Nutrient Criteria The state of Missouri is currently in the process of developing nutrient criteria for reservoirs, which would identify when a reservoir has too much phosphorus, nitrogen or chlorophyll. The challenges in developing criteria are many; they include dealing with a very wide range of reservoirs (in terms of size, hydrology, etc.) and considering multiple uses that can have different optimal nutrient and chlorophyll levels (swimming versus fishing).

The current proposed approach to determining criteria first involves dividing the state into physiographic regions (to account for differences in soils, topography and historic land cover). Reservoir morphology, hydrology and historic land cover (alone or in combination, depending on region) are then used to predict reservoir phosphorus concentrations. This allows the state to take into account some of the difference among the reservoirs (such as depth, hydrology, etc.) that were set in place when the impoundments were constructed. Finally, “reference” reservoirs were identified and the phosphorus concentrations of these water bodies are used to gauge the range of phosphorus concentrations found in nominally impacted reservoirs.
A target phosphorus concentration is identified for each reservoir using the above information. A reservoir’s nitrogen and chlorophyll criteria are then calculated using the target phosphorus value and desired ratios of nitrogen-phosphorus and chlorophyll-phosphorus.
The proposed reservoir nutrient criteria values have been incorporated into the trends graphs throughout this report.

Nutrient and chlorophyll criteria are identified by a white background within the trends graphs. Comparison of the long-term mean value (horizontal line within graphic) to the top of the white background will show you whether a reservoir is below, at or above the proposed criterion. It should be noted that not all reservoirs currently have proposed criteria. If the needed morphology/hydrology information is not available for a given reservoir then a site-specific criterion has not been calculated. In these cases the values shown are the regional “reference” values. Also, criteria were designed to describe desired water quality at the dam site of the reservoir. Comparisons of water quality at sites other than at the dam to criteria should be made with the knowledge that criteria are not applicable at these sites.

The determination of whether a reservoir meets or exceeds criteria will be made using all available data. In many cases the University of Missouri has additional data, which may change the long-term mean value shown in the graphs within this report.

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