Environmental Engineering Reference
In-Depth Information
changes in dissolved organic carbon (DOC) exports (
Meyer et al. 1998
), secondary produc-
tion of aquatic insects (
Wallace et al. 1997
), and nutrient dynamics (
Webster et al. 2001
).
Changing just one of those pathways of interaction with the terrestrial ecosystem (leaf litter
inputs) had profound impacts on the stream ecosystem. Modern humans are changing
many pathways.
Streams in human-dominated landscapes commonly have high concentrations of nutri-
ents, and nutrient export can be directly related to the fraction of the catchment no longer
in native vegetation (
Allan 2004
). Increased input of nutrients (e.g., fertilizers and sewage)
is generally considered the primary cause. Yet landscape alteration affects not only nutri-
ent delivery to streams, but also the biogeochemical processes that remove and transform
nutrients in streams.
Our understanding of biogeochemical processes in streams has grown considerably over
the decades since my doctoral research on phosphorus dynamics. The concept of nutrient
spiraling (
Box 16.1
) represented a considerable advance over the budget approach I used in
my earlier research, because spiraling deals explicitly with transport and uptake and elimi-
nates the problem of budget sensitivity to the ecosystem boundaries selected (
Fisher et al.
2004
). Nutrient spiraling length is the downstream distance traveled by a nutrient molecule
(transport) before its removal from the water column (uptake) by biotic uptake or physical
sorption; some of the nutrient taken up is later released and transported downstream,
resulting in a nutrient spiral rather than the more conventional nutrient cycle (See Box 5.2).
The increased availability of analytical and chemical tools for analysis, such as mass
spectrometers and stable isotopes of nitrogen, has enabled stream researchers to explore
the impacts of human land use on nutrient spiraling in streams in the Lotic Intersite
Nitrogen eXperiment (LINX). I have been fortunate to be a part of this team of several
dozen colleagues and students, who have explored nitrogen dynamics in streams in eight
biomes, each with catchments in three different land covers: natural vegetation, agricul-
ture, or urban. After demonstrating the effectiveness of small streams in removing ammo-
nium from the water column (
Peterson et al. 2001
), we showed that nitrate uptake and
denitrification rates increased but efficiency of uptake and denitrification decreased with
the increasing nitrate concentration associated with agricultural and urban land uses
(
Mulholland et al. 2008
). As excess nitrate enters the stream from human activities, the
fraction of nitrate exported downstream increases (
Mulholland et al. 2008
). Human activi-
ties in the valley change both inputs to and biogeochemical pathways in flowing waters.
We must, in fact, not divorce the stream from its valley in our thoughts at any time. If we do, we lose
touch with reality. The real lake is not a basin with two vertical sides as in the textbook. One that is like
that, Loch Ness, is so out of line that it harbours monsters. Somewhere, in Australia, there must be a
stream with a channel like a gutter, fed by runoff from a landscape paved like a parking lot. There,
I predict, will be found the legendary river creature of the aboriginals—the Bunyip.
(Hynes 1975)
We do not need to go to Australia to find streams like gutters fed by a parking lot.
Most college campuses have streams like that (does yours?), although few stream ecolo-
gists study them. Engagement with a local watershed group and a personal resolve
to study urban streams (
Figure 16.4
) have added to my conviction that an ecosystem
perspective is essential to understand and reduce human impacts on stream ecosystems.
