Freshwater Ecosystems - Freshwater Ecology: Concepts and Environmental Applications - page 456

disastrous, but recovery does occur (Lamberti et al., 1991). Flood distur-
bances in small streams can control primary producers (Biggs, 1995, 2000)
and, thus, ecosystem function. Flooding and flow are becoming important
to ecosystem and river management, such as in efforts to protect the en-
dangered whooping crane (Sidebar 22.1). Stream ecologists must consider
the ramifications of flooding in management plans.
The concept of allochthonous (organic material provided from outside
the system) versus autochthonous (organic material from photosynthetic
organisms within the system) production has been stressed in streams be-
cause of the potentially strong influence of terrestrially derived organic ma-
terial and a substantial standing stock and production of periphyton (Min-
shall, 1978) and macrophytes (Hill and Webster, 1983) in some systems.
The source of organic material is important because different invertebrates
specialize in different types of carbon (Cummins, 1973), and varied sources
of carbon can alter pathways of carbon transfer through the food web. For
example, invertebrates that process leaf litter would be expected to provide
important routes for energy flux into the food web in a small forested
stream (allochthonous input). Exclusion of litter from forest streams has a
profound effect on stream invertebrate communities (Wallace et al., 1999).
Wood inputs can be tremendous, but not all wood is available to con-
sumers. In the Queets River, Washington, most large wood is less than 50
years old, but some in the channels is up to 1400 years old (Hyatt and
Naiman, 2001).
The relative contributions of primary production and external sources
of organic carbon can be difficult to identify. In small streams that are
heavily wooded, the input of leaves and wood is high, and shading limits
primary production; some slow-growing mosses may be abundant. In
larger streams, algal biomass is high when sufficient light can reach the
substrate. However, dissolved and particulate organic carbon enters the
stream from the surrounding terrestrial areas and from upstream. One way
to establish the relative importance of internal versus external supplies of
carbon is to compare the respiration and photosynthesis occurring in the
stream (stream metabolism).
Estimates of stream metabolism can be used to determine the ratio of
photosynthesis to respiration (P:R), which serves as an index to the degree
of autotrophy (relative autochthonous production) in the system. Two
methods have been used to make such estimates based mainly on rates of
O 2 production and consumption: isolation of shallow benthic substrata in
sealed recirculating chambers and measurements of whole-stream diurnal
O 2 flux (see Chapter 11). In general, chamber methods have indicated that
primary production exceeds respiration in well-lighted streams (Minshall
et al., 1983; Naiman, 1983; Bott et al., 1985). Whole-stream estimates sug-
gest that production over a 24-h period rarely exceeds respiration, and that
P:R is usually less than 1, even in lighted streams (Young and Huryn,
1999). The discrepancy between these two methods occurs because the
chambers include only the top layer of benthos, whereas the whole-stream
methods include significantly more subsurface respiration (the influence of
the hyporheic component). Because the hyporheic metabolic activity is
linked to instream O 2 dynamics, it makes sense to include it in estimates
of whole system metabolic activity.