Environmental Engineering Reference
In-Depth Information
creased average water velocity, decreased uptake rates, increased distur-
bance of benthos, and increased insect drift. Spiral lengths of inorganic nu-
trients that are in high demand ( S w ) are generally short; uptake lengths of
phosphate and ammonium are often less than 100 m (Mulholland et al.,
1990; Hart et al., 1992; Butturini and Sabater, 1998). The concept of re-
source spiraling is powerful because it allows comparison of how nutrients
are retained by a variety of streams (Elwood et al., 1983), a process that
is particularly important in small streams (Peterson et al., 2001).
The river continuum concept (Vannote et al., 1980) has been one of the
most influential ideas in stream ecosystem theory. This concept views flow-
ing waters as a connected continuum from small forested headwater streams
to large rivers. It uses the associated gradient in abiotic and riparian char-
acteristics to make specific predictions about the biological community. The
concept posits that the dense canopy cover and low light found in small
streams supply leaf material as the primary carbon source, and the inverte-
brate community is dominated by shredders. As the stream increases in
width downstream, light increases and leaf input becomes less important.
Benthic algal productivity and fine organic
material washed from upstream contribute
most heavily to production of available car-
bon, and grazing and collecting invertebrates
dominate. In the largest rivers, benthic pro-
duction is low, suspended particulate mate-
rial is high, zooplankton and phytoplankton
can become established in the water column,
and collectors dominate the invertebrate
community (Fig. 22.7). This is a simple de-
scription of the possible ecosystem parame-
ters that can vary from headwaters to large
rivers. An expanded view (Table 22.2) con-
siders other abiotic (e.g., temperature and in-
organic substrate) and biotic (e.g., woody de-
bris) factors. This model is powerful in part
because it encourages consideration of stream
ecosystems across landscapes and as influ-
enced by the watershed (including terrestrial
and in-stream processes) above each point.
There are clear exceptions to the gener-
alizations of the river continuum concept (as
there are to any general ecological model).
For example, streams that are frequently dis-
turbed do not exhibit spatial trends in func-
tional feeding groups of insects (Winterbourn
et al., 1981). Also, small streams can flow di-
rectly into oceans without ever moving into
higher order rivers. Further, grassland rivers
may have limited leaf input in the smallest
streams (Gurtz et al., 1988), as may tundra
streams and rivers. Finally, the idea of serial
discontinuity (Ward and Stanford, 1983) sug-
nerable to damage by ice. Lack of June flooding
has encouraged establishment of vegetation on
sandbars. Since the 1960s the width of the chan-
nel has stabilized, vegetation has trapped sedi-
ments, and riparian forests have developed.
Channel modification and flow alteration in-
fluence the extent and ecosystem characteris-
tics of riparian wetlands. Cranes obtain much
of their nutrition in wet riparian meadows, ac-
quiring needed fat for continued migration.
These wet meadows have a unique assem-
blage of organisms associated with them and
they are rapidly disappearing (Whiles et al.,
1999). Areas with narrow channels have fewer
associated wetlands and lower than historical
usage rates by whooping cranes and other wa-
terfowl (U.S. Fish and Wildlife Service, 1981).
The management of this ecosystem requires
knowledge of how hydrology relates to habitat
and organisms over long timescales. Vegeta-
tion removal to widen the channel may not be
advisable because it causes sudden large sed-
iment releases (Johnson, 1997). Mimicking the
natural discharge regime to discourage estab-
lishment of riparian vegetation may be the pre-
ferred alternative. Given the tremendous de-
mand for water from the Platte River basin, it
may be difficult to obtain a discharge regime
similar to that occurring historically to main-
tain the desirable biotic features of the Platte.
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