Benchmarking
See Hickson Rowden and Perrie (2018) for the finer details on the application of the assessments outlined below
Trophic Lake Index (TLI)
Classification of lake trophic status using the TLI Burns et al. (1999)
| Trophic status (nutrient enrichment) | TLI | Chlorophyll a (mg/m3) | Secchi depth (m) | Total phosphorus (mg/L) | Total nitrogen (mg/L) |
|---|---|---|---|---|---|
| Ultra-microtrophic (practically pure) | 0.0-1.0 | 0.13-0.33 | 33-25 | 0.00084-0.0018 | 0.016-0.034 |
| Microtrophic (very low) | 1.0-2.0 | 0.33-0.82 | 25-13 | 0.0018-0.0041 | 0.034-0.073 |
| Oligotrophic (low) | 2.0-3.0 | 0.82-2.0 | 15-7.0 | 0.0041-0.0090 | 0.073-0.157 |
| Mesotrophic (medium) | 3.0-4.0 | 2.0-5.0 | 7.0-2.8 | 0.0090-0.0200 | 0.157-0.337 |
| Eutrophic (high) | 4.0-5.0 | 5.0-12 | 2.8-1.1 | 0.0200-0.0430 | 0.337-0.725 |
| Supertrophic (very high) | 5.0-6.0 | 12-31 | 1.1-0.4 | 0.0430-0.0960 | 0.725-1.558 |
| Hypertrophic (extremely high) | >6.0 | >31 | <0.4 | >0.0960 | >1.558 |
Proposed Natural Resources Plan (PNRP)
Selected attributes and narratives from GWRC’s PNRP as well as suggested measures and thresholds for attributes taken from Greenfield et al. (2015)
| Attribute | Narrative | Measure | Threshold |
|---|---|---|---|
| Nutrients | Total nitrogen and phosphorus concentrations do not cause an imbalance in aquatic plant, invertebrate or fish communities | Total nitrogen (mg/L) | Median <0.0725 |
| Total phosphorus (mg/L) | Median <0.043 | ||
| Phytoplankton | Phytoplankton communities are balanced and there is a low frequency of nuisance blooms | Chlorophyll a (mg/m3) | Median <5.0 and Maximum ≤60 |
National Policy Statement for Freshwater Management (NPS-FM)
Attribute states and guideline values taken from the National Objectives Framework (NOF)
Chlorophyll a (mg/m3)
| Attribute state | Annual median | Annual maximum |
|---|---|---|
| A | ≤2 | ≤10 |
| B | >2 and ≤5 | >10 and ≤25 |
| C | >5 and ≤12 | >25 and ≤60 |
| D | >12 | >60 |
Total phosphorus (mg/L)
| Attribute state | Annual median |
|---|---|
| A | ≤0.010 |
| B | >0.010 and ≤0.020 |
| C | >0.020 and ≤0.050 |
| D | >0.050 |
Total nitrogen (mg/L)
| Attribute state | Annual median: Polymictic lakes | Annual median: Seasonally stratified & brackish lakes |
|---|---|---|
| A | ≤0.30 | ≤0.16 |
| B | >0.30 and ≤0.50 | >0.16 and ≤0.35 |
| C | >0.50 and ≤0.80 | >0.35 and ≤0.75 |
| D | >0.80 | >0.75 |
pH adjusted ammoniacal nitrogen (mg/L)
| Attribute state | Annual median | Annual maximum |
|---|---|---|
| A | ≤0.03 | ≤0.05 |
| B | >0.03 and ≤0.24 | >0.05 and ≤0.40 |
| C | >0.24 and ≤1.30 | >0.40 and ≤2.20 |
| D | >1.30 | >2.20 |
