Setting concentration and/or load targets for nutrients (mainly nitrogen, N, and phosphorus, P) in lakes is problematic for several reasons. There are few numeric standards for specific forms of N and P, and those that exist do not adequately reflect the range of observed responses
in aquatic environments. Regional criteria developed for the USEPA can provide higher reliability in predictions of system response to changes in nutrient levels, but a lake-specific evaluation is still needed in most cases. Meeting numeric federal criteria or state standards is not necessarily
coincident with protecting beneficial uses. Narrative standards generally reflect the desire to limit human impacts and are intended to protect beneficial uses of aquatic systems, but do not provide the numeric guidance necessary to develop a valid TMDL. Furthermore, it is not the nutrients
themselves that pose a threat in the vast majority of cases, but rather their processing in the aquatic system that leads to elevated primary production and possible biomass accumulation. Such chemical-biological coupling is not always inconsistent with beneficial uses, as elevated productivity
can enhance fish and wildlife propagation. However, some adverse biological impacts often occur, and elevated productivity often negatively impacts uses such as contact recreation, water supply, and aesthetics. Determining TMDL target conditions is therefore a function of clear goal setting
and determining the linkage between nutrient levels and goal achievement. In setting TMDL targets for lakes, available approaches include: Use of numeric criteria or standards, as is or adjusted on a site-specific basis as warranted
by more detailed knowledge of local conditions and use goals Estimation of “pristine” conditions based on pre-settlement land use applied in loading and response models Estimation of potential future
conditions based on current or future land uses and practical Best Management Practice application applied in loading and response models Conversion of narrative standards to numeric criteria based on the intent of the narrative standard (protective of
what use or feature?) and knowledge of local linkages between nutrient levels and uses Back-calculation of loads or concentrations necessary to meet desired uses at some acceptable frequency, typically using empirical relationships and uncertainty
or reliability analyses Each approach has merits and drawbacks, and applying all of them initially allows assessment of the range of possibilities. Careful comparison of goals and possible targets can foster determination of the most appropriate target. Final
target selection should be tempered by realistic management options and commitment to adaptive management through iterative rounds of assessment, management, and evaluation. In general, lakes listed for non-attainment of uses are impacted by human activities to the extent that restoration
of “pristine” conditions is not practical, if such conditions ever existed. Additionally, achievement of many use goals is often possible at nutrient levels well above the postulated “natural background” level. Numeric standards or criteria established from broad databases
tend to correspond to conditions that support designated uses, but have an implicit margin of safety that is often excessive and impractical from a management perspective. Comparison of levels achievable by BMPs with loads derived from either conversion of narrative standards or relationships
between nutrients and targeted conditions (e.g., chlorophyll, Secchi transparency) has provided useful insight in many cases. This approach also facilitates consensus among interested parties and lends itself to adaptive management. A focus on matching TMDL targets to desired uses, expressed
as a distribution of expected conditions or variable values, is strongly recommended.
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