Environmental Engineering Reference
In-Depth Information
BOX 5.2
NUTRIENT SPIRALING
In streams, currents move materials
downstream at the same time that they are
cycling between compartments. Stream ecol-
ogists think of this combination of processes
as drawing out a nutrient cycle into a coil
or spiral ( Figure 5.3 ; see also Chapter 16)
and have developed a formal framework to
analyze nutrient spiraling, the combination
of transport and cycling.
Three parameters are commonly calcu-
lated to describe nutrient uptake and trans-
port in this framework. Areal uptake (U)is
the rate at which a dissolved nutrient is
removed from stream water, per area of
streambed (in units of mass per area per
time). This uptake could be biotic or abiotic.
Uptake length (S w ) is the distance that a dis-
solved nutrient travels before it is taken up
by the streambed or the biota. S w obviously
depends on both the intensity of nutrient
uptake and the velocity of water flow; a
short uptake length could result from rapid
nutrient uptake, slow water flow, or both.
Uptake velocity (v f ) is the apparent vertical
velocity at which the nutrient is lost from
the water column (as if it were sinking out
from the water). Because U and v f do not
depend directly on the discharge or water
velocity of a stream, they can be especially
useful in removing the effects of these
variables when comparing stream ecosys-
tems. The spiraling parameters are related
Water flow
Downstream
transport
Uptake
Release
Stream
Stream bed
FIGURE 5.3 Nutrient spiraling,
in which downstream transport draws out nutrient cycles into
spirals.
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