Freshwater Ecosystems - Freshwater Ecology: Concepts and Environmental Applications - page 457

The trophic dynamics of stream invertebrates and the relationship be-
tween standing stocks (biomass) and production have received attention
with regard to stream ecosystems. Observations of invertebrate biomass
have yielded examples of greater biomass of secondary and higher con-
sumers than of primary consumers. This inverted biomass pyramid has
been termed the Allen Paradox following the observation that fish in a
stream required 100 times more benthic prey than was available at any one
time (Hynes, 1970). The Allen Paradox is an example of problems that can
arise when biomass is assumed to be directly proportional to production.
The production of primary consumers per unit biomass can be very high
and can support the secondary and tertiary consumer biomass, even when
the biomass of primary consumers is relatively low (Allan, 1983; Benke,
1984). Such analyses have become very detailed, including description of
carbon flux associated with each species in invertebrate communities. This
detailed description involves determination of the trophic basis of produc-
tion, or which individual food sources are responsible for production of
each species (Benke and Wallace, 1980).
Materials cycle as they move down-
stream. Cycling of materials in unidirectional
flow environments is termed nutrient spiral-
ing (Webster, 1975). Each molecule is in the
water column for an average amount of time
while it moves downstream. It then is taken
up or adsorbed by the benthos and moves
downstream more slowly. Thus, the nutrient
cycle that is typically conceptualized as a
wheel in lakes becomes a spiral in streams.
The spiral length ( S ) for a nutrient is the sum
of the distance that a molecule travels in the
water column in the dissolved form ( S w ) and
how far it is transported in the primary par-
ticulate compartments (algae, microbes, sus-
pended particles, and animals; S p ):
S

Sidebar 22.1.
Management of the Platte River for
Water Quality, Water Quantity, and
Species Preservation

The Platte River and its tributaries drain a large
portion of Nebraska and about one-third of
Wyoming and Colorado. The endangered pallid
sturgeon (Scaphirhynchus albus) is found in the
river. The Platte serves as a major stopover for
migrant waterfowl and provides vital habitat for
endangered whooping cranes (Grus ameri-
cana), piping plovers (Charadrius melodus), and
least terns (Sterna antillarum). About 80% of all
sandhill cranes (Grus canadensis) stop there
during their migrations. Approximately 70% of
the Platte's discharge is diverted by consump-
tive uses in Colorado, Wyoming, and western
Nebraska. There is concern about how this di-
version will influence the Platte River ecosys-
tem (U.S. Fish and Wildlife Service, 1981).
A major result of flow reduction and control
has been an alteration of channel morphology
since the early 1900s. Historically, the river was
sandy and braided with a wide, shallow chan-
nel. Flow modification has resulted in the inva-
sion of woody species ( Populus and Salix ) and
loss of much of the initial channel width (John-
son, 1994). It has also decreased water in the
channel during winter, when seedlings are vul-

S p
The relationship can be described graphi-
cally (Fig. 22.6) or in more mathematical
detail (Newbold et al., 1981). In practice,
modeling spiraling length can be difficult
(Stream Solute Workshop, 1990) because
understanding the movement of nutrients
and environmental contaminants through
streams requires detailed descriptions of the
processes of dilution, uptake, and reminer-
alization. Description of the influence of the
hyporheic zone may be particularly impor-
tant (Mulholland and DeAngelis, 2000).
However, some generalizations are possible
with regard to spiral length: Length should
be greater with greater stream discharge, in-

S w