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Setting the scene
New Zealand is an island nation, comprised of two large and many smaller islands, located in the South Pacific Ocean. It has a warm to cool temperate climate with moderate rainfall, and a landscape that was originally largely forested, with a wealth of pristine lakes and rivers.
However the arrival of people heralded a new era for the land, as forests were cleared, wetlands drained, farming and agricultural practices were developed and intensified over the years, and cities were built. Today over 90% of New Zealand’s original wetlands have been drained and only 23% of New Zealand has an indigenous forest cover (Taylor and Smith 1997).
Such dramatic changes in land use have had a significant impact on the lakes and waterways. For example, increased nutrient enrichment and siltation, from changing land catchment use has lead to the degradation of many lakes, especially smaller lakes at low elevation. Water clarity in particular has decreased due to increasing phytoplankton abundance and suspended inorganic particles. Such significant change to the aquatic environment has also resulted in subsequent stress on the native aquatic flora and fauna, leaving the once pristine lakes and waterways – our national treasures – under threat.
The threat to our national treasures is not from land-use practises alone, but the invasion of alien plants and animals that has accompanied man’s arrival in New Zealand. Settlers introduced a succession of new plants and animals: for food and cultivation as crops, as well as for ornamental and aesthetic reasons, and accidentally as ‘hitchhikers’ with other imports.
By the 1940s about 500 alien plant species had become established in New Zealand. By March 1998, 2068 alien plants had naturalised (formed self-perpetuating populations) out of the more than 19,000 terrestrial and freshwater introduced plant species currently present (Owen 1998). The invasive spread of uncontrolled alien species has become a major management issue on land and in water.
Aquatic plant species have had spectacular success in invading New Zealand lakes. Amongst the native aquatic flora there are 38 endemic species (which is relatively few compared with other temperate land masses) and there are a limited range of life form types (Coffey and Clayton 1988). The continued introduction and spread of invasive alien species has resulted in few water bodies retaining their natural or original indigenous aquatic vegetation.
Of the invasive aquatic plants that have established in New Zealand, it is the alien submerged species that have arguably been the most successful, difficult to control, and continue to pose the most significant threat to our national treasures. In particular, members of the Hydrocharitaceae (i.e. oxygen weed species) and the Ceratophyllaceae families have had a significant impact on the biodiversity and amenity values of the lakes and waterways they invade. The adverse changes associated with their establishment are threefold.
- Their superior competitive abilities see these weeds exclude and replace native plants, reducing native representativeness and biodiversity values.
- The tall, surface-reaching nature of the invasive alien weed beds interferes with recreational uses of water bodies.
- Economic utility of waterways for hydroelectric generation and water extraction can be compromised
For example, a native aquatic plant community (see diagram above) might include a ‘shallow-water zone’ with emergent and/or low-growing submerged turf species, a ‘mid-depth zone’ of taller submerged species such as milfoils and pondweeds amidst charophytes, and in clear water there is normally a ‘deep zone’ with charophytes and bryophytes, that extends as deep as 70 m in some clearwater glacial lakes. (Click on the images at the right for a closer view and more detailed description.)
However, a major change occurs to this typical vegetation pattern whenever invasive submerged species become established in a lake. All of the main invasive weed species impacting on lake vegetation structure are tall-growing angiosperms (Hydrocharitaceae and Ceratophyllaceae), but they have one distinctive difference from the native milfoils and pondweeds. These invasive species can form extremely dense growths that exclude all other vegetation. They typically occupy the shallow to mid-depth range of lakes and are most common from two to eight metres water depth, although they can grow to a depth of ten metres where they exclude almost all native species. This in turn reduces wildlife habitat, and leads to further decline of the aquatic system.
Taylor, R.; Smith, I. (1997). The state of New Zealand’s environment 1997. Ministry for the Environment, Wellington.
Owen, S.J. (1998). Department of Conservation strategic plan for managing invasive weeds. DoC, Wellington. 86 p.
Coffey, B.T.; Clayton, J.S. (1988). New Zealand waterplants: a guide to plants found in New Zealand freshwaters. Ruakura Agricultural Centre, Hamilton.
Lakes unmodified by catchment development, human settlement and public access still retain much of their original status in terms of water quality and aquatic vegetation. Large lakes have a greater buffering capacity compared to small lakes, but even big clearwater lakes such as Taupo and Wanaka are now showing disturbing signs of human impacts that include progressively reducing water clarity, increasing frequency of algal blooms and biodiversity impacts from invasive weed species.
Deterioration in the condition of Rotorua Lakes has been occurring for many years. Apart from records on water quality decline, scientific papers have also been published comparing vegetation from the 1960s to the 1980s that showed parallel deterioration in abundance scores for total vegetation and key submerged species.
This paper describes the status of the Rotorua lakes based on information revealed from their aquatic vegetation and discusses further threats to these water bodies and what individual lake users can do to help reduce the risk of further deterioration.
Read 'Rotorua Lakes: Plants tell the tale' (PDF 2.2 MB)
Presented at Rotorua Lakes 2003: A Public Symposium on Practical Management for Good Lake Water Quality, Rotorua, October 2003.
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