Report: TR06/13

Author: Tracey Edwards, John Clayton and Mary de Winton (NIWA)

Abstract

NIWA was contracted by Environment Waikato to assess the condition of thirty-three lakes using LakeSPI; a method that uses Submerged Plant Indicators (SPI) to assess, monitor and report on lake condition. LakeSPI assessments were carried out on each lake to describe:

• pristine condition (lake plant communities in pre impacted times)
• historical condition (lake condition as described by historical data)
• present day condition (most recent data).

From the lakes assessed for this report, only two lakes were classified in excellent condition (LakeSPI index of equal to or above 80 per cent); a further 15 lakes are in a satisfactory condition (impacted vegetation and/or weed invasion); 15 lakes were classified as unsatisfactory (devegetated, LakeSPI index of zero per cent) and the only hydro lake (Hinemaiaia B) was not suitable for LakeSPI assessment. Lakes were also considered according to their type as peat, riverine, volcanic or dune lakes.

The most notable of all the lakes were Rotopiko North and Rotopiko East, since they were the only lakes to have retained close to their original pristine condition and are ranked in 'excellent' overall condition. Despite their high LakeSPI scores, it should be noted that these two lakes still show distinctive signs of stress, consistent with the type of historical changes that are known to have taken place in most of the other lakes.

Lake Rotoroa had the highest score of the 'satisfactory' lakes, which may seem surprising, the results reflecting substantial regeneration of submerged native vegetation in recent years. Unfortunately this may be temporary, as an invasive weed species (Egeria) has re-established and will soon impact negatively upon lake condition. Of the remaining lakes in the 'satisfactory' group, some had relatively sparse native vegetation and little or no impact by invasive species, while others had well developed vegetation that was dominated by invasive species.

All lakes have shown a significant reduction in LakeSPI scores from the pre-1900 'pristine' state. More of the peat and riverine lakes have deteriorated and, by the year 2000, half of the peat lakes and all the riverine lakes in this study had become devegetated. The dune and volcanic lakes of the region deteriorated more slowly and only one volcanic lake is now devegetated (Ngahewa).

LakeSPI enables the condition of small shallow waterbodies to be compared with larger and deeper lakes (such as volcanic lakes). Although the latter have a greater buffering capacity against land use effects, recent impacts are associated with the widespread establishment of Ceratophyllum demersum (hornwort), the most invasive submerged weed in New Zealand.

LakeSPI indices for these Waikato lakes will provide valuable inter-lake comparisons and enable long term monitoring of future changes in their condition. For lake managers, LakeSPI provides relevant information for regional and national reporting requirements and can be used to help assess the effectiveness of catchment and lake management initiatives.

The Condition of Lakes in the Waikato Region Using LakeSPI [PDF, 1.2 MB]