Colorado Benthic Macroinvertebrates

Benthic Macroinvertebrate (aquatic insects) found in stream bends can indicate the health of a stream or river. Monitoring and assessing of the benthic macroinvertebrates is important in order to target resources to achieve the goals of the federal Clean Water Act (CWA) and the Colorado Water Quality Control Act (CWQCA), which is to restore, maintain, protect, and improve the quality of our water resources ¹ .  ¹   _{See Colorado Water Quality Control Act, § 25-8-102(2), C.R.S.; Federal Clean Water Act, § 101(a), 33 U.S.C. § 1251(a).}  

The Water Quality Control Division (WQCD) monitors and assesses benthic macroinvertebrates to assist in determining the status of the Aquatic Life Use. The assigned classified uses for a waterbody can also be for the expected or future condition of that waterbody. For Colorado, there are five different categories of classified uses that were adopted by the  Colorado Water Quality Control Commission . The five categories are: 1) Agriculture; 2) Domestic Water Supply; 3) Recreation; 4) Wetlands; and 5) Aquatic Life.

For further information regarding the Colorado Water Quality Control Division's benthic macroinvertebrate monitoring and assessment program, please contact the following WQCD staff: Assessments and Collection- Scott Garncarz (scott.garncarz@state.co.us) Policy 10-1 - Chris Theel (christopher.theel@state.co.us) Standards - Blake Beyea (blake.beyea@state.co.us)

These are the laws, regulations, and policies that guide the WQCD's use of benthic macroinvertebrates for the assessment of the Aquatic Life Use.

The following timeline shows the major developmental moments from initiation of the aquatic life workgroup to the approval of the updated Policy 10-1, which is currently in use.

In total there were 41 stakeholder meetings, and took over 8 years to establish Policy 10-1.

Policy 10-1 will expire on 12/31/2025, updates on when the stakeholder process will occur will be posted when available.

 2024 Section 303(d) Listing Methodology 

The Listing Methodology is a document that establishes the framework for determining the status of the water quality using the designated uses and standards adopted for that waterbody.

Information specifically for the assessment of the Biological and Physical Data (benthic macroinvertebrates) is located in  Section V.E . The assessment of the benthic macroinvertebrates starts with the calculation of a Multi-Metric Index (MMI) score, which is the unitless score from 0-100. This score is calculated for each sample that is collected. A single sample is considered representative of a waterbody. The calculated MMI score is then compared to thresholds for the associated biotype to determine attainment or impairment. High MMI scores reflect a healthy benthic macroinvertebrate community.

Assessment decisions are based on the information provided in the Compilation of Macroinvertebrate Data, which can be retrieved by clicking the following button.

The Compilation of Macroinvertebrate Data has multiple tabs. The Introduction tab is self explanatory, the Rationales tab is the tab used for the assessments of benthic macroinvertebrates (contains the MMI score and WBID), the All tab provides all the station information and assessment determinations, and the All-Benthics tab contains all the "raw" data for the stations. The raw data consists of the benthic macroinvertebrate species count as reported by the taxonomist, and the other is the 300-fixed count that operates the MMI tool. This compilation of data includes all benthic macroinvertebrate data submitted to the WQCD.

A Biotype is calculated by using the elevation, stream slope and ecoregion of a sample location, and is similar in community composition. Theses factors are used to predict the biotype of new sites.

There are three Biotypes: Biotype 1 (Transitional); Biotype 2 (Mountain); Biotype 3 (Xeric and Plains)

A biological threshold is a numeric value or narrative expression that describes the expected biological condition of the aquatic community in waters with a given classified aquatic life use. These are based on a direct measurement of the aquatic community. The development of the biological thresholds are multifaceted and involves both statistical analysis, and policy related actions from the WQCC. The goal of the thresholds are to characterize biological conditions that demonstrate support of the aquatic life use. A 144 references sites were used in the statistical analysis to determine the "expected condition" for each of the 3 biotypes.

Biotype Thresholds:

Threshold for High Scoring Waters. More than a 22 point decline in the MMI score results in an impairment conclusion.

The Gray Zone are the MMI scores that fall between the attainment and impairment thresholds for each Biotype. The Gray Zone only applies to waters that are classified as Warm or Cold Class 1. These waters are able to sustain a wide variety of biota, including sensitive species. If an MMI score falls between the attainment and impairment threshold, and is a class 1 waterbody, the auxiliary metrics are used to determine attainment or impairment. Waters designated as Warm or Cold Class 2 are not capable of sustaining a wide variety of biota, which includes sensitive species, and therefore do not need further confirmation of attainment or impairment from the auxiliary metrics.

Hilsenhoff Biotic Index - Measure the tolerance of composition of the sample Shannon Diversity Index - Measures the diversity of the sample.

The auxiliary metrics both have to pass in order for the station to be considered attaining. The HBI score needs to be below the thresholds, and the SDI has to be above the thresholds indicated below.

Using the MMI score and comparing it to the thresholds for the specific biotype, an assessment determination is made by the WQCD. When assessing the benthic macroinvertebrates, the WQCD always uses the MMI score from the most recent sample collected within the current period of record. So, if there are multiple samples collected on the same segment, but collected in different years, the most recent sample's MMI score will be used.

Once an assessment determination is made by the WQCD, the data supporting information is submitted to the WQCC through a formal rulemaking hearing process. The WQCC makes the final attainment decision during the Regulation No. 93 rulemaking process.

Listing Determinations:

• When assessing benthic macroinvertebrates and there are multiple samples collected in the same year on the same waterbody, if the MMI scores are conflicting (one station is attaining and one station is impaired) the waterbody will be placed on the M&E List.

• If a benthic macroinvertebrate sample is collected near a biotype boundary and there is a question as to which biotype the station belongs in, the WQCD will use the designated biotype that is calculated in EDAS. If the assigned biotype is challenged, the WQCD will assess the station using both biotypes. If both MMI scores are determined to be impaired, the segment will be considered impaired. If one MMI score is determined to be impaired and the other MMI score is determined to be attaining, the biotype that was determined in EDAS will be used for the assessment decision.

• If the waterbody is determined to be impaired, and there are no other apparent pollutant(s) causing the benthic macroinvertebrates to be impaired, the segment will be identified as "provisional". An example would be, if the MMI score is determined to be impaired, and the waterbody's aquatic life use is also impaired for dissolved zinc, then the provisional tag will not be used. If the benthic macroinvertebrates are the only parameter listed for a WBID, then the WBID will be provisionally listed.

For the WQCD to propose a 303(d) listing, clear and convincing evidence is required to show impairment. The WQCC has previously determined that one MMI score is sufficient to show clear and convincing evidence for the assessment decision.

Once an assessment determination is made, and if the determination changes the status of the segment, it is proposed to the WQCC for a final decision. Based on this decision the segment is either placed, retained or removed from Regulation No. 93, Colorado's Section 303(d) List of Impaired Waters or Monitoring and Evaluation List.

This regulation establishes Colorado’s Lists of Impaired Waters. These waters include Water-Quality-limited Segments Requiring Total Maximum Daily Loads (“TMDLs”), Impaired Water Bodies with Approved TMDLs and 4b Plans, and Colorado’s Monitoring and Evaluation List. The WQCD is required by the EPA to submit a list of impaired waters every two years, and this would include waterbodies with MMI scores below the attainment thresholds.

This presentation was used to update the WQCC on the MMI tool version 4.0 and to give a general briefing on the history and status of the MMI. Slide 6 shows an MMI update in 2023, the WQCC approved an extension of Policy 10-1 to December 31, 2025.

 MMI 101 Presentation