
acquire and store this phosphate, which provides nutrients for future
blooms. Several strategies have been devised to combat this resuspension
of phosphate. These strategies are discussed in detail elsewhere (Cooke
et al., 1993) and summarized here and in Table 17.2.
One method to counteract the symptoms of eutrophication is to pro-
vide O 2 to the hypolimnion so phosphate remains in the sediments. This
hypolimnetic aeration requires large amounts of energy; thus, it can be pro-
hibitively expensive in any but the smallest of lakes. If the main goal is to
protect a cold-water fishery, then only a small part of the hypolimnion
needs to be oxygenated and care must be taken to not break stratification.
This approach provides low-temperature, oxygenated water as a refuge for
salmonid species.
Aeration does not always lower algal biomass (Soltero et al., 1994).
However, aeration for many consecutive years has been used successfully
to mitigate water quality problems in shallow urban lakes (Lindenschmidt
and Hamblin, 1997). Destratification can also keep phosphate in the sedi-
ments and can have an inhibitory effect on nuisance cyanobacteria by in-
creasing mixing depth. This mixing can select for green algae and diatoms
instead of the toxic cyanobacterium Microcystis (Visser et al., 1996).
The use of copper as a method to control algae has been widespread.
Copper is particularly toxic to cyanobacteria and thus removes the objec-
tionable algae. In hard waters copper can precipitate as copper carbonate
so that repeated applications are necessary to achieve results. The copper
contaminates the sediments and can eventually poison other aquatic life,
such as crustaceans, if pH is acidic. Furthermore, the copper may break the
cells of cyanobacteria and release toxins into the water (Lam et al., 1995).
Extended treatment with copper may become more problematic than the
condition it was supposed to cure (Cooke et al., 1993).
Recently, addition of barley straw has been proposed as a way to con-
trol phytoplankton blooms. Some studies of this method have shown mea-
surable lowering of algal biomass (Barrett et al., 1996; Everall and Lees,
1996; Ridge et al., 1999). Repeated treatment with barley straw was
demonstrated to be effective in lowering cyanobacterial populations and
decreasing taste and odor problems in one drinking water supply reservoir
(Barrett et al., 1999). The mechanism for this control is not well estab-