Environmental Engineering Reference
In-Depth Information
strategies to tackle the issue of human alteration of the P cycle, and the problems caused by
that alteration, in the final section of this chapter.
THE PHOSPHORUS CYCLE AT THE
WATERSHED SCALE
Nested within the global cycle of P are smaller (spatial)-scale P cycles. For relatively
pristine forested and grassland systems, the inputs of P include weathering and (limited)
atmospheric inputs in the form of dust, but also P transferred by animal movement and
migration. Organisms feeding in the terrestrial environment or in the coastal oceans can
transfer P directly from one ecosystem to another. For some lakes, geese have been shown
to be important in the transfer of P from grasslands to aquatic systems and salmon are
well cited as an example of transfer from coastal oceans and estuaries to rivers, lakes, and
even terrestrial systems. In the case of Pacific salmon, more than 95% of their body mass is
accumulated in the marine environment, yet this material is deposited in freshwater
systems where the salmon return to spawn and die. This represents an important nutrient
subsidy from marine to freshwater systems (
Naiman et al. 2002
). These same transfers can
happen in terrestrial systems. For example, roe deer that graze on crops and then defecate
in nearby forests may bring a significant source of nutrients to those forest patches (Abbas
et al. 2012).
The P transferred from terrestrial to aquatic ecosystems is of particular interest because
of the extreme sensitivity of many aquatic systems to P inputs. The P cycling in inland
waters and the supply from the watershed are controlled by the interaction of hydrology,
chemistry, and biology. Phosphorus typically enters aquatic systems from terrestrial ones
bound to soil particles carried there by erosion or via air as dust. However, P can also
enter water bodies from sewage, manure runoff, industrial wastes, seepage from septic
tanks, and other sources.
Table 8.1
shows the relationship of lake productivity to P concen-
trations as well as typical P concentrations in pristine and human-impacted lakes.
The amount of P transferred to aquatic systems from pristine watersheds depends on a
number of factors including soil texture and chemistry, the slope of the watershed, and
the amount and pattern of precipitation and water runoff. Human-induced climate change
can alter P outputs through its impact on water runoff. Additionally, atmospheric
pollution may alter these transfers. In particular, acid precipitation is thought to decrease
soil pH and increase binding of P to soils and, hence, decrease P transfer from terrestrial
TABLE 8.1
Relationship between lake productivity
and average epilimnetic total P concentrations.
Lake Productivity
Total P
Oligotrophic
,
5
µ
g/L
Mesotrophic
5
30
µ
g/L
Eutrophic
30
100
µ
g/L
Hypereutrophic
100
g/L
.
µ
