Environmental Engineering Reference
In-Depth Information
The long-term decline in nitrate concentration and export in New England streams
illustrates the need to consider both terrestrial and aquatic ecosystems when seeking a
mechanistic understanding of observed patterns (
Bernhardt et al. 2005
). Current models of
terrestrial ecosystems (e.g.,
Aber et al. 2002
) cannot explain the marked decline in nitrate
concentration and export that has been observed in many New England watersheds
(
Bernhardt et al. 2005
). An analysis of the 40-year record of observations and experiments
on nutrient dynamics in streams at the Hubbard Brook Experimental Forest provides
a plausible mechanism: Changes in the stream ecosystems have resulted in increased rates
of nitrate uptake, benthic storage, and denitrification; and these changes can explain at least
30% of the long-term decline (
Bernhardt et al. 2005
). Clearly, an ecosystem perspective that
extends beyond the boundaries of a single ecosystem is needed to understand the mechan-
isms leading to declining nitrate export. Although “the valley rules the stream” (
Hynes
1975
), the stream alters the linkage between the valley and downstream lakes or estuaries.
The publication of the River Continuum Concept (RCC;
Vannote et al. 1980
) revolution-
ized stream ecosystem research because it emphasized those up- and downstream linkages
(
Box 16.1
). The RCC manuscript was one of the first I reviewed as a young professor.
When I read it, I knew it would change the perspective of stream ecologists, who at the
time tended to work at the scale of a single stream reach. The RCC stimulated stream
BOX 16.1
THREE CONCEPTS THAT ENHANCE
UNDERSTANDING OF STREAM ECOSYSTEMS
Try this exercise, which I learned from
Stuart Fisher: Take a minute and sketch
your concept of a stream ecosystem; do it
before reading any further!
When I have used this exercise in clas-
ses, students have often drawn a diagram
of a reach of a stream, showing inputs and
exports and some of the organisms and
processes in the ecosystem (much like
Figure 16.1
). That is what we see when
standing on a stream bank. What is missing
from this diagram is the recognition that
each stream reach is part of a larger net-
work—the many tributaries and channels
that comprise a river network. The concept
of stream order is used to describe the posi-
tion of a stream reach in that larger network.
A first-order stream is the first channel
formed when water comes out of the ground
(identified as solid blue lines on 1:24,000
topographic maps); when two first-order
streams converge, a second-order stream is
formed. Stream order increases only when
two streams of equal order converge.
Stream order influences the physical,
chemical, and biological forms and pro-
cesses occurring in a stream ecosystem,
a fundamental idea in the RCC (
Vannote
et al. 1980
;
Figure 16.2
). Headwater streams
(orders 1
3) in forested landscapes are
dependent on inputs from the surrounding
forest (e.g., leaf litter) and have invertebrate
assemblages dominated by benthic species
that feed on those inputs and export fine
detrital particles downstream. As streams
get
6), more light is
available to support autochthonous produc-
tion and the benthic consumers that feed on
larger (
B
orders 4
