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Assessment Approaches

The ambient concentrations of nutrients sometimes can provide an indication of the level of eutrophication. Often the limiting nutrient is reduced to very low concentrations, while nutrients less in demand have higher concentrations. However, nutrients are present in different forms, which vary in their relevance to assessing eutrophication. In most studies of rivers and standing waters, the forms of phosphorus and nitrogen are operationally defined based on available analytical methods. The distinction between particulate and dissolved forms depends on the porosity of the filter used to separate the two fractions; filters with porosities approximately 0.5 µm are commonly used. Total dissolved phosphorus is often divided into soluble reactive phosphorus, which can sometimes be considered dissolved inorganic phosphorus, and dissolved organic phosphorus. Similarly, total dissolved nitrogen includes dissolved inorganic ammonium, nitrate, and sometimes nitrite and urea, and dissolved organic nitrogen. Total particulate phosphorus and nitrogen are determined as particulate inorganic phosphorus and nitrogen and particulate organic phosphorus and nitrogen. In some cases, concentrations of total phosphorus or nitrogen are measured; these include all the dissolved and particulate forms. However, only a portion of the total phosphorus or nitrogen is biologically available.

The nitrogen to phosphorus ratio in particulate organic matter suspended in lakes is a potentially valuable index of the nutritional status of the phytoplankton. Healthy algae contain approximately 16 atoms of nitrogen for every atom of phosphorus. Ratios of nitrogen to phosphorus less than 10 often indicate nitrogen deficiency and ratios greater than 20 can indicate phosphorus deficiency. Often nitrogen to phosphorus ratios are low in eutrophic lakes and high in mesotrophic and oligotrophic ones, and blooms of nitrogen-fixing cyanobacteria have been induced experimentally in lakes after reducing nitrogen to phosphorus ratios in inflows.

The rate of uptake of radioactive phosphate by particulate matter suspended in lakes is a widely used index of phosphorus demand by the plankton. Turnover times are typically rapid when phosphorus is in short supply and are slow when supply is adequate.

Nutrient limitation can be assessed by experimental manipulation of nutrient levels. Experiments can be carried out on scales ranging from small flasks to enclosures containing many liters to whole lakes. Large volume experiments provide more realistic conditions than small containers. Enclosures with volumes ranging from tens to thousands of liters can be replicated with experimental designs that permit discrimination of interacting factors leading to changes in water quality.

Phytoplankton species composition changes in response to eutrophication. Although general trends in the development of certain assemblages of phytoplankton are associated with trophic status, particular phyla or classes cannot be assigned exclusively to one level of eutrophication. While cyanobacteria are commonly observed under eutrophic conditions, other species can be important.

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