Indicators 2.6.11–2.6.13

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System

Water Type

Feed Dependent

Species

Indicators 2.6.11–2.6.13

by Contact on 8 October 2026

Requirement

The UoC shall, based on the monitoring of the water quality parameters, ensure that there is no decline in trophic status compared to the water quality baseline determined, following the method outlined in Appendix 8 (8.8).

The UoC shall, based on the monitoring of the water quality parameters, ensure that there is no upward rate of change >25% over the previous 24 months, for any of the water quality parameters listed below, following the method outlined in Appendix 8 (8.8): – TN; and, – TP; and, – Chl-a; and, – Biological Oxygen Demand (BOD).

The UoC shall implement a nutrient reduction plan when at least one of the following is identified: – TN, TP or Chl-a has increased by ≥5 index points according to Table 2 in Appendix 8 (8.8), compared to the water quality baseline; or, – TN, TP or Chl-a increased by ≥15% over the past 24 months; or, – The distance from the surface to the zone of oxygen depletion or to the zone ofanoxia has decreased by ≥25% over the past 24 months; or, – BOD has increased by ≥15% over the past 24 months; or, – ≥1 adverse turnover event or ≥1 harmful algal bloom within the past 10 years.

How Do I Interpret This Requirement?

  • Quarterly monitoring of TN, TP and Chl-a will determine whether there has been a change in the status of the limiting nutrients or a decline in trophic status. BOD will also need to be calculated annually. The calculated data will be compared to the water quality baseline.

  • Quarterly monitoring data will be submitted to the ASC Water Quality Calculator at least annually. The ASC Water Quality Calculator will determine rates of change in the limiting nutrient(s) and trophic status compared to the baseline.

  • A decline in trophic status will be determined when the next Trophic Status Index (TSI) breakpoint has been exceeded, in accordance with Table 2 in Appendix 8 (8.8).

  • Exceedance of the next TSI breakpoint will occur when one parameter falls into the next TSI category.

  • The ASC Water Quality Calculator also determines whether one of the scenarios in Indicator 2.6.13 has occurred based on the quarterly monitoring data.

  • If one of scenarios has occurred, an action plan needs to be implemented to reduce nutrients released by the site and/or to have improved the efficiency of the feed used.

  • The action plan should be specific, measurable, achievable, relevant and time bound.

Evidence of Implementation

  • Monitoring data has been submitted annually to the ASC Water Quality Calculator and results have been obtained.

  • Results will determine the trophic status and status of TN, TP, Chl-a and BOD, in accordance with Indicator 2.6.12. Results will also be compared to the water quality baseline after the first annual submission.

    • Any upward transitions in trophic status at the compared to the water quality baseline will be determined based on the results from the ASC Water Quality Calculator.

    • Results from the ASC Water Quality will determine rate of change for TN, TP, Chl-a and BOD compared to the water quality baseline, specifically whether this exceeds 25% over the previous 24 months.

  • Results from the ASC Water Quality Calculator will be obtained to determine whether one of the scenarios in Indicator 2.6.13 has occurred.

    • This will be based on the TN, TP, Chl-a and BOD data submitted to the ASC Water Quality Calculator.

  • It will also be based on submissions of the number of HABs and ATEs to the ASC Water Quality Calculator.

  • If one of the scenarios has occurred, an action plan will have been documented which will reduce nutrient nutrients released by the site and/or to have improved the efficiency of the feed used.

Auditors Should Confirm

  • The UoC presents evidence demonstrating monitoring data has been submitted annually to the ASC Water Quality Calculator.

  • The UoC presents the outputs from the ASC Water Quality Calculator, which demonstrates whether there has been a decline in trophic status or upward rate of change >25% for TN, TP, Chl-a and BOD.

  • The UoC presents evidence which determines whether one or more of the scenarios in Indicator 2.6.13 have occurred.

  • A specific, measurable, achievable, relevant and time bound action plan has been documented by the UoC, and has been presented, if one of the scenarios in Indicator 2.6.13 has occurred.

Useful Resources

  • Trophic status is determined using the Carlson Index values (0-100), with ‘breakpoints’ set at minimum internals of 10 index points.

  • Carlson’s Index is used to classify Type A waterbodies and consists of four separate normalised indices: Chlorophyll-a, Total Phosphorous, Total Nitrogen and Secchi Disk transparency measurements.

  • Normalised TSI values range from 0 (ultra-oligotrophic) to 100 (hypereutrophic). A TSI of less than 40 is indicative of oligotrophic conditions, while TSI values greater than 50 are indicative of eutrophic conditions. TSI values between 40 and 50 are indicative of mesotrophic conditions.

  • More information about the equations used for TSI assessment, including the relevant peer-reviewed scientific literature, is provided in Carlson, R.E. and Simpson, J. (1996) A Coordinator’s Guide to Volunteer Lake Monitoring Methods. North American Lake Management Society, 96 p.