Environmental Engineering Reference
In-Depth Information
6 Biota of Rivers and Streams
An Introduction
6.1 SPATIAL SCALE AND DISTRIBUTION
The physical and water quality characteristics of rivers and streams vary widely from their headwa-
ters to their terminus, over their cross sections and over time, as described in Chapters 2 through 5.
These physical and water quality variations also directly impact the presence and distribution of
the biota of streams and rivers. Organisms may have speciic ranges of depths, velocities, sub-
strate types, temperatures, dissolved oxygen, and other physical and chemical factors that they can
tolerate. The range of conditions from a preferred or optimal condition that can be tolerated may
be broad or narrow depending on the organisms and, in many cases, depending on the particular
life stage of the organisms. For example, organisms found in headwater or low-order streams may
be less tolerant to high temperatures and temperature variations, than those found in high-order
streams. In low-order, high-energy streams, the velocity may be suficient to armor the bed, so the
majority of the substrate may consist of gravel, rocks, and boulders. Some organisms may be more
tolerant of higher velocities and resist being washed away by attaching to this coarse substrate, or
by living in interstitial waters.
One of the goals of the management or restoration of rivers and streams is the maintenance or
the establishment of a healthy biotic community, where a healthy biotic community it typically
composed of a diverse assemblage of organisms, each with speciic instream habitat requirements.
The instream habitat is impacted by changes that occur at a variety of spatial and temporal
scales. For example, Frissell et al. (1986) described a hierarchical classiication of instream habitats,
ranging from the microhabitat where variations occur on the order of a foot or less, to the stream
itself that may vary over scales of thousands of feet (Figure 6.1) (FISRWG 1998). The microhabitat
could support fungi, small epiphytic plants, insects, and other biota and could include leaves and
debris, rocks and cobbles, small patches of gravel, or other small-scale habitats as illustrated. Over
a larger scale on the order of tens of feet are variations such as in steps and pools in steep systems,
or pools and rifles in less steep systems (FISRWG 1998). Different organisms may inhabit the
pool versus the rifle areas, and the frequency of the pools versus rifles, and their size, shape, and
other factors affect the habitat and the ability of the organisms to move between habitats. On larger
scales, changes may occur over scales of hundreds of feet (reaches) to thousands of feet (segments;
Figure 6.2).
Changes occurring on an even larger scale resulting from physical and landscape changes are
illustrated by the river continuum concept (RCC) developed by Vannote et al. (1980) (Figure 6.2).
The RCC illustrates how watersheds and streams are connected and how variations in watershed
and stream characteristics impact biological communities from the headwaters to the mouths of
streams. For example, according to the RCC, productivity in low-order streams is more limited due
to shading by riparian vegetation, so that most of the energy coming into these systems is from out-
side sources (allocthonous), such as leaf fall. As a result, the habitat favors organisms that can grow
or feed on these coarse organic sources. Toward the mouth, rivers are wider, slower, deeper, and
more of their productivity is derived from internal sources (autochthonous), such as phytoplankton.
The effect of scale on the distribution of organisms in rivers in streams is a critical component
in evaluating aquatic health, and in maintaining or restoring that health. Healthy aquatic systems
are typically diverse. For example, a habitat with a large number of few species is not considered
 
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