Newsletter and Technical Publications
Lakes and Reservoirs vol. 3
Water Quality: The Impact of Eutrophication
Where Nutrients Come From and How They Cause Entrophication
Where Nutrients Come From
What are the sources of nutrients causing eutrophication of lakes and
reservoirs? There are many sources. All activities in the entire drainage area
of a lake or reservoir are reflected directly or indirectly in the water quality
of these water bodies. A lake or reservoir may, however, be naturally eutrophied
when situated in a fertile area with naturally nutrient enriched soils. In many
lakes and reservoirs wastewater is the main source since untreated wastewater or
wastewater treated only by a conventional mechanical- biological methods still
contains nitrogen (25-40 mg per liter) and phosphorus (6-10 mg per liter). Both
nitrogen and phosphorus can be removed by well-known technology - phosphorus by
addition of a chemical that precipitates phosphate though a chemical reaction,
and nitrogen usually by biological means through micro- organism activity.
Nitrogen costs more money and also, technically speaking, is more difficult to
remove than phosphorus.
|Photo 20: General view of a wetland.
Drainage water from agricultural land also contains phosphorus and nitrogen.
It usually has much more nitrogen because phosphorus is usually bound to soil
components. Extensive use of fertilizers results in significant concentrations
of nutrients particularly nitrogen, in agricultural runoff. If eroded soil
reaches the lake, both phosphorus and the nitrogen in the soil contribute to
eutrophication. Erosion is often caused by deforestation which also results from
unwise planning and management of the resource.
Wetlands are increasingly used to solve the problem of diffuse pollution from
agriculture which cause eutrophication (Photo 20). Nitrate is converted in
wetlands to free nitrogen and released to the air. This is not harmful, as free
nitrogen compromises about 4/ 5ths of the atmosphere. Phosphorus is adsorbed by
wetland soils and, like nitrogen, is taken up by the plants. Both nitrogen and
phosphorus may therefore be removed by wetlands. In addition, it is often also
necessary to control fertilizer usage in agricultural practices as the majority
may end up in the drainage area, if the diffuse pollution from nutrients is to
be reduced sufficiently to improve water quality.
Photo 21: Aerial view of fish pens in Laguna de Bay, Philippines.
Rain water contains phosphorus and nitrogen from air pollution. As nitrogen
is more mobile in the atmosphere than phosphorus, it is usually over 20 times
more concentrated than phosphorus. Nitrogen can only be reduced in rain water by
extensive controls of the air pollution in the entire region. One can safely say
that the main sources of pollution in the atmosphere are from industries and
automobile exhaust without proper filtering systems.
When lakes are used for aquaculture, excess fish food pollutes the water as
complete use of the food cannot be achieved (Photo 21). Nitrogen and phosphorus
present in the excess food is dissolved or suspended in the water. The use of
lakes for aquaculture therefore needs careful environmental planning and
management practices by the owners and workers.
The sediment of a lake -its muddy bottom layer -contains relatively high
concentrations of nitrogen and phosphorus. These can be released to water,
particularly under conditions of low oxygen concentrations. The nutrients in the
sediment come from the past settling of algae and dead organic matter. The
nutrients released from sediments are referred to as the lake’s
Figure 5 sketches the sources of nutrients: externally from wastewater,
agricultural drainage water, erosion and rain, and internally from activities in
the lake itself, e.g. aquaculture and sediment release.
It is possible but very expensive to remove the upper nutrient-rich layer of
sediment. Covering sediments with clay to seal them and thereby reduce internal
loading has also been tried. Even when nutrients are removed in large amounts
from wastewater, agricultural drainage water and rain, it often takes much time
before nutrient concentrations fall in the upper sediment layer because they are
still present in the water environment. Early reduction or elimination of
nutrient sources is therefore very important.
Fig. 5: Major sources of nutrients in lakes.
Lakes and reservoirs can be classified according to the extent of their
eutrophication (or nutrients enrichment) into four main classes: oligotrophic,
mesotrophic, eutrophic and hypereutrophic (Table 3). This classification results
from extensive examination of eutrophication in countries within the
Organization for Economic Cooperation and Development (OECD) in the 1970s and
early 1980s. It is based on concentrations of phosphorus, nitrogen and
chlorophyll a (the green plant pigment needed
in photosynthesis). Chlorophyll a roughly indicates the concentration of plant
biomass (on average 1% of algae biomass is chlorophyll a).
Factors that limit Eutrophication
Table 4 shows the average composition of freshwater plants on a wet basis
(when they are not dried): the plants require all listed components in the
approximate percentages indicated. Generally, nitrogen (0.7%) and/or phosphorus
(0.09%) are usually the first components depleted when plants form following
photosynthesis. These two nutrients are less abundant in water than other
elements needed, relative to their composition in plants. About eight times more
nitrogen is required than phosphorus. Phosphorus thus limits eutrophication if
nitrogen is more than eight times as abundant as phosphorus, while nitrogen
limits eutrophication if its concentration is less than eight times as abundant
|Photo 22: Growth of blue-green algae on the shore of a lake.
Untreated wastewater and wastewater treated by
mechanical-biological methods contain about
32mg/L nitrogen and about 8 mg/L phosphorus on average. In a lake heavily loaded
with wastewater, eutrophication is limited by nitrogen, as the nitrogen
concentration in the discharged wastewater is only four times the phosphorus
concentration. Such lakes often display extensive blooms of blue-green algae as
unsightly surface scum (Photo 22). Some species of blue-green algae use nitrogen
directly from the air and grow, although dissolved nitrogen is limiting. Lakes
that receive natural tributaries and drainage water from agriculture, however,
have high nitrogen concentrations and are therefore usually limited by
The central question is not to determine which nutrient is limiting but to
determine which nutrient can most easily be made limiting. As phosphorus is more
easily and less expensively removed from wastewater than nitrogen, in many cases
(but not all) the best environmental management strategy for lakes and
reservoirs is to remove as much phosphorus as possible from wastewater.