Section NR 102.60 - Combined assessment procedure for phosphorus(1) GENERAL. (a) This section establishes a combined assessment approach for making total phosphorus attainment determinations for surface waters in cases specified in par. (b). This approach is designed to account for variability in how waterbodies respond to phosphorus. The combined approach evaluates a waterbody's quality by considering the total phosphorus concentration in the surface water in conjunction with an evaluation of the phosphorus response indicators specified in subs. (2) to (4). The phosphorus response indicators characterize the condition or abundance of aquatic organisms that are responsive to total phosphorus to determine whether aquatic life and recreation uses are being met. Together, the total phosphorus criteria and response indicators may be used to determine whether the phosphorus water quality standards are attained or whether the waterbody should be listed as impaired for total phosphorus on the section 303 (d) list.(b)1. If a waterbody's calculated total phosphorus concentration exceeds its total phosphorus criterion using the assessment procedure under s. NR 102.07 and the waterbody's calculated phosphorus concentration is within the combined assessment range shown in Table 10, the department may make the total phosphorus attainment or impairment determination using phosphorus response indicators specified in subs. (2) to (4) if sufficient biological data are available to conduct these assessments. In that case, the following decision protocols apply: a. A waterbody that attains all of its applicable phosphorus response indicators under subs. (2) to (4) may be excluded from the section 303 (d) listing of waters impaired for phosphorus. Note: If a waterbody is not considered impaired using the combined approach, it may be a candidate for a less stringent phosphorus site-specific criterion under ch. NR 119. If a waterbody attains its phosphorus criterion but one or more phosphorus response indicators are not attained, it may be a candidate for a more stringent site-specific phosphorus criterion under ch. NR 119.
b. If a waterbody does not attain one or more of the applicable phosphorus response indicators in subs. (2) to (4) or if the department does not have sufficient data to evaluate all of the applicable response indicators, then the waterbody shall be considered impaired for total phosphorus and the department shall propose inclusion of the waterbody on the section 303 (d) list as not attaining its phosphorus criterion. As part of the public comment period for the section 303 (d) list, the department shall provide a list of waterbodies needing additional data to determine whether phosphorus response indicators are met. If sufficient phosphorus response indicator data becomes available in the future, the waterbody may be reassessed.2. If a waterbody's calculated phosphorus concentration exceeds its total phosphorus criterion using the assessment procedure under s. NR 102.07 and the waterbody's calculated phosphorus concentration also exceeds the upper limit of the combined assessment range shown in Table 10, then the waterbody shall be considered impaired for total phosphorus regardless of attainment of phosphorus response indicators, and the department shall propose to include the waterbody on the section 303 (d) list. Table 10
Range for applying combined assessment for total phosphorus1
Waterbody type | Total phosphorus criterion (ug/L) | Combined approach range2 (ug/L total phosphorus) |
Stream or its impounded flowing water | 75 | 75 to [LESS THAN]150 |
River or its impounded flowing water | 100 | 100 to [LESS THAN]200 |
Unstratified reservoirs, unstratified drainage or seepage lakes | 40 | 40 to [LESS THAN]60 |
Stratified reservoirs, stratified drainage lakes | 30 | 30 to [LESS THAN]45 |
Stratified seepage lakes | 20 | 20 to [LESS THAN]30 |
Two-story fishery lakes | 15 | 15 to [LESS THAN]22.5 |
1 To determine whether a waterbody falls into the combined approach range, compare the lower confidence limit of the waterbody's two-sided 80% confidence interval around the mean (for lakes/rivers) or median (for rivers/streams) total phosphorus concentration to the ranges in the table.
2 For streams and rivers the combined criteria range is between the applicable total phosphorus criterion and two times that criterion. For lakes, the range is between the applicable total phosphorus criterion and 1.5 times that criterion. If a waterbody has an approved site-specific phosphorus criteria, the combined criteria range for that waterbody shall be calculated using these multiplication factors.
(2) LAKE AND RESERVOIR PHOSPHORUS RESPONSE INDICATORS. A lake or reservoir 5 acres or greater for which the total phosphorus concentration is within the combined approach range specified in Table 10 shall be listed on the section 303 (d) list as impaired for phosphorus unless it attains all of the following phosphorus response indicators: (a)Frequency of moderate algae levels. The biological assessment thresholds for frequency of moderate algae levels to attain recreation uses as specified in s. NR 102.56 (2).(b)Chlorophyll a. The chlorophyll a biological assessment threshold to attain aquatic life uses as specified in s. NR 102.56 (1) (a).(c)Aquatic plants. The aquatic plant phosphorus response indicator for aquatic life use in this paragraph. Thresholds for assessing macrophyte community response to phosphorus levels in a lake or reservoir are shown in Table 11. Thresholds indicate the acceptable percentage of a lake or reservoir's vegetated area supporting species that are phosphorus-sensitive or phosphorus-tolerant. Non-attainment of a threshold indicates that an aquatic plant community is considered degraded by phosphorus concentrations in the surface water. Assessment methods are the same as those specified in s. NR 102.56 (1) (b) 2. except percentages are compared against thresholds in Table 11. Table 11
Lake aquatic plant community phosphorus response indicator
Lake subcategory1 | Macrophyte assessment of condition for phosphorus is attained if: |
Northern seepage | Phosphorus tolerant [LESS THAN OR EQUAL] 44% |
Northern drainage | Phosphorus sensitive [GREATER THAN] 51% |
Southern seepage | Phosphorus sensitive [GREATER THAN] 26% |
Southern drainage | Phosphorus sensitive [GREATER THAN] 42% |
1 In Table 11, northern lakes are those north of 44.84707° N latitude, and southern lakes are those south of that latitude. This plant phosphorus response indicator does not apply to the Great Lakes or lakes less than 5 acres in surface area.
(d)Oxythermal layer thickness. The oxythermal layer thickness criteria specified in s. NR 102.04 (4) (am). This paragraph applies only to two-story fishery lakes.(3) RIVER AND IMPOUNDED FLOWING WATERS PHOSPHORUS RESPONSE INDICATOR. A river listed in s. NR 102.06 (3) (a), or its impounded flowing waters, for which the total phosphorus concentration is within the combined approach range specified in Table 10 shall be listed on the section 303 (d) list as impaired for phosphorus unless it exceeds 20 ug/L chlorophyll a for fewer than 30 percent of days during the summer sampling period of July 15 to September 15, as calculated following s. NR 102.56 (2) (c).(4) STREAM PHOSPHORUS RESPONSE INDICATORS. (a)General. A stream for which the total phosphorus concentration is within the combined approach range specified in Table 10 shall be listed on the section 303 (d) list as impaired for phosphorus unless it attains the phosphorus response indicators specified in this subsection. When applying the phosphorus response indicators for streams, the department may apply the benthic algal biomass indicator under par. (b) as a screening tool before determining whether the benthic diatom assessment under par. (c) is necessary for an attainment determination. If available, benthic diatom assessment results under par. (c) supersede results from the benthic algal biomass screening under par. (b).(b)Benthic algal biomass screening. Benthic algal biomass is a measure of primary productivity in streams, and is quantified using a viewing bucket assessment method along stream transects. The benthic algal biomass phosphorus response indicator is applicable to both the aquatic life use and the recreational use, and may be used to make an initial use attainment determination as specified in Table 12. If results from the benthic algal biomass assessment conclusively demonstrate attainment or non-attainment of the benthic algal biomass indicator, no benthic diatom analysis under par. (c) is necessary for the attainment decision. If the benthic algal biomass assessment is inconclusive according to Table 12, or in cases where the assessment is inappropriate due to silted substrate, additional benthic diatom analysis under par. (c) is required to make the aquatic life use attainment determination. If a stream's benthic algal biomass score is inconclusive and a benthic diatom sample is not available, the stream shall be proposed for inclusion on the section 303 (d) list. Table 12
Stream benthic algal biomass phosphorus response indicator
Benthic algal biomass, viewing bucket score (0-3) | Attainment decision |
Aquatic life use | Recreational use |
[LESS THAN] 1 | Attained1 | Attained |
1 - 2 | Inconclusive; assess benthic diatoms |
[GREATER THAN] 2 | Not attained | Not attained |
1 If the mean score is [LESS THAN] 1 but 20% or more of individual transect points score a 3, a benthic diatom assessment under par. (c) is required to make an attainment determination.
Note: Wisconsin's benthic algal viewing bucket methods are available on the department's website in the Electronic Guidance and Documents (EGAD) system at https://apps.dnr.wi.gov/water/egadSearch.aspx by searching for Viewing Bucket Method for Estimating Algal Abundance in Wadeable Streams.
(c)Benthic diatoms. Benthic diatoms are an algal taxonomic group that represents primary producer community structure, and are used for assessment of the aquatic life use. This assessment is needed only if the benthic algal biomass assessment for aquatic life under par. (b) is inconclusive or inappropriate due to siltation. A stream's diatom taxa are statistically analyzed using Wisconsin's weighted average Diatom Phosphorus Index, or DPI. To determine use attainment, the DPI result shall be compared to the stream phosphorus criterion of 75 ug/L phosphorus. If only one diatom sample per site is available, the confidence interval approach described under s. NR 102.52 (2) (c) is applied. If the DPI is below 75 as specified under s. NR 102.52 (2) (c) 1., the phosphorus response indicator is attained. If more than one sample is available from the most recent 5 years, the mean score of the surveys is calculated and compared to the threshold of 75 ug/L without applying confidence intervals.Wis. Admin. Code Department of Natural Resources NR 102.60
Adopted by, CR 19-094: cr. Register September 2022 No. 801, eff. 10/1/2022Amended by, correction in (Note) made under s. 13.92 (4) (b), Stats., Register January 2024 No. 817, eff. 2/1/2024The statistical code to run the Wisconsin DPI calculation can be obtained through the department's Water Evaluation Section by contacting the department's call center at 1-888-WDNRINFo (1-888-936-7463) or using options provided on its website at https://dnr.wi.gov/contact/.