NIWA's underwater health check
For Joe Butterworth and his Te Arawa iwi, lake restoration goals are encapsulated in the footsteps of the kōura, our native freshwater crayfish.
“The kōura migrate to the shallows to feed, and that’s when they’re easiest to catch. With the water quality degrading in some lakes, you’re unable to see them even in the shallows. If the water quality improves, we’ll be able to see those kōura again.”
Butterworth, a scientific advisor for the Te Arawa Lakes Trust, says it’s not just about the clarity of the water – it’s about the whole lake ecosystem. “Kōura are scavengers and live on the lake bed. They need oxygen and rely on other species to feed. If the ecosystem is healthy, you’ll get lots of kōura. But for them to feed in the shallows, they need to be able to get there. In some lakes invasive plants form almost a solid wall preventing them migrating to the edges.
“The Rotorua Te Arawa lakes are used widely, not only by the local community, but also by lots of people from other regions for recreational activity. They’re pretty important to Te Arawa for cultural reasons. These lakes are the food basket, and so we want to keep that going for the future.”
LAKE MEASUREMENT – THE FACTS
NZ has 3,820 lakes that are 1 hectare or more and just over 50% of those lakes are partially or completely in conservation areas.
“We monitor the lakes with the greatest problems, which are primarily large lowland lakes and are more impacted by human activity,” says NIWA Emeritus Scientist Dr Clive Howard-Williams. “More than half of our lakes are on DOC land, so much less affected by pollutants and weed invasions.”
Butterworth says being able to contribute to his iwi and the whole community in trying to restore the Rotorua Te Arawa lakes is what gets him up in the morning. Making a difference and contributing to the health of New Zealand’s lakes is also what inspires NIWA freshwater ecologist and scientific diver Tracey Burton. Both Burton and Butterworth, along with other NIWA colleagues, have been working underwater over autumn to assess the health of the Rotorua Te Arawa lakes using the LakeSPI (Submerged Plant Indicators) survey method.
LakeSPI was developed by NIWA scientists 18 years ago, and uses the underwater plants growing in a lake as indicators of its ecological health. The divers don cold water drysuits, dive masks, fins and scuba gear before heading beneath the surface with waterproof field sheets to assess the state of the vegetation on the lake bed at pre-defined locations. Their observations feed into the national LakeSPI database and build a picture of lake condition and change over the years. The submerged plant indicators help measure two major influences on lake ecology: increased sediments and nutrients and the impact of invasive weeds on the lake’s native vegetation.
Lake owners and managers such as the Te Arawa Lakes Trust, the Bay of Plenty Regional Council (BOPRC) and Rotorua District Council can use LakeSPI to assess the status of the lakes and monitor changes within them.
NIWA has been conducting regular LakeSPI surveys on 12 Rotorua Te Arawa lakes (Ōkāreka, Ōkaro, Ōkataina, Rerewhakaaitu, Rotoehu, Rotokākahi, Rotomā, Rotomāhana, Rotoiti, Rotorua, Tarawera, and Tikitapu) since 2005 – work funded by BOPRC.
"If the water quality improves, we’ll be able to see those kōura again.”Joe Butterworth
Burton has been diving in the lakes for 20 years and says she has seen both positive and negative changes in the lakes.
“Many of the negative changes are attributable to the spread of alien invasive weed species such as hornwort, egeria and lagarosiphon which have had a devastating impact on some of the lakes. But increased awareness of lake issues and new management initiatives have seen some lakes improve.”
Targeted interventions by BOPRC, such as the efforts to eradicate hornwort in Lake Ōkāreka have met with success. LakeSPI results show improvements in lake condition over the last six years. But this lake and others are still at high risk of new invasions. Only two of the 12 Rotorua Te Arawa lakes remain free of major weeds and constant vigilance is needed to prevent the spread of weeds throughout New Zealand’s waterways. This is especially relevant for Lake Rotomā, currently one of the highest ranked lakes in the region. LakeSPI shows Rotomā has remained stable, with little change for more than 20 years. However, Lake Rotoehu is just one kilometre to the east and it is classified in poor condition, with a heavy infestation of hornwort and water quality issues.
“A piece of weed the size of your thumbnail would be all that it takes for hornwort to be transferred to Rotomā. Once in, it would displace the native vegetation in the lake – we’ve already seen this happen in Lake Tarawera and areas of Lake Taupo,” says Burton. “That’s why the ‘check, clean, dry’ message is so important.”
As well as blocking the passage of kōura, and smothering the native vegetation, NIWA divers have noticed other less well-known impacts from invasive weeds. For example, where there are large dense weed beds, there are fewer kākahi (native freshwater mussels). Kākahi are highly valued both as mahinga kai for Māori communities and for the role they play in maintaining ecosystem health as biofilters.
Of the 300+ lakes monitored using LakeSPI
34% are rated as excellent/high (close to pristine or with minor impacts from invasive species)
14% poor (extensive invasions)
22% non-vegetated (degraded – not suitable for plant growth)
LakeSPI has been used to assess the condition of 305 of New Zealand’s 3,820 lakes, with many of them, such as the Rotorua Te Arawa lakes, being surveyed multiple times. The Ministry for the Environment uses the data to help describe the state of our freshwater, and it links directly to the Land And Water Aotearoa database, which also uses StatsNZ data collected by NIWA and regional councils.
Bay of Plenty Regional Council’s senior biosecurity officer Hamish Lass says LakeSPI has proved invaluable. “It gives us trends over time which show where we’ve got issues in some lakes, and where other lakes are getting better through good management. It also lets us know what plant species we have in the lakes and what impact invasive species are having on overall lake condition.”
He says LakeSPI also illustrates the impact of people. “The lakes that have the most human contact have got most of the invasive weeds. For example, Lake Rotorua, with six boat ramps, has all four of the worst invasive weeds, whereas Lake Rotokakahi, which is a private lake with no public access has only one weed in it.”
“One of the most valuable things about LakeSPI is that we can use it as a tool to aid in decisions as to what our lakes should look like compared to how they once were, and what we would find acceptable in the future,” says Burton. “We can use this information to set limits and define targets for our lakes, and these kinds of discussions are already underway in the Bay of Plenty.”
Joe Butterworth says the Trust wants to be able to use a combination of matauranga Māori and western science to achieve its goal, to improve the lakes.
“At the end of the day, both have the same goal, don’t they? When the lakes were taken, they were in good health, but since Te Arawa’s taken back ownership of the lake beds, they’re not in good health. So we want to put that right using matauranga alongside western science.
“We may not be able to restore some of them back to the way they once were – we’ve got to be realistic about that, but we’ve got to do something.”