Eutrophication: Consequences and Correctives
3
One can therefore expect radically different effects on water quality depending on the relative proportions of the phosphorus supply that are accumulated in large, inedible algae or propagated through an efficient grazer
food chain.
Phosphorus Loading Models. From both large regional surveys and wholelake manipulations it has become clear that phosphorus can usually be
considered the key limiting nutrient in freshwater plankton systems, and
that the phosphorus supply sets an upper bound on the phytoplankton
yield of a given lake (Schindler 1977, 1978). Eutrophication control has
therefore to a large extent been a question of reducing phosphorus inputs
to lakes.
The simple one-box model introduced by Vollenweider (1968) has
proven its value as a management tool in predicting the total phosphorus
concentration resulting from a given phosphorus loading. The main idea
behind this model and its descendants (cf. Reckhowand Chapra 1983) is
that the net result of all biotic and abiotic phosphorus transport processes
within the lake can be described by a single parameter, the phosphorus
retention, which is defined as the ratio of the amount of phosphorus held
back in the lake to the amount of phosphorus put into the lake. The crucial
assumption behind practical application of these models is that the phosphorus retention can be taken to be a static entity for a given lake, and that
it can be predicted from easily observable parameters like water renewal
time or flushing rate.
By reducing the individual characters of lakes to just a set of morphometric and hydraulic parameters, all aspects of lakes as living ecosystems
are neglected. This probably will put strong limitations to what predictive
power can ever be gained from this approach, such that further refinement
of predictive water quality models will have to include some aspects of
biology. On the other hand, the kind of mass-balance philosophy inherent
in the simple loading models has served an important purpose in emphasizing lakes as open dynamic systems where the trophic state is a property
of the whole watershed and not only of the pelagic zone of the lake (cf. Hutchinson 1973). As such, the dynamic structure of the simple loading
models should also provide a sound framework for constructing more
complex models where some of the input and output processes are given a
biological interpretation.
Water Quality Improvement Through Biomanipuiation. Although total
phosphorus typically explains more than 80% of the chlorophyll a variance
in log-log regressions from regional studies, the residual variance in the
backtransformed variables will still be sufficient to let the phytoplankton
yield vary by an order of magnitude among lakes with the same total phosphorus level. Zooplankton grazing has been identified as a strong modulator of the realized phytoplankton yield in a given locality (Shapiro 1980).
3
One can therefore expect radically different effects on water quality depending on the relative proportions of the phosphorus supply that are accumulated in large, inedible algae or propagated through an efficient grazer
food chain.
Phosphorus Loading Models. From both large regional surveys and wholelake manipulations it has become clear that phosphorus can usually be
considered the key limiting nutrient in freshwater plankton systems, and
that the phosphorus supply sets an upper bound on the phytoplankton
yield of a given lake (Schindler 1977, 1978). Eutrophication control has
therefore to a large extent been a question of reducing phosphorus inputs
to lakes.
The simple one-box model introduced by Vollenweider (1968) has
proven its value as a management tool in predicting the total phosphorus
concentration resulting from a given phosphorus loading. The main idea
behind this model and its descendants (cf. Reckhowand Chapra 1983) is
that the net result of all biotic and abiotic phosphorus transport processes
within the lake can be described by a single parameter, the phosphorus
retention, which is defined as the ratio of the amount of phosphorus held
back in the lake to the amount of phosphorus put into the lake. The crucial
assumption behind practical application of these models is that the phosphorus retention can be taken to be a static entity for a given lake, and that
it can be predicted from easily observable parameters like water renewal
time or flushing rate.
By reducing the individual characters of lakes to just a set of morphometric and hydraulic parameters, all aspects of lakes as living ecosystems
are neglected. This probably will put strong limitations to what predictive
power can ever be gained from this approach, such that further refinement
of predictive water quality models will have to include some aspects of
biology. On the other hand, the kind of mass-balance philosophy inherent
in the simple loading models has served an important purpose in emphasizing lakes as open dynamic systems where the trophic state is a property
of the whole watershed and not only of the pelagic zone of the lake (cf. Hutchinson 1973). As such, the dynamic structure of the simple loading
models should also provide a sound framework for constructing more
complex models where some of the input and output processes are given a
biological interpretation.
Water Quality Improvement Through Biomanipuiation. Although total
phosphorus typically explains more than 80% of the chlorophyll a variance
in log-log regressions from regional studies, the residual variance in the
backtransformed variables will still be sufficient to let the phytoplankton
yield vary by an order of magnitude among lakes with the same total phosphorus level. Zooplankton grazing has been identified as a strong modulator of the realized phytoplankton yield in a given locality (Shapiro 1980).