Assessment of the ecological condition of lakes in the Waikato region using LakeSPI – 2010

Report: TR 2010/24
Author: Tracey Edwards, Mary de Winton, John Clayton (NIWA)

Abstract

NIWA was contracted by Environment Waikato to assess the condition of selected lakes within the Waikato Region using LakeSPI; a method that uses Submerged Plant Indicators (SPI) to assess, monitor and report on lake condition. A total of fifty-two Waikato lakes have been assessed for this report using LakeSPI 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).

All lakes of the Waikato Region have shown a significant reduction in LakeSPI scores from the pre 1900 ‘pristine’ state. Lakes displayed a wide range of current LakeSPI scores from having no submerged vegetation (0%) to a maximum LakeSPI Index of 89%. From the lakes assessed for this report, one lake was classified in ‘excellent’ condition (LakeSPI Index ≥ 75%); four lakes in ‘high’ condition, eleven in ‘moderate’ condition, seven in ‘poor’ condition and the remaining twenty-nine lakes were classified as ‘non-vegetated’ (LakeSPI Index 0%).  Lakes were also considered according to their lake type as peat, riverine, volcanic, hydro or dune lakes.

Lake Serpentine North, was the only lake to have retained close to its original pristine condition and was ranked in “excellent” overall condition. Despite the high LakeSPI score, it remains under threat from invasion by exotic weeds and the status of this lake will fall in the event that the alien bladderwort Utricularia gibba establishes. This is now likely as this weed is widely spread by waterfowl and is present in the adjacent Lakes Serpentine South and East.  Vegetation in Serpentine North also shows advanced signs of stress, consistent with the type of changes that have taken place in most other Waikato lakes.

From the four lakes ranked in ‘high’ condition, two were from the volcanic group of lakes (Rotopounamu and Opouri) and two were peat lakes (Serpentine East and Rotoroa).  Lake Rotopounamu was the highest ranked lake in this group because of the absence of any invasive species. However, a recent loss of the charophyte beds that previously dominated the submerged vegetation has resulted in lower LakeSPI scores than would otherwise be expected for this lake.  A resurvey is recommended to assess charophyte recovery, as a positive change in charophyte covers could see this lake return to ‘excellent’ condition in the future. Lake Serpentine East recently dropped from an ‘excellent’ to ‘high’ category due to invasion by the alien bladderwort Utricularia gibba, and reduced native plant diversity.

Eleven lakes were ranked in ‘moderate’ condition and although all of these lakes still supported native plant communities, all had significant impact by invasive weed species. Lake Harihari was the highest ranked lake in this category, with only the relatively benign weed Elodea canadensis recorded, and significant deep-water charophyte beds were present beyond the range occupied by E. canadensis.  Inclusion of the dune lake Lake Otamatearoa in this category reflects recent weed control initiatives, but this status will not be retained if the invasive weed Ceratophyllum demersum is allowed to recover its former abundance. All seven dune lakes surveyed for this report were ranked in this ‘moderate’ category, which also included three volcanic lakes and the uppermost hydro-lake in the Waikato River chain that is immediately downstream from Lake Taupo.

The remaining seven hydro-lakes in the Waikato River chain were ranked as being in ‘poor’ condition. All had similar LakeSPI scores that are influenced by the almost complete domination of vegetation by C. demersum, New Zealand’s worst submerged weed species.  It is however noted that maintaining invasive species in a lake is preferable to macrophyte collapse and algal dominance.

The remaining twenty-nine lakes assessed in this report were categorised as being ‘non-vegetated’, either devoid of submerged vegetation or having submerged vegetation covers that did not exceed the 10% threshold for LakeSPI assessment.  All nine of the riverine lakes surveyed were categorised as ‘non-vegetated’ as were eighteen of the twenty-seven peat lakes assessed in this report.  Recently vegetation in several peat lakes (Lakes Serpentine South, Mangakaware, and Kainui) has fallen below the plant abundance (10% cover) where a LakeSPI score is generated and lakes are now termed ‘non-vegetated’. These lakes may be at the threshold between a vegetated and non-vegetated state and would be sensitive to future management initiatives.

LakeSPI enables the condition of different types of lakes to be compared, for example, small shallow water bodies with larger deeper lakes. The Waikato Region, compared with lakes nationally, have a higher proportion of non-vegetated lakes indicating a high level of degradation from catchment activities such as intensive agriculture. A smaller portion of Waikato lakes fall into the ‘poor’ category, representing extensive invasion and dominance by one of the country’s worst weeds, C. demersum. Nationally, most lakes fall into the ‘moderate category, which in the Waikato Region contains most of the volcanic and dune lakes that are impacted to varying degrees by invasive weeds.

LakeSPI indices for the 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. We recommend that a schedule for LakeSPI surveys, with priority and timing of future assessments, is developed that reflects current knowledge gaps, perceived lake value, stability and known threats.

Assessment of the ecological condition of lakes in the Waikato region using LakeSPI – 2010 [PDF, 3.7 MB]