The term oligotrophic has been used to describe lakes low in nutrients and
consequently low in productivity. Lakes high in productivity are termed eutrophic.
As a general rule, lakes proceed from oligotrophic to eutrophic as the lake ages.
Some researchers add the word mesotrophic to designate lakes on the verge of
becoming eutrophic. These terms are not intended to imply that all eutrophic lakes
are undesirable or that all oligotrophic lakes are desirable. The desirability of a
specific level of productivity is a function of the specific use of the lake. Probably
what is desirable is a mixture of lakes of the different types. The long-range problem
is that as lakes age, the nutrients accumulate within the lake. New nutrients are
brought into the lake from allochthonous inputs. Siltation may decrease the volume
of water within the lake, thus concentrating the nutrients. Anthropogenic inputs such
as wastewaters and fertilizers add significantly to the nutrient level. Deforestation
results in more rapid runoff, which carries both silt and nutrients into the lake. All
these combine to increase eutrophication in a lake.
4 Considerations in Remediation
In order to prolong the life of a lake, actions must be taken to reduce the rate of
eutrophication. Very little can be done to overcome the natural process of eutrophication. However, much can be done to overcome the anthropogenic impacts. It is
easy to say just stop any human activities that contribute to the eutrophication, but
that is difficult to achieve. The best that can be done is to determine what activity will
provide the best return for the effort and/or expenditure.
Sakamoto [2] showed a direct correlation between the phosphorus concentration
in a lake at the time of spring turnover and the amount of productivity as measured
by the amount of chlorophyll-α present during the summer (Fig. 7.3). Correspondingly, the greater the chlorophyll-α content, which indicates the presence of algae,
the greater the turbidity of the water, and, therefore, the lower the clarity of the water
as measured by the Secchi disk depth. Whereas there was good coordination
between the phosphorus content and the chlorophyll-α, there was poor correlation
between chlorophyll-α and the clarity of the water. Substances other than chlorophyll-α can impact the turbidity of the water. These include the presence of zooplankton that feed on the phytoplankton and particulate matter, such as fine clay or
silt that is carried into the lake in the runoff.
Numerous models have been derived to correlate certain specific parameters with
the trophic state of a lake. Two stand out as being quite reliable and simple. Both
relate total phosphorus loading to the trophic state of the lake as a function of the
body of water. In the original work by Vollenweider [3], he showed a correlation
between the total phosphorus loading and the mean depth of the lake. Many lakes
were studied and there was a good correlation between these two parameters. Later
Vollenweider and Dillon [4] improved the model by comparing phosphorus loadings
with the mean depth and the retention time of the lake (Fig. 7.4). The correlation was
poor with lakes that were not phosphorus limited.
7 Lake Restoration
265
consequently low in productivity. Lakes high in productivity are termed eutrophic.
As a general rule, lakes proceed from oligotrophic to eutrophic as the lake ages.
Some researchers add the word mesotrophic to designate lakes on the verge of
becoming eutrophic. These terms are not intended to imply that all eutrophic lakes
are undesirable or that all oligotrophic lakes are desirable. The desirability of a
specific level of productivity is a function of the specific use of the lake. Probably
what is desirable is a mixture of lakes of the different types. The long-range problem
is that as lakes age, the nutrients accumulate within the lake. New nutrients are
brought into the lake from allochthonous inputs. Siltation may decrease the volume
of water within the lake, thus concentrating the nutrients. Anthropogenic inputs such
as wastewaters and fertilizers add significantly to the nutrient level. Deforestation
results in more rapid runoff, which carries both silt and nutrients into the lake. All
these combine to increase eutrophication in a lake.
4 Considerations in Remediation
In order to prolong the life of a lake, actions must be taken to reduce the rate of
eutrophication. Very little can be done to overcome the natural process of eutrophication. However, much can be done to overcome the anthropogenic impacts. It is
easy to say just stop any human activities that contribute to the eutrophication, but
that is difficult to achieve. The best that can be done is to determine what activity will
provide the best return for the effort and/or expenditure.
Sakamoto [2] showed a direct correlation between the phosphorus concentration
in a lake at the time of spring turnover and the amount of productivity as measured
by the amount of chlorophyll-α present during the summer (Fig. 7.3). Correspondingly, the greater the chlorophyll-α content, which indicates the presence of algae,
the greater the turbidity of the water, and, therefore, the lower the clarity of the water
as measured by the Secchi disk depth. Whereas there was good coordination
between the phosphorus content and the chlorophyll-α, there was poor correlation
between chlorophyll-α and the clarity of the water. Substances other than chlorophyll-α can impact the turbidity of the water. These include the presence of zooplankton that feed on the phytoplankton and particulate matter, such as fine clay or
silt that is carried into the lake in the runoff.
Numerous models have been derived to correlate certain specific parameters with
the trophic state of a lake. Two stand out as being quite reliable and simple. Both
relate total phosphorus loading to the trophic state of the lake as a function of the
body of water. In the original work by Vollenweider [3], he showed a correlation
between the total phosphorus loading and the mean depth of the lake. Many lakes
were studied and there was a good correlation between these two parameters. Later
Vollenweider and Dillon [4] improved the model by comparing phosphorus loadings
with the mean depth and the retention time of the lake (Fig. 7.4). The correlation was
poor with lakes that were not phosphorus limited.
7 Lake Restoration
265