616 LIMNOLOGY
When one level of the food chain of a lake is altered, it
affects all other levels, sometimes positively and sometimes
adversely. For example, in Lake Washington, Daphnia popu-
lations increased in the 1970s.Why? The longfin smelt popu-
lation increased in the 1960s when flood control activities in
the main inlet stopped and spawning beds were no longer
damaged. Longfin smelt feed on a large crustacean called
Neomysis which feeds on Daphnia. Predation on Daphnia
was thereby reduced. This had a positive effect on Lake
Washington because Daphnia grazed on the algae, resulting
in improved water clarity in the lake.
Another example of food chain manipulation is stock-
ing a lake with piscivorous fish. When the fish are removed
by anglers, there will be more planktivorous fish which will
result in a decreased zooplankton population. Fewer zoo-
plankton will mean more algae in the lake, which could have
adverse effects on water clarity in the lake. In sum, altering
one part of a lake’s ecosystem has repercussions throughout
the ecosystem.TROPHIC STATUS/EUTROPHICATIONLakes are characterized according to their level of biological
productivity, or trophic status. The trophic status of a lake
depends on the concentration of chlorophyll a (the pigment
found in green plants that traps energy from the sun to enable
the plants to produce their own food by the process of photo-
synthesis), frequency of algal blooms, the concentrations of
nutrients, particularly phosphorus, and water clarity (trans-
parency). The phosphorus concentration determines how
many algae and other plants will grow in the lake. The clar-
ity of the water is influenced by a variety of factors including
algae, turbidity from sediments or other suspended particles,
and the natural color of the water in the lake. Water clarity
is measured with the use of a Secchi disk, a 20-centimeter
plastic or metal disk that is divided into alternating black
and white quadrants. The disk is lowered into the water until
the observer can no longer see it. The distance between the
lake surface and the point at which the disk disappears from
view is called the “Secchi transparency” or “Secchi depth”
of the lake.
Three trophic classifications are commonly used for
lakes. An oligotrophic lake is one in which there is clear
water, low levels of chlorophyll a and nutrients, and hencelittle aquatic life. Oligotrophic lakes tend to be found in
alpine and other wilderness areas. The lakes are beautiful to
look at and are fine swimming and boating lakes, but are not
good fishing lakes unless they are stocked with fish. There
are few naturally occurring fish in oligotrophic lakes because
there are few plants or insects for fish to eat.
At the other end of the scale are eutrophic lakes. A eutro-
phic lake has murky water, high levels of chlorophyll a and
nutrients, and is full of aquatic life. Many lakes in urban and
suburban areas are eutrophic, as evidenced by algal blooms.
A mesotrophic lake is in between, i.e., is moderately trans-
parent, with moderate levels of chlorophyll a and nutrients,
and some aquatic life.
Transparency, chlorophyll a and total phosphorus (both
organic and inorganic forms of phosphorus) are most fre-
quently used to assign trophic status to lakes. The general
relationship between these lake water quality parameters and
trophic status index (TSI) is summarized in Table 2.
A lake’s natural level of productivity is determined by
a combination of factors, including the geology and size of
the watershed, depth of the lake, climate, and water sources
entering and leaving the lake. Some lakes are naturally eutro-
phic based on their inherent physical attributes and watershed
characteristics.
Increases in a lake’s natural productivity over time, a pro-
cess called eutrophication, occurs naturally in some lakes, and
may be accelerated in others by human activities. For many
small lakes, natural eutrophication typically occurs over hun-
dreds or thousands of years, and is hence not observable in
a single lifetime. What is observable in a single lifetime is
the human-induced, or cultural eutrophication of lakes. Our
land-based activities, including home-building, agriculture,
forestry, resource extraction, landscaping, gardening, and
animal husbandry, all contribute nutrients and sediments to
surface waters, which in turn contribute to increasing a lake’s
biological productivity. Land erosion and forest clearcutting
contribute sediments to lakes. Surface water runoff from
impervious surfaces such as construction sites, parking lots,
and pavement contributes nutrients and pollutants to lakes.
Agricultural practices such as horses grazing near lakes, cows
wandering in streams, and extensive pesticide use contribute
nutrients and toxic pollutants to lakes. If oil or other toxic
chemicals are poured down storm drains, these end up in the
nearby lake, stream, or bay. Gardening chemicals such as fer-
tilizers and household toxic chemicals can end up in stormTABLE 2
Trophic status and associated values (Carlson, 1977; Cooke et al., 1993b;
Porecella et al., 1980)Trophic StatusTransparency
(meters)Chl. a
(mg/L)Total Phosphorus
(mg/L)TSI
(average)Oligotrophic 4 3 4 40
Mesotrophic 2–4 3–9 14–25 40–50
Eutrophic 2 9 25 50mg/L micrograms per liter (parts per billion).C012_002_r03.indd 616C012_002_r03.indd 616 11/18/2005 10:36:48 AM11/18/2005 10:36:48 AM