Page 129 - Soil and water contamination, 2nd edition

116 Soil and Water Contamination

Fertiliser
Atmospheric Produce
Animals
Runoff
and
erosion
Vegetation / crops Surface water
solution
Algae / Aquatic
Adsorbed vegetation
Soil organic ions
Adsorbed Soil solution matter Fish
ions
Adsorbed Detritus
Pore
Soil organisms ions solution
Weathering Groundwater seepage 6642 6642 6642
Leaching
Figure 6.1 Schematic overview of nutrient cycling in terrestrial and aquatic ecosystems.

some soil bacteria, and P may be released by slightly soluble inorganic minerals. Nutrients are
withdrawn from local cycling due to crop harvest, leaching to deeper soil horizons beyond
the reach of plant roots, release of gaseous compounds to the atmosphere (only nitrogen ),
storage in peat or other organic deposits, or lateral runoff and erosion to downstream areas.
Most of these local nutrient sources and sinks are part of the nutrient cycle at the regional or
global scale .
Nutrient cycling is a natural process, but it may be enhanced or accelerated by external
nutrient inputs of anthropogenic origin, such as fertilisers and wastes. Obviously, the
enhancement is intentional in farming, to increase agricultural production, but nutrients
of anthropogenic origin may also enter natural ecosystems via various hydrological or
atmospheric pathways (e.g. surface runoff, atmospheric deposition , groundwater discharge ).
The resulting enhanced nutrient cycling is also referred to as eutrophication .
The condition of an ecosystem can be described in terms of its trophic state, i.e. its
degree of eutrophication or the lack thereof. Three designations are used: 1) oligotrophic , i.e.
low productivity, ecosystems; 2) mesotrophic , i.e. intermediate productivity ecosystems, and
3) eutrophic , i.e. high productivity ecosystems. In both terrestrial and aquatic ecosystems,
eutrophication causes an increase of biomass production and a loss of biodiversity.
Furthermore, in surface waters, the increased nutrient levels may result in an excessive growth
of aquatic weeds and algae (phytoplankton ). The excessive growth of phytoplankton is also
known as algal bloom . This increased production of aquatic plants has several consequences
for water uses (Thomann and Mueller, 1987):
1. Large diurnal variations in dissolved oxygen (DO): by day phytoplankton produce
oxygen through photosynthesis , but at night they consume oxygen through respiration .
The excess occurrence of algae then causes a rapid depletion of DO, which, in turn, can
result in fish dying.
2. Phytoplankton and weeds settle to the bottom of the water system and create a sediment
oxygen demand (SOD). In deep lakes and reservoirs, this results in low values of DO in
the hypolimnion .
3. Extensive growth of rooted aquatic macrophytes interferes with channel carrying capacity,
aeration , and navigation.
4. Aesthetic and recreational drawback: algal mats, decaying algal clumps, odours, and
discolouration may occur.
5. Public health drawback: some blue algae (cyanobacteria ) species secrete toxins that can
be harmful to the liver. Blooms of these algae in summer may make surface water bodies
unsuitable for swimming.

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