and animal wastes, will wash out of soils and into rivers. The seasonal variation
of NO 3 - concentrations in many temperate rivers is caused mainly by fluctuation
in supply of NO 3 - from soil. In summer, NO 3 - concentrations are low because soil-
water flushing by rainfall is low. In the autumn, soil moisture content increases,
allowing nitrate to wash out of the soil into rivers (Fig. 5.14). In nitrate-rich rivers
like the Great Ouse in eastern England (N : P about 30 : 1 in winter), biological
production makes little initial impact on NO 3 - levels until later in the season as
the NO 3 - supply decreases due to reduced runoff. A NO 3 - minimum is reached in
summer as a result of the reduced supply from soils and increased biological
uptake, before rising again in the autumn (Fig. 5.14). DIP concentrations, in
contrast, show more erratic behaviour (Fig. 5.14), reflecting the influences of
biological and dilution control working out of phase, but are generally higher
during low-flow conditions in summer.
Apart from biological uptake, denitrification in low-oxygen environments is
the most important way that NO 3 - is removed from soil, rivers and groundwater.
It has been estimated that, in the rivers of northwest Europe, half of the total
nitrogen input to the catchment is lost by denitrification before the waters reach
the sea. Thus, under low redox conditions, DIP is mobilized during iron(III)
reduction and NO 3 - is lost, again emphasizing the importance of redox processes
in environmental chemistry.168 Chapter Five
MAM J J A S ON D JMAM J J A S ON D J
MAM J J A S ON D JMAM J J A S ON D J80
60
40
2020015010050
Chlorophylla (mg l–1
)Phosphorus (mmol l–1
)200100Silicon (mmol l–1
)700
600
500
400
300
200
100Nitrate (mmol l–1)1990199019901990Fig. 5.14Seasonal variations in dissolved phosphorus, silicon, nitrate and particulate chlorophyll a(as a
measure of phytoplankton abundance) in the River Great Ouse (eastern England). Data from Fichez et al.
(1992).