New research into Rotorua lakes aims to find vital clues into their health and history

Lake Rotorua..
Lake Rotorua.. Photo credit: MfE / Supplied

By Andrew McRae of RNZ

Obtaining vital clues to the health and history of lakes in Aotearoa-New Zealand is the focus of research being carried out on three Rotorua lakes.

Scientists from GNS Science, the Cawthron Institute, Victoria University of Wellington and the University of Otago in conjunction with Te Arawa Lakes Trust are currently taking both sediment and water samples from lakes Rotorua, Ōkataina and Okareka to provide a full 1000-year history.

It is part of a project called 'Lakes 380 - Our lakes' health - past, present, future' which aims to find out about the health, wellness and history of around 10 percent of New Zealand's 3800 lakes which are greater in size than a hectare.

The team of scientists first visited the Rotorua region in 2019 and have returned to collect longer core samples from the bottom of the lakes.

Co-project leader Marcus Vandergoes from GNS Science said the team is using a unique type of lake sediment corer, called a Mackereth corer.

It is 12-metres long and uses a compressed air system. It is lowered over the side of a boat and settles on the bottom of the lake and sucks up the samples in a tube.

It is then hauled back to the surface and the core sample is taken off and the process is then repeated.

It allows them to collect samples up to 6m, which equates to a 1000-year history.

''These are 2m long tubes of mud in most cases and we try and capture the history of the lake and how it is has changed over time by analysing indicators of lake health and water quality change trapped in the mud over time.

''The mud at the bottom of a lake layers year by year and accumulates over time so we can take the mud cores and basically use them in a way to go back in time.''

He said it will provide information from prior to humans arriving in Aotearoa-New Zealand and any changes that have occurred as a result of human interaction with lake systems and landscape changes.

''Prehuman is sort of a reference state of its natural condition and how a lake has changed in response to a volcanic event, so the historic component gives us a reference baseline condition of what the lake was like.''

Vandergoes said the Rotorua lakes are of particular interest because of local volcanic activity.

''We can investigate how some of those naturally occurring events have impacted on the lakes in their history.''

He said scientists can then see if or how much the lake has changed since that time and why.

''Whether we see marked changes in the last 50 years that might be associated with landscape changes due to different things like farming, intensification of housing or even the introduction of exotic fish species which can change the ecosystem and influence the health of a lake.''

The water samples taken at the same time as the core samples provide information on water conditions including algae and bacteria which are often a problem in lakes.

Te Arawa Lakes Trust said the research is an exciting opportunity to see how factors such as human arrival and the 1886 Mt Tarawera eruption have impacted on lake ecosystems.

Trust strategic manager (taiao) Mariana Te Rangi said it will help identify what the lakes and their surroundings were once like and to where they are today.

''Using that data to look forward to the next 50 years to put barriers or safety buffers around our lakes in order to protect them or to mitigate and isolate any further potential risks and poor water quality to our lakes.''

Te Rangi said the Tarawera eruption hugely impacted the iwi.

''It will absolutely make for an exciting future for our lakes and surrounding land.

''It indicates to us the vegetation change throughout the 500 to 1000-year history.''

She suggests the possibility of potentially bringing the original vegetation back, such as beech forests.

Te Rangi said it will take a very long time to return the lakes to their original pristine condition.

''It has been a strong focus for the Te Arawa Lakes Trust for many years.''

She said the iwi will need to decide if it wants the lakes returned to how they were prior to human arrival, prior to the arrival of pests, farming and forestry.

''A wider discussion will need to be had.

''It's looking at a total picture and pin-pointing the highest risk factors impacting on our water body and then just targeting one at a time and isolate them and slowly this will happen, but of course the aim is to eliminate.''

Lakes 380 - Our lakes' health - past, present, future is a five-year project which will be completed by the end of 2022.