Report: TR 2009/14

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).

Lakes of the Waikato Region displayed a wide range of current LakeSPI scores from having no submerged vegetation (0%) to a maximum LakeSPI Index of 91%. From the lakes assessed for this report, two lakes were classified in ‘excellent’ condition (LakeSPI Index ≥ 75%); four lakes in ‘high’ condition, eleven in a ‘moderate’ condition, eight in a ‘poor’ condition and the remaining twenty seven 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.

The most notable of all the lakes were Lakes Serpentine North and Serpentine 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 lakes still show advanced 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 four lakes ranked in ‘high’ condition, two were from the volcanic group of lakes (Rotopounamu and Opouri) and two were peat lakes (Mangakaware and Rotoroa).  Lake Rotopounamu was the highest ranked lake in this group because of the absence of any invasive species. A recent loss of the charophyte beds that previously dominated the submerged vegetation in Rotopounamu has resulted in a reduction of the Native Condition score which has resulted in a lower LakeSPI result than would otherwise be expected for this lake.  A resurvey has been recommended to assess charophyte recovery, as a positive change in covers could see this lake being ranked as in ‘excellent’ condition in the future.

Eleven lakes were ranked in ‘moderate’ condition and although all of these lakes still supported native plant communities, all but one 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 beyond the main depth of impact from E. canadensis.  Six of the seven dune lakes surveyed for this report were ranked in this ‘moderate’ category.

Of the eight lakes ranked as being in a ‘poor’ condition, all had similar LakeSPI scores that are influenced by the almost complete domination of vegetation by Ceratophyllum demersum, New Zealand’s worst submerged weed species.  This group of lakes included seven of the eight Waikato River hydro lakes and one dune lake, Otamatearoa. It is however noted that maintaining invasive species in a lake is preferable to macrophyte collapse and algal dominance.

The remaining twenty-seven lakes assessed in this report were categorised as being ‘non-vegetated’, either devoid of submerged vegetation or having 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 sixteen of the twenty-seven peat lakes assessed in this report. 

LakeSPI enables the condition of different types of lakes to be compared, for example, small shallow water bodies to be compared with larger and deeper lakes. Compared nationally the Waikato Region has a higher proportion of non-vegetated lakes, indicating a high level of degradation from catchment modification and 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, Ceratophyllum demsersum. Nationally, the largest proportion of lakes fell into the ‘moderate category, which in the Waikato Region contains most of the volcanic and dune that are impacted to varying degrees by invasive weeds. A relatively small proportion of Waikato lakes in the high to excellent group represent small lakes that remain close to the maximum ecological condition expected for their type.

All lakes have shown a significant reduction in LakeSPI scores from the pre 1900 ‘pristine’ state. The most significant recent change recorded for any of the Waikato lakes was the loss of submerged vegetation in Serpentine South. In the last year, (2008/09) the submerged vegetation within the lake collapsed with only sparse remnants of native vegetation still remaining at covers that did not exceed the 10% required to generate a LakeSPI score.  The most recent survey at Lake Serpentine South also gave the first record of the alien bladderwort Utricularia gibba for lakes of the Waikato Region, with a subsequent record at a nearby peat lake, Lake Milicich.

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. We recommend that a schedule for Lake SPI 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 - 2009 [PDF, 1.6 MB]