Principles of terrestrial ecosystem ecology.pdf

streams, are not strongly nutrient limited, in
part because turbulence reduces diffusion
limitation. In addition, uptake of nutrients by
stream organisms does not influence the supply
of nutrients from upstream (Newbold 1992).
The relative importance of nitrogen and
phosphorus limitation varies among streams,
depending on watershed parent material, landscape
age, and land use. Phosphorus limitation
of stream production, for example, is more
common in the eastern United States, where
the parent material is relatively old and weathered,
than on younger parent materials, where
phosphorus inputs are larger and nitrogen is
more likely to limit production (Home and
Goldman 1994).
There is a strong interaction between top–
down and bottom–up controls over primary
production in streams. Nutritional controls over
the energy available to support higher trophic
levels is generally the dominant control over
stream productivity, but the types of predators
present strongly influence the pathway of
energy flow, just as in lakes (see Chapter 12).
Carbon and nutrients spiral down streams
and rivers and the groundwater beneath them,
rather than exchange vertically with the atmosphere
and groundwater. Streams are not
passive channels that carry materials from land
to the ocean. The streams and their riparian
zones process much of the material that enters
them. The strong directional flow of water in
streams and rivers carries the resulting products
downstream, where they are repeatedly
reprocessed in successive stream sections.
Energy and nutrients therefore spiral down
streams, rather than cycle vertically as they tend
to do in most terrestrial ecosystems (Fisher et
al. 1998). This leads to open patterns of nutrient
cycling, in which the lateral transfers are
much greater than the internal recycling (Giller
and Malmqvist 1998). Stream productivity
therefore depends highly on regular subsidies
from the surrounding terrestrial matrix and is
quite sensitive to changes in these inputs due to
pollution or land use change. The spiraling
length of a stream is the average horizontal distance
between successive uptake events. It
depends on the turnover length (the downstream
distance moved while an element is in
Streams and Rivers 241
organisms) and the uptake length (the average
distance that an atom moves from the time it is
released until it is absorbed again). A representative
spiraling length of a woodland stream
is about 200m. Of this distance, about 10%
occurs as microorganisms flow downstream
attached to CPOM and FPOM, 1% as consumers
move downstream, and the remaining
89% after release of the nutrient by mineralization
(Giller and Malmqvist 1998). A unit of
nutrient therefore spends most of its time with
relatively little movement, but moves rapidly
once it is mineralized and soluble in the water.
Spiraling is therefore not a gradual process but
occurs in pulses. The patterns of drift of stream
invertebrates is consistent with these generalizations.
Invertebrates drift downstream when
they are dislodged from substrates or disperse.
Drift is a an important food source for fish but
represents only about 0.01% of the invertebrate
biomass of stream at any point in time. In
other words, stream invertebrates are so effective
in remaining attached to their substrates
that carbon and nutrients spiral downstream
primarily in the dissolved phase.
Headwater streams less than 10m in width
are particularly important in nutrient processing
because they are the immediate recipient of
most terrestrial inputs and account for up to
85% of the stream length within most drainage
networks (Peterson et al. 2001). Small streams
are particularly effective in cycling nitrogen
(have shorter uptake lengths) because their
shallow depths and high surface to volume
ratios enhance nitrogen absorption by algae
and bacteria that are attached to rocks and sediments.
Uptake lengths for ammonium range
from 10 to 1000m and increase exponentially
with increases in stream discharge (Peterson et
al. 2001). Streams generally have much higher
nitrate than ammonium concentrations, even
when they occur in ammonium-dominated
watersheds, because of preferential uptake of
ammonium over nitrate by stream organisms
and because nitrification rates are frequently
high in riparian zones and in streams. For these
reasons, the uptake length of nitrate is about
10-fold greater than that of ammonium. Thus
nitrate is much more mobile than ammonium
in streams, as on land, but for different reasons.