Aquatic eutrophication (from the
Greek, meaning 'well
nourish') is the enrichment of natural waters with plant nutrients,
which results in the stimulation of an array of symptomatic changes.
These include the increased production of algae and other aquatic
plants, affecting the quality of the water and the balance of organisms
present within it. Such changes may be undesirable and interfere with
water uses.
The nutrient status of any natural
water is determined by the
supply of nutrients from its catchment, which in turn is influenced by
the geology. Thus waters will vary in their natural nutrient status.
Some waters are naturally eutrophic. However enrichment and
consequential impacts can occur if there are changes to the nature of
the catchment. This becomes a concern when human activities accelerate
the enrichment process, and it is this artificial eutrophication, which
warrants attention.
- Causes of eutrophication
- The growth of plants in natural waters is influenced by the
supply of nutrients, light, temperature, flow regime, turbidity,
zooplankton grazing and toxic substances. The properties of the
catchment and the water body and the impact of human activities affect
these factors. While it is important to understand the role of all
these factors, it is widely accepted that the most important is the
supply of nutrients.
In temperate freshwater systems, phosphorus is generally
the key limiting nutrient. However, silicon may limit blooms of diatoms
in spring and, in areas where phosphate levels are naturally high
because of the underlying geology, waterbodies may be nitrate-limited,
e.g. the meres of Shropshire and Cheshire. Estuarine ecosystems tend to
show phosphorus-limitation at their freshwater extreme, grading through
to nitrogen-limitation at their seaward end. However, the common
occurrence of suspended sediments in estuaries with resulting turbidity
often means that light limits algal growth. Where nutrients are the
limiting factor in coastal waters, nitrogen is generally believed to be
the key nutrient.
- Sources of nutrients
- Nutrient sources can be broadly segregated into two
categories: readily identifiable point sources (such as sewage
effluents), and diffuse sources (such as the run-off from agricultural
land), with the relative contribution of each varying between
catchments. The contributions (in percentage terms) of the main sources
of phosphorus entering surface waters in the UK have been estimated as:
-
| Agricultural |
43% |
| Human & household waste including detergents |
43% |
| Industry |
8% |
| Background source |
6% |
The source of these figures is Morse et al., The
economic & environmental impact of phosphorus
removal from wastewater in the European Community, Selper
Publications, London 1993. The actual contributions in any given
catchment will depend on the nature of the catchment and the human
activities within it.
For nitrogen, inputs to fresh waters in Europe come
principally from diffuse sources, particularly agriculture, although
point sources (usually urban wastewater) also contribute significantly
in many regions. In England and Wales 70% of the total input of
nitrogen to inland surface waters is estimated to come from diffuse
sources (agriculture, precipitation and urban run-off, in order of
decreasing importance). The remaining 30% comes from sewage effluent
and industrial discharges. As for P, the actual contributions in any
given catchment will depend on the nature of the catchment and the
human activities within it.
- Impacts of eutrophication
Eutrophication can have both temporary and long-term
effects on aquatic ecosystems. Large fluctuations in dissolved oxygen
concentrations can occur between day and night. Low oxygen levels, the
result of plant respiration, may lead to the death of invertebrates and
fish. This process can be compounded when algal blooms, through their
decay, further reduce the oxygen content of water. The growth or decay
of benthic (bottom-dwelling) mats of macro-algae can also lead to the
deoxygenation of sediments.
Certain algal species, particularly freshwater blue-green
algae, and marine dinoflagellates can produce toxins, which may
seriously affect the health of mammals (including humans), fish and
birds. This occurs either through the food chain, or through contact
with, or ingestion of, the algae. Algal species also cause fish deaths,
for example by physically clogging or damaging gills, causing
asphyxiation. Eutrophication ultimately detracts from biodiversity,
through the proliferation and dominance of nutrient-tolerant plants and
algal species. These tend to displace more sensitive species of higher
conservation value, changing the structure of ecological communities.
Eutrophication can also adversely affect a wide variety of
water uses such as water supply (e.g. algae clogging filters in
treatment works), livestock watering, irrigation, fisheries,
navigation, water sports, angling and nature conservation. It can give
rise to undesirable aesthetic impacts in the form of increased
turbidity, discolouration, unpleasant odours, slimes and foam
formation.
