Geography Reference
In-Depth Information
surface waters originates from diffuse rather than
point sources. Increasing nitrate concentrations in
freshwaters are therefore thought to reflect
intensification and expansion of agricultural
production, but the linkages between land-use
change, increased fertiliser application and rising
nitrate levels are not always direct or simple
(Heathwaite
et al
. 1993).
Less information is available about total
nitrogen and the relative importance of different
N species in freshwaters, especially for river
systems. A recent study of four contrasting UK
rivers (Russell
et al
. 1998) showed that total
nitrogen loads were dominated by the dissolved
total oxidised fraction (nitrate and nitrite), which
accounted for 76 to 82 per cent of the annual flux
in 1995 (Table 11.2). Dissolved organic N and
particulate N did not contribute more than 16 and
8 per cent of the annual load, respectively, and
dissolved inorganic nitrogen in the form of
ammonium was a minor component of total N in
these predominantly rural and not highly polluted
systems. In contrast, information for UK lakes
(Heathwaite 1993) shows that the organic fraction
often dominates the dissolved component of
nitrogen, especially in upland environments,
where lentic waters are poor in nutrients and in
plant life.
The particulate fraction is of more importance
in the transport of phosphorus in freshwater
systems, because erosion of sediment from
agricultural land provides an important pathway
by which P bound to the soil matrix can be
introduced to river systems. In the study of the
four UK catchments referred to above, total
particulate P, comprising both inorganic and
organic fractions, accounted for 26-75 per cent
of the annual total P flux (see Table 11.2).
Increased use of fertilisers containing inorganic
P, which has for example tripled in the USA
between 1945 and 1993 (Puckett 1995), and
greater dissolved P inputs from sewage, livestock
wastes and detergents (Withers 1994) have led to
increased phosphorus concentrations in rivers
and lakes of Europe and North America over the
last fifty years (Heathwaite
et al.
1997). However,
improvements to waste water treatment facilities
and the banning of phosphate-based detergents
have markedly reduced total phosphorus
concentrations during the last 15 years in some
rivers and lakes (e.g. Edmonson 1985; Peters
et al
.
1997).
Enrichment of water bodies with plant
nutrients, especially phosphorus and nitrogen,
which leads to changes in biological structure and
function, is the process of eutrophication
(Vollenweider 1968; Harper 1992; Rast and
Thornton 1997). This process takes place naturally
over geological time but may be greatly
accelerated by human disruption of catchment
nutrient cycles. Excessive algal and rooted plant
growth occurs, which in turn results in
Table 11.2
The percentage contribution of individual fractions to nutrient fluxes in selected UK drainage basins.
Notes:
TDN=total dissolved nitrogen; NH4-N=dissolved ammoniacal nitrogen; TON=dissolved total oxidisable nitrogen
(nitrate and nitrite); DON=dissolved organic nitrogen; TPN=total participate nitrogen; TDP=total dissolved phosphorus;
DIP=dissolved inorganic phosphorus; DOP=dissolved organic phosphorus; TPP=total particulate phosphorus;
PIP=particulate inorganic phosphorus; POP=particulate organic phosphorus.
