Increase or decrease of one species in relation to another can give useful information about changes in systems. Depending on the problem and the system that has to be investigated, different indices are applicable.
Nygard's algal index
This index is used to evaluate the nutritional status of lake ecosystems and their fertility. The index can be calculated as a compound quotient out of:
Myxophycean + chlorococcales + centric + Euglenophyceae/desmids
Oligotrophic systems have a quotient between 0.01 and 1.0, eutrophic systems between 1.2 and 2.5. Further Nygard's indices are based on ratios between Myxophyceae and desmids, chlorococcales and desmids, centric and pinnate diatoms or Euglenophyta and Myxophyceae + chlorococcales.
In the context of altering nutrient charges in aquatic systems, the ratio of the major phytoplankton groups diatoms versus flagellates (diatoms-nondiatoms ratio) can be used as indicator. Regarding for example eutro-phication, nutrient reductions can be seen in a decrease in flagellate abundance.
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