Graphical Display of Reference and Current Trophic Level Index of New Zealand Lakes

Mean annual Trophic Level Index (TLI) values predicted for lakes prior to human disturbance ('reference' conditions) were estimated and compared to predicted current TLI for lakes that could be modelled. The results, expressed as deviation from reference, are displayed below as an interactive graphical representation for 1,030 mapped lakes >1 ha.

National Water Quality Monitoring Network lake data were used to develop statistical models to predict average values of the four constituent TLI variables: total nitrogen (TN), total phosphorus (TP), chlorophyll a and Secchi depth. Separate models were developed to predict water quality corresponding to current and reference states. Multiple predictor variables were used. For estimating current water quality, these included estimated TN and TP concentrations for lake inflows that were calculated using output from the Catchment Land Use for Environmental Sustainability (CLUES) model. McDowell et al. (2013) was used to predict reference nutrient concentrations for lake inflows under reference conditions. 

Model predictions of TLI variables were then used to estimate TLI for reference and current states (see interactive map below), with the difference between these values interpreted as a measure of change in trophic status from reference conditions. Current TLI was greater than reference TLI in 89% of lakes. On average, lake TLI was 0.67 units greater under current conditions than reference conditions (standard deviation = 0.67 units). Based on predictions, 32% of lakes were oligotrophic under current conditions, compared with 68% of lakes under a reference state. Under current conditions, 38% of lakes were mesotrophic, compared with 24% of lakes under a reference state. The predominant lake trophic status under a reference state was oligotrophic and the predominant trophic state under current conditions was mesotrophic. Under current conditions, 28% of lakes have a trophic status of eutrophic or greater (TLI > 4) compared with 5% of lakes under reference conditions. 


Lake Trophic Level Index Scale and description

 Lake Trophic Status Lake Trophic Level Index Description
 Microtrophic 1 - 2Lakes are very clean and often have snow or glacial sources (e.g., Lake Pukaki in Canterbury).
 Oligotrophic 2 - 3Lakes have moderate levels of nutrients and algae (e.g., Lake Rerewhakaaitu in Bay of Plenty).
 Mesotrophic 3 - 4Lakes have moderate levels of nutrients and algae (e.g., Lake Rerewhakaaitu in Bay of Plenty).
 Eutrophic 4 - 5Lakes are murky, with high levels of nutrients and algae (e.g., Lake Rotorua in Bay of Plenty). 
 Supertrophic 5 - 6 Lakes are fertile and saturated in phosphorus and nitrogen, often associated with poor water clarity. Excessive phytoplankton growth can occur in ideal conditions - when there's a calm, hot and sunny period of a few weeks (e.g., Lake Whangape in Waikato).
 Hypertrophic 6 - 7 Lakes are highly fertile and supersaturated in phosphorus and nitrogen. They have excessive phytoplankton growth which contributes to poor water clarity, poor suitability for recreational uses, and restricts the habitat for desirable fish (e.g., Lake Ellesmere (Te Waihora) in Canterbury)

A full description of this work can be downloaded here or by clicking on the image opposite.

Please cite this work as:

Abell, J. and van Dam-Bates, P. 2018. Modelling Reference and Current Trophic Level Index for NewZealand Lakes. Consultant's report prepared for the University of Waikato by EcofishResearch Ltd, Ecofish Research Ltd., Courtenay, B.C. pp 70.


Disclaimer:

In using these predictions  you agree that you rely on the data at your own risk; the University of Waikato offers no warranty or representation that the data is fit for any particular purpose, or free from defects, and will not be liable for any losses or damages whatsoever including any indirect, special, incidental or consequential
damages incurred by you arising out of the use of the database or in reliance on the data retrieved. You agree that data is not of a kind ordinarily supplied for personal or domestic purposes, and that the Consumer Guarantees Act 1993 does not apply.

Notes:
1. Predictions are based on models derived from data for a small subset of lakes nationally. Interpretation of these predictions should be made in conjunctionwith reading the report Abell et al. (2018)

2. Predictions should be used for understanding regional- or national-scale patterns rather than lake-specific conditions.

3. "Reference" values indicate the conditions that can potentially be
achieved in the absence of human-induced eutrophication

4. Caution should be applied to interpreting reference state for (i) glacial lakes because of naturally low clarity, (ii) geothermal lakes because of the low number of modeled lakes, and (iii) dammed lakes because they are not naturally formed, although 'reference' can indicate management targets for water quality.