Environmental Engineering Reference
In-Depth Information
Sizes, shapes, and spatial heterogeneity of ecosystems are important and may determine
or influence important properties of ecosystems (
Lovett et al. 2005
). For example, the
depth of the upper mixed layer for lakes that temperature-stratify is a key property.
Mixed layer depth is influenced by the clarity of the water. Greater clarity leads to deeper
light penetration and hence a deeper uniform heating of the surface waters. The upper
mixed layer is deeper in clear lakes than in more turbid lakes. Water clarity is measured
by light transmission (in units of inverse meters
m
2
1
) and so a relationship between per-
cent
light
transmission (water clarity) and depth of
the mixed layer is expected
(
Figure 9.4
). The relationship, however, differs for small (
500 ha)
lakes (
Fee et al. 1996
;
Figure 9.4
). It is possible to predict mixed layer depth from a mea-
sure of light transmission, but this important property of lake ecosystems is also scale-
dependent. Mixing depth depends on the size of the lake. Large lakes differ from small
lakes because of the differential effects of wind energy exerted over the greater length
(fetch) of a large lake (
Fee et al. 1996
). In simple terms greater wind mixing in large lakes
tends to promote deeper mixed layers and causes a different relationship between clarity
and mixed layer depth.
The lake example represents two important outcomes. The first outcome is that general-
izations about ecosystems are possible. This topic contains many examples of patterns
within and among ecosystems as well as explanations of these patterns. The development
and evaluation of patterns is a fundamental scientific activity. Despite the abstract aspects
of the ecosystem concept noted at the beginning of this chapter, the ability to generalize
about ecosystems indicates this unit of nature is a worthy topic for scientific study. The
second outcome is that variation in properties within and among ecosystems may be
related to scale (see Chapter 17). Variation in ecosystem size in the lake example is directly
500 ha) versus large (
,
.
16
14
12
10
8
6
4
2
0
0
20
40
60
80
100
Light transmission (%/m)
FIGURE 9.4
Relationship between water clarity (as % light transmission) and mixing depth in small (
500 ha,
,
closed circles) and large (
500 ha, x's) lakes.
.
