EXERCISE 27
Historical Records of Changes
in the Productivity of Lakes
Changes in climate and in the geomorphology of
drainage basins in the past have altered water
and nutrient budgets and, as a result, productivity and rates of eutrophication of lake ecosystems. In many cases, human activites have
accelerated greatly these changes, but on a much
shorter time scale. A record of the resulting
alterations in chemistry, flora, and fauna is left in
the sediments as static derivatives of dynamic
systems. Paleolimnology assesses the sedimentary record and the diagenetic processes that may
alter it. An ultimate goal is to gain insight into the
past conditions that caused a lake to enter a
different level of productivity.
Interpretations about past levels and conditions of productivity of an aquatic ecosystem
can be made from analyses of the: (1) physical
structure and mineralogy of sediments, (2)
inorganic and organic chemical constituents of
the sediments, and (3) preserved morphological
remains of organisms. Paleontological records
rarely are complete. Moreover, lake sediments
contain materials from the atmosphere and the
drainage basin. Sediments within the basin can
be redistributed by water and wind movements.
Some remains of interred organisms preserve at
different rates under changing lake conditions;
others do not peserve at all. In spite of these
difficulties, which demand critical interpretation,
much information about changes in lake metabolism can be gleaned from the sedimentary
record. Accurate interpretation depends on
thorough understanding of ongoing biological
and physicochemical processes of lakes.
The amount of permanent sedimentation in a
lake is the result of the difference between the
various inputs from the drainage basin and from
the atmosphere (including solar radiation) and
the losses to the atmosphere and to drainage. The
form and rate of sedimented materials are regulated largely by biogeochemical transformations
within the lake basin. Rates of sedimentation
may be estimated from: (1) an analysis of the
vertical age distribution in sediments, (2) measurement of seston sedimented, and (3) difference
in input/output budgets [e.g., Wetzel et al. (1972),
White and Wetzel (1975), Moeller and Likens
(1978), and Davis and Ford (1985)].
In this exercise, we shall examine relatively
recent sediments in lake or reservoir systems.
From analyses of selected chemical and biological characteristics of the sediments, insight
into processes that have induced changes III
productivity of the systems may emerge.
337
Historical Records of Changes
in the Productivity of Lakes
Changes in climate and in the geomorphology of
drainage basins in the past have altered water
and nutrient budgets and, as a result, productivity and rates of eutrophication of lake ecosystems. In many cases, human activites have
accelerated greatly these changes, but on a much
shorter time scale. A record of the resulting
alterations in chemistry, flora, and fauna is left in
the sediments as static derivatives of dynamic
systems. Paleolimnology assesses the sedimentary record and the diagenetic processes that may
alter it. An ultimate goal is to gain insight into the
past conditions that caused a lake to enter a
different level of productivity.
Interpretations about past levels and conditions of productivity of an aquatic ecosystem
can be made from analyses of the: (1) physical
structure and mineralogy of sediments, (2)
inorganic and organic chemical constituents of
the sediments, and (3) preserved morphological
remains of organisms. Paleontological records
rarely are complete. Moreover, lake sediments
contain materials from the atmosphere and the
drainage basin. Sediments within the basin can
be redistributed by water and wind movements.
Some remains of interred organisms preserve at
different rates under changing lake conditions;
others do not peserve at all. In spite of these
difficulties, which demand critical interpretation,
much information about changes in lake metabolism can be gleaned from the sedimentary
record. Accurate interpretation depends on
thorough understanding of ongoing biological
and physicochemical processes of lakes.
The amount of permanent sedimentation in a
lake is the result of the difference between the
various inputs from the drainage basin and from
the atmosphere (including solar radiation) and
the losses to the atmosphere and to drainage. The
form and rate of sedimented materials are regulated largely by biogeochemical transformations
within the lake basin. Rates of sedimentation
may be estimated from: (1) an analysis of the
vertical age distribution in sediments, (2) measurement of seston sedimented, and (3) difference
in input/output budgets [e.g., Wetzel et al. (1972),
White and Wetzel (1975), Moeller and Likens
(1978), and Davis and Ford (1985)].
In this exercise, we shall examine relatively
recent sediments in lake or reservoir systems.
From analyses of selected chemical and biological characteristics of the sediments, insight
into processes that have induced changes III
productivity of the systems may emerge.
337