Streams and Their Valleys - Fundamentals of Ecosystem Science - page 259

nutrients and organic matter transported
from upstream are a significant component
of the ecosystem in all downstream reaches.
Downstream transport is a characteris-
tic of flowing waters that sets them apart
from other ecosystems. The concept of
nutrient spiraling incorporates down-
stream transport into more traditional
concepts of nutrient cycling ( Newbold
et al. 1981 ). Following a path of an atom of
nitrogen serves to illustrate the nutrient
spiraling concept: Let's say the nitrogen
atom enters the stream as nitrate in
ground water and is transported down-
stream a distance before being taken up by
a diatom, which is consumed by a mayfly,
which excretes the atom of nitrogen as

dissolved organic nitrogen, which is once
again transported downstream before
being taken up by bacteria on a decompos-
ing leaf. The path just described is a spiral
(a cycle plus downstream transport) from
water column to benthos back to water
column again (as depicted in Figure 16.1 ).
The spiraling length can be measured
using tracers (e.g., Newbold et al. 1981 )as
the average downstream distance traveled
by an atom during one spiral from being
dissolved in the water, to uptake into an
organism in the streambed, to release back
into the water. Spiraling length is a mea-
sure of the efficiency with which nutrients
are utilized in a stream; it is shorter when
nutrients are more intensely used.

ecologists to assess not only the influence of the valley on the stream but also the effect
of location in the stream network on the structure and function of the ecosystem. That per-
spective provided direction for a study of a blackwater river that I was developing with
my colleagues in Georgia.
The waters of the Ogeechee River are stained with humic acids leached from the flood-
plain forests that line its meandering path through the Coastal Plain. The tea-colored
water reduces light penetration, and primary productivity is relatively low ( Meyer and
Edwards 1990 ); yet secondary productivity is very high (e.g., Hauer and Benke 1991 ).
The food web of the Ogeechee River ( Figure 16.3 ) is supported by inputs of bacteria and
other organic matter from its floodplain ( Meyer 1990; Wainright et al. 1992 ). Not only are
the up- and downstream linkages important, as emphasized in the RCC, but also lateral
inputs from floodplains are significant, particularly in larger rivers that have not been
isolated from their floodplains by levees or other human actions. Once again, to more
fully understand the structure and function of an ecosystem, its context and interactions
with surrounding ecosystems must be considered (see chapter 10).

... changes in the valley wrought by man may have large effects. (Hynes 1975)

The impacts of forestry practices (e.g., clear-cutting) on stream ecosystems provided
an early illustration of the tight linkage between human activities on the land and the
structure and function of stream ecosystems (e.g., Likens et al. 1970 ). Cutting trees
increases the amount of water that runs off a watershed, as well as water temperature