Condition of aquatic plant communities in lakes

Waikato Regional Council contracts the National Institute of Water and Atmospheric Research Ltd (NIWA) to carry out the aquatic plant assessments in lakes.  This data provides an assessment of the condition of aquatic plant communities in selected lakes throughout the Waikato region.

Surveys are undertaken using LakeSPI which requires the use of trained divers to make underwater aquatic plant assessments.  Surveys are carried out in summer and through to autumn to make the most of the time period when plant growth is at its highest.

Monitoring sites

A total of 64 lakes have been assessed for aquatic vegetation throughout the region.

These lakes are:

Monitoring frequency

Repeat aquatic plant assessments are likely to be undertaken every 5 years at some or all of these sites.  New sites will also be added as is appropriate.

Monitoring history

The LakeSPI technique was developed by NIWA as one of the responses to the Environmental Performance Indicators (EPI) programme coordinated by the Ministry for the Environment.  The development of this technique occurred during the late 1990s and early 2000s.  Waikato Regional Council first commissioned work using this method in 2001. 

Measurement technique

The LakeSPI method attempts to characterise both the native and invasive condition of vegetation in any lake that contains submerged plants.  The technique individually assesses and measures both of these conditions using structural and compositional features of lake vegetation.

The LakeSPI method is based on selection and assessment of five representative baseline sites within each lake.  The baseline sites are chosen to best capture the full suite of characteristics of each lake.  SCUBA divers are used to collect and record aquatic plant data from the five sites within each lake.

Data captured during the survey is used to generate three scores for each site – native condition score, invasive condition score and LakeSPI score.  Adjustments to these scores result in the final indices calculation for: Native Condition Index, Invasive Condition Index and LakeSPI Condition Index.  The indices are expressed as a percentage of each lake’s individual potential condition, i.e. its current condition compared with its own best possible condition.  Therefore, LakeSPI assesses “condition” rather than “value”.

Quality control procedures

NIWA has developed and documented processes for conducting its aquatic plant assessments in lakes.  Data is collected consistently and in the same manner across lakes.  These processes ensure quality control of the information collected.

How is the raw data stored?

Raw data is stored at NIWA and made available via their website.

NIWA calculates three indices based on the aquatic plant assessments undertaken at each lake.  They are:

Native Condition Index reflects the native character of vegetation in a lake.  It is based on the range and quality of native plant communities found in each lake.  The higher the score the healthier, deeper and more diverse the plant communities in the lake.

Invasive Condition Index reflects the invasive character of vegetation in a lake.  It is based on the degree of impact from exotic species.  A higher score means greater impact which is less desirable in most lakes.

LakeSPI Index is a combination of parts of both the native condition and invasive condition indices and provides an overall indication of the lake’s condition.  The higher the score, the better the condition.

From the calculation of these indices, NIWA was asked to classify each lake as either excellent, satisfactory or unsatisfactory based on the LakeSPI Index value.  These categories are relevant only for lakes in the Waikato region.

Excellent: A LakeSPI score greater than 75%.  This means the lake is close to the pristine condition expected for a waterbody of that type.

High: A LakeSPI score between 50-75%

Moderate: A LakeSPI score between 20-50 %

Poor: A LakeSPI score of 0% - 20%.

Non-vegetated: A Lake SPI score of 0%.  Insufficient plants present to apply the LakeSPI technique.  A significantly degraded environment for plant growth.

Systems used to analyse the data

The LakeSPI technique has been used by NIWA to calculate these indices and is presented in this indicator. Microsoft Excel was used to analyse the data.

When using the LakeSPI method to assess the condition of aquatic plant communities the following limitations apply:

• not all lakes are suitable for LakeSPI assessments
• for an assessment to be feasible, there must be submerged plants in the lake
• it is not suitable for lakes where submerged plants cover less than 10% of the site.  However, sites are still assessed if there is less than 10% cover of the site and given a score of 0
• the lakes are not influenced by brackish or estuarine conditions
• the LakeSPI method is not designed to detect the early stages of any new invasive species establishment
• the LakeSPI categories used in this indicator apply only to lakes in the Waikato region
• the current assessment was made over the period 2001-2015. This may limit direct comparisons between lakes if changes occurred during that time.

Other lakes are likely to be surveyed and data added to this indicator as and when needed.  Repeat surveys will be undertaken at some of these sites on a rotational basis over the next 5 years.

This graph shows the percentage of sampled lakes that met or exceeded the criteria for excellent, high, moderate, poor or non-vegetated aquatic plant communities in the Waikato.