The research will study how human arrival and the 1886 Mt Tarawera eruption impacted lake ecosystems. Photo / File
The research will study how human arrival and the 1886 Mt Tarawera eruption impacted lake ecosystems. Photo / File
A team of researchers will this week delve deeper into the history of Rotorua Te Arawa lakes to collect vital clues to the health of the lakes over the past 1000 years.
Scientists from GNS Science, the Cawthron Institute, Victoria University and the University of Otago, in partnership with Te Arawa Lakes Trust, will sample three Rotorua lakes as part of a nationwide project called "Lakes 380 - Our lakes' health – past, present, future", which aims to determine the history and environmental health of around 10 per cent of New Zealand's 3800 lakes.
Lake Ōkataina. Photo / File
Co-project leader Dr Marcus Vandergoes from GNS Science said lake sediments were natural archives that continuously recorded environmental history, providing measures of current and historical aquatic communities and water quality.
The team visited the Rotorua region in 2019 to take samples from the 13 lakes and are returning this year to collect longer core samples from Lakes Rotorua, Ōkataina and Okareka to provide a full 1000-year history.
Te Arawa Lakes Trust environment manager Nicki Douglas said the research was an exciting opportunity to see how factors such as human arrival and the 1886 Mt Tarawera eruption impacted the lake ecosystems.
"Our knowledge from ancestors only goes so far, so having the research from deep within the Earth is not only a welcomed insight into the history of our lakes, it also enables us to see how the lake health and ecosystems have changed over time, which will help us future-proof our work in protecting the taonga for generations to come."
Vandergoes said the 1886 Mt Tarawera volcanic eruption deposited large amounts of ash in many of the Rotorua Te Arawa lakes, and in some lakes the two-metre sediment cores – tubes of mud – they collected in 2019 were not long enough to provide a 1000-year history.
Scientist Dr Susie Wood of the Cawthron Institute. Photo / File
This time the team will use a unique type of lake sediment corer, a Mackereth corer, allowing them to collect cores up to six metres, equating to 1000 years of history.
The results from the 2019 sampling have provided many new insights into the scientific history of the Rotorua Te Arawa lakes. The data covers six lakes - Tutaeinanga, Ngāhewa, Ngāpouri, Ōkaro, Ōkataina and Rotoehu - with more data on the remaining lakes expected by mid-2021.
One significant finding is the varying impacts the 1886 Mt Tarawera eruption had on the lakes. This has been discovered through the analysis of the tephra (rock fragments and particles, or ash ejected during a volcanic eruption) and in-wash (tephra that is washed into a lake) in the core samples.
"For example in Lake Rotoehu, the tephra is composed of gravel-size pieces and the layer is only about 5cm deep, whereas in Lake Ōkaro the core is dominated by a more muddy tephra in-wash which is several meters thick," Vandergoes said.
Lake Okareka. Photo / File
In particular, researchers have focused on how the eruption affected the biological communities at Lake Ōkataina, which was still mostly surrounded by native forest.
Co-project leader Dr Susie Wood from Cawthron Institute said the data so far showed that the bacterial communities, the smallest organisms in the lake and those at the base of the food web, were very different following the eruption.
"Despite this event occurring over 130 years ago, the communities have not returned to their pre-eruption composition," she said.
Using a combination of DNA and scanning methods, the team also explored how algae densities had changed in the lakes. In most, but not all lakes, the amount of algae had increased in the last 200 years, most likely associated with land-use change and introduced animals and plants.
Wood said of most concern was the recent increase in some lakes of bloom-forming and potentially toxic cyanobacteria (blue-green algae).
Douglas said the findings were alarming but expected.
"Having the 1000-year data to show how our lake health has deteriorated enforces the biosecurity mahi that we do to protect the lakes, as even the smallest piece of weed or tiny fish egg can have devastating effects by creating a major biosecurity issue," she said.
