614 LIMNOLOGY
Aquatic plants provide many benefits, including sedi-
ment and shoreline stabilization; food source and habitat
for benthic invertebrates, fish, and wildlife; oxygenation of
the water column; and aesthetics. Most rooted macrophytes
obtain their nutrients from lake sediments rather than the
water column, and take up phosphorus that would otherwise
have been available for algal growth, thus preventing the
overgrowth of algae.
However, when there are too many aquatic plants, par-
ticularly non-native plants, the advantages turn into disad-
vantages. When a lake is shallow and nutrient-enriched, then
there can be too many macrophytes. Too many aquatic plants
can decrease the quality of fish and wildlife habitat, interfere
with beneficial uses of a lake such as swimming and boating,
and even create safety problems, i.e., swimmers can become
entangled in milfoil and other plants. When the plants decay,
they deplete the lake waters of oxygen and release nutrients
into the water column which can promote algal growth. Too
many macrophytes can make a lake look unsightly. The
advantages and disadvantages of aquatic plants are opposite
sides of the same coin; it’s a matter of degree and balance.
Many types of animals are found in lakes. Zooplankton
are microscopic animals found in the lake water column.
Examples are rotifers and water fleas, e.g., Daphnia. They
are visible to the naked eye on close inspection of a glass of
lake water. Zooplankton are important in the food web of a
lake because they eat algae and, in turn, are eaten by plank-
tivorous fish. The types and number of zooplankton pres-
ent are also indicative of lake water quality. Generally, large
grazing species improve water quality by eating algae. On
the other hand, a general decrease in the size of zooplank-
ton species, with their reduced capacity to graze the phyto-
plankton, is a response to the greater availability of bacterial
detritus resulting from the relatively ungrazed algae (Welch,
1992). Therefore, the presence of larger zooplankton in a
lake usually indicates good water quality, while the presence
of smaller zooplankton generally indicates more nutrient-
rich waters.Benthic infauna are small invertebrate animals such as
molluscs, worms, and midges that live in the bottom sedi-
ments of lakes. They feed on detritus in the sediments and
recycle nutrients to the water. The species of benthic animals
found in a given area are usually indicative of the surround-
ing water quality. Some invertebrates, such as mayflies, are
intolerant of low dissolved oxygen conditions; their presence
in large numbers in lake ecosystems indicates good water
quality. Other invertebrates, such as oligochaetes and chi-
ronomids, are more tolerant of low dissolved oxygen condi-
tions; their presence in large numbers in a lake may indicate
the presence of pollutants or degraded water quality.
The greatest density and diversity of benthic inverte-
brates is usually found in the littoral zone of a lake, where
ample vegetation and oxygen are present. The benthic com-
munities, in turn, provide food for larger invertebrates, fish,
amphibians, and birds.
The types of fish found in a lake are influenced by water
temperature and dissolved oxygen levels. Fish such as perch,
bass, and smelt are warmwater fish and thrive in lakes where
the summer water temperature exceeds 65F. The dissolved
oxygen level needs to be at least five parts per million (ppm)
in order for the fish to remain healthy. Coldwater fish such as
salmon and trout are found in lakes where the summer water
temperature is less than 65F. The dissolved oxygen level
needs to be at least seven ppm for these fish. If the summer
water temperature is too high, the dissolved oxygen level is
often too low to support healthy fish populations.
Each organism in a lake is dependent on other organisms
for its food. Each lake has a natural food chain. Algae are
eaten by zooplankton. In some lakes, the efficient grazing
of zooplankton by algae can help to maintain water clarity.
Zooplankton are eaten by planktivorous fish such as long-
fin smelt and perch. Planktivorous fish are eaten by larger,
piscivorous fish such as northern squawfish and largemouth
bass. The larger fish are eaten by birds and by mammals,
including humans. Figure 5 illustrates the aquatic food chain
found in many lakes.Free-floatingEmergentPlanktonic algae
SubmergentsRooted, floating-leavedFIGURE 4 Macrophyte community types.C012_002_r03.indd 614C012_002_r03.indd 614 11/18/2005 10:36:47 AM11/18/2005 10:36:47 AM