Report: TR 2008/36

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

Abstract

NIWA was contracted by Environment Waikato to assess the condition of forty-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, three lakes were classified in excellent condition (LakeSPI Index ≥80%); a further sixteen lakes are in a satisfactory condition (impacted vegetation and/or weed invasion); twenty four lakes were classified as unsatisfactory (essentially de-vegetated, LakeSPI Index 0%). Lakes were also considered according to their type as peat, riverine, volcanic or dune lakes.

The most notable of all the lakes were the three Serpentine (Rotopiko) Lakes, 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 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. From the lakes ranked “satisfactory”, Lake Rotoroa (Hamilton) had the highest score which may seem surprising but the results reflect the 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 78%, of those in this study are currently devegetated. The dune and volcanic lakes of the region have deteriorated more slowly and only one volcanic lake is now devegetated (Tutaeinanga).

LakeSPI enables the condition of small shallow water bodies to be compared with larger and deeper lakes (e.g., 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 43 Lakes in the Waikato Region Using LakeSPI [PDF, 845 KB]