Trophic State and Eutrophication - Freshwater Ecology: Concepts and Environmental Applications - page 342

lake, would likely be very short relative to the entire geological life span.
In long-lived lakes, long-term changes related to geological processes (e.g.,
deforestation related to glaciation) may lead to periods when lakes are
mesotrophic or eutrophic and others when they are oligotrophic. Pale-
olimnological methods utilizing isotopic dating and preserved remains of
algae in the sediments (primarily diatoms) can be useful for estimating the
history of a lake's trophic state (Anderson, 1993). Such methods often re-
veal that lakes thought to be naturally eutrophic were more oligotrophic
thousands of years ago (Anderson, 1995).
Natural eutrophication can occur with watershed disturbances. In an
interesting case, Spirit Lake was altered greatly following the volcanic
eruption of Mount St. Helens. The eruption occurred on May 18, 1980,
and was the equivalent of a 10-megaton nuclear explosion leading to mas-
sive input of downed timber, volcanic ash, and an abrupt temperature in-
crease from 10 to 30°C. Spirit Lake was deep and oligotrophic before the
blast. The eruption altered the lake to a shallower basin with a large sur-
face area, ultimately leading to increased macrophyte growth and produc-
tion (Larson, 1993). Such rapid and drastic
changes are rare in most natural lakes on
human timescales of observation.
Cultural eutrophication is common in
the United States and other countries in
which there are moderate to high densities
of human activity. Cultural eutrophication
occurs rapidly (relative to most geological
processes) and can be difficult to reverse.
Human activities that lead to cultural eu-
trophication include use of agricultural fer-
tilizers, livestock practices, watershed distur-
bance such as deforestation, and release of
nutrient-rich sewage into surface waters
(Loehr, 1974). Road building also leads to
increased erosion and infilling of lakes. His-
torical examples of eutrophication caused by
watershed disturbance include road con-
struction of the Via Cassia by the Romans
(Sidebar 17.1) and eutrophication caused by
agriculture in early Mexico (O'Hara et al.,
1993).
Eutrophication control can be costly;
thus, political battles over the relative im-
portance of phosphorus control to solve eu-
trophication problems caused by humans can
be intense (Edmundson, 1991). Perhaps the
most important scientific verification of the
role of phosphorus in eutrophication was
the work headed by David Schindler (Biog-
raphy 17.1) at the Experimental Lakes Area
in Canada. These whole-lake experiments
demonstrated that phosphorus additions,

shed (Ward-Perkins, 1970), which resulted in
settlement and deforestation in the watershed.
Analyses of lake sediments dated with 14 C
to the time when Via Cassia was built reveal a
marked increase in the rate of sedimentation,
a decrease in the amount of tree pollen, a de-
crease in the amount of aquatic plant pollen,
and increased carbon and nutrient content of
the sediments. These and other characteris-
tics are consistent with deforestation of the
watershed, increased sedimentation, more nu-
trient input associated with increased runoff,
and greater productivity of the lake. This eu-
trophic state abated somewhat after the fall of
the Roman Empire, and the lake has maintained
a moderately eutrophic state since that time.
This study has several important points. It
demonstrates that humans have a long history
of causing eutrophication and impacting habi-
tats on a watershed scale. It also demonstrates
that lakes do not necessarily undergo a con-
stant succession from oligotrophy to eutrophy
over geological time. Finally, this is an early
example of study of a limnological problem that
was best accomplished by assembling a team
of specialists. It illustrates that limnology is a
holistic subject, and that observations from
both “hard” and “social” sciences can be used
to study ecologically and environmentally rel-
evant questions.