EXERCISE 25
Diurnal Changes in Lake
Systems
Environmental conditions may change markedly
within aquatic ecosystems during a 24-h period.
Solar radiation varies from high intensity at
midday to darkness. Surface temperatures and
concentrations of dissolved gases also may
fluctuate between extremes of day and night,
especially in shallow, littoral areas. Organisms
within the lake may move vertically or otherwise
relocate themselves in response to these environmental changes. For example, various species of
zooplankton undergo marked vertical migrations during a 24-h cycle [cf., Hutchinson (1967)
and Wetzel (1983)]. These movements are keyed
to light, food availability, and predation pressures. In this exercise, we shall examine some of
the relationships between these factors and the
vertical movements of zooplankton in a lake.
LOCATION AND MOVEMENT OF THE AVERAGE INDIVIDUAL
Since variability in response is common among living organisms, it is often convenient
to follow the movement of the average individual (Worthington, 1931):
.
. .
XlZ l + X 2 Z 2 + ... + XnZn
Depth of average mdIVIdual = ---=---=--~=--------"--"
Xl + X 2 + ... + Xn
where Z 1 = a depth of 1 m, Z 2 = a depth of 2 m, Xl = number of individuals/m 3 at a
depth of 1 m, x 2 = number ofindividuals/m 3 at a depth of2 m, and so on to a maximum
depth, n. The velocity and amplitude of vertical migration then can be calculated on the
basis of the depth of the average individual.
Obviously, by following only the "average individual" the detailed pattern of
movement for the entire population is obscured. Detailed tabular data or histograms
can be used but often are difficult to interpret because of the bulk of data involved.
Pennak (1943) proposed a method of plotting quartile curves which is useful and
particularly well suited for illustrating movements in populations with relatively slow
movement. Quartiles can be calculated readily from tabulation of cumulative numbers
of organisms with depth (e.g., Table 25.1). As an example, the 25,50, and 75% quartiles
for the data presented in Table 25.1 fall between 2 and 3 m, 4 and 5 m, and 6 and 7 m,
respectively. By interpolation, 25 % of the individuals in this hypothetical species were
found between the surface and 2.25 m, 50% were found between the surface and 4.32 m,
325
Diurnal Changes in Lake
Systems
Environmental conditions may change markedly
within aquatic ecosystems during a 24-h period.
Solar radiation varies from high intensity at
midday to darkness. Surface temperatures and
concentrations of dissolved gases also may
fluctuate between extremes of day and night,
especially in shallow, littoral areas. Organisms
within the lake may move vertically or otherwise
relocate themselves in response to these environmental changes. For example, various species of
zooplankton undergo marked vertical migrations during a 24-h cycle [cf., Hutchinson (1967)
and Wetzel (1983)]. These movements are keyed
to light, food availability, and predation pressures. In this exercise, we shall examine some of
the relationships between these factors and the
vertical movements of zooplankton in a lake.
LOCATION AND MOVEMENT OF THE AVERAGE INDIVIDUAL
Since variability in response is common among living organisms, it is often convenient
to follow the movement of the average individual (Worthington, 1931):
.
. .
XlZ l + X 2 Z 2 + ... + XnZn
Depth of average mdIVIdual = ---=---=--~=--------"--"
Xl + X 2 + ... + Xn
where Z 1 = a depth of 1 m, Z 2 = a depth of 2 m, Xl = number of individuals/m 3 at a
depth of 1 m, x 2 = number ofindividuals/m 3 at a depth of2 m, and so on to a maximum
depth, n. The velocity and amplitude of vertical migration then can be calculated on the
basis of the depth of the average individual.
Obviously, by following only the "average individual" the detailed pattern of
movement for the entire population is obscured. Detailed tabular data or histograms
can be used but often are difficult to interpret because of the bulk of data involved.
Pennak (1943) proposed a method of plotting quartile curves which is useful and
particularly well suited for illustrating movements in populations with relatively slow
movement. Quartiles can be calculated readily from tabulation of cumulative numbers
of organisms with depth (e.g., Table 25.1). As an example, the 25,50, and 75% quartiles
for the data presented in Table 25.1 fall between 2 and 3 m, 4 and 5 m, and 6 and 7 m,
respectively. By interpolation, 25 % of the individuals in this hypothetical species were
found between the surface and 2.25 m, 50% were found between the surface and 4.32 m,
325