Changes in land use have turned the sand country lakes around Whanganui into cot cases - but we still want to swim, fish, boat and gather kai in them. Laurel Stowell dives into the complexities of how these water bodies work.

A fresh crop of annoying algal blooms may be about to strike Whanganui's small but precious lakes.

Our local lakes have a "legacy load" of nutrients, especially nitrogen and phosphorous, from pastoral farming. Last spring those nutrients, warmed by spring sunshine, fed algal blooms at both Dudding and Wiritoa.

Our lakes are dune lakes, formed when a drift of sand blocks a gully and traps water. Dudding is used for boating, swimming and camping, Rotokawau/Virginia is the centre of a premium Whanganui park, Pauri is used for boating and Wiritoa is used for boating and swimming.

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Rotokawau/Virginia Lake has had algal blooms in most years recently, and Whanganui District Council installed ultrasound and wave churners to try to prevent them.

Rotokawau/Virginia Lake was a bright algal green in January 2018. Photo / file
Rotokawau/Virginia Lake was a bright algal green in January 2018. Photo / file

Dudding, Pauri and Wiritoa all have more nutrients than they should, an October 2018 Cawthron Institute report to Horizons Regional Council says. It forms part of a Lake Management Update considered by Horizons councillors earlier this year.

The regional council doesn't monitor Rotokawau/Virginia Lake, which may be even worse in terms of nutrient levels.

The lakes' circumstances are a long way from favourable. Lakes stay in the best condition if they are in a forested catchment, especially indigenous forest, with a large wetland system and a diverse population of native plants and animals.

Our lakes risk becoming dominated by blue-green algae, also known as cyanobacteria - a weird type of bacteria that can photosynthesise like a plant. Some can even fix their own nitrogen.

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These tiny ancient organisms come in lots of different kinds and can colour water, usually green and sometimes blue. They thrive with nutrients, especially phosphorous, and in the layer of warm water that forms at the top of a lake.

If they proliferate into an algal bloom, they can cause skin irritations and respiratory issues for swimmers. They also smell and taste bad, and can kill fish. When they die, they sink to the bottom of the lake and decompose, robbing the water of oxygen.

At really high density they can block light from reaching water plants lower down.

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Some also produce toxins, which can kill a dog or cause vomiting, diarrhoea, abdominal pain, cramps and nausea in a person. However, that hasn't happened much around here. The toxins from a microcystin cyanobacteria in Wiritoa in December 2018 were the first there in 10 years. READ MORE:
People swim at popular Whanganui lake despite warning signs
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Algae turn Virginia Lake brighter shade of green
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A wetsuit is no defence against toxic cyanobacteria. Filtering, disinfecting or boiling the water can even make it worse.

The worst possible scenario is a bloom of toxic cyanobacteria in the upper warm layer of a lake in still weather. Deeper down a layer of cold water forms, with no oxygen. Nothing can survive there and in those conditions phosphorous is released from the sediment.

When wind mixes the water layers together again, the phosphorous comes to the surface and feeds a fresh bloom of cyanobacteria, in a repeating cycle.

Horizons Regional Council freshwater manager Logan Brown notes the result of water monitoring. Photo / file
Horizons Regional Council freshwater manager Logan Brown notes the result of water monitoring. Photo / file

Lakes are like sinks, with water and nutrients continually coming in and not as much going out. Kaitoke is the only lake near Whanganui that has a permanent outflow, Horizons freshwater manger Logan Brown says.

Left alone, any lake will eventually fill with sediment to become a wetland, then dry land.

Fencing and planting the streams that feed them reduces the amount of nutrients and sediment coming into Pauri and Wiritoa lakes - but they still have that "legacy load" of nutrients from the past.

Source: Horizons Regional Council
Source: Horizons Regional Council

Dudding and Rotokawau/Virginia also have introduced weeds and fish, which displace native species. Rank weeds can entrap swimmers. When the weeds die and rot they make bad smells and tie up oxygen other organisms need.

Introduced fish can eat native ones. Some, such as the grass carp found at Dudding Lake this year, eat water plants. A permit is required by anyone who wants to release an introduced fish.

Overall a lake dominated by plants, even if they are introduced weedy ones, is healthier than one dominated by cyanobacteria.

Lakes vary a lot, and there's no one-size-fits-all solution to keeping them healthy. To find the right solution you first need to know a lot about the lake, Brown says. All of ours need more research.

Lakelands Holiday Park owners Jackie and Darryl Dowman have an interest in the health of Lake Wiritoa. Photo / Bevan Conley
Lakelands Holiday Park owners Jackie and Darryl Dowman have an interest in the health of Lake Wiritoa. Photo / Bevan Conley

Scientists focused on Dudding Lake in one intensive day in May, and Wiritoa may get the same treatment in the next few years. Both are being monitored more intensively by Horizons.

In the meantime, limiting the nutrients and sediment going into them can only help, and doesn't need research to prove it's a good idea. Other options are more debatable.

Harvesting pest weeds and fish is possible.

Improving a lake that has become dominated by cyanobacteria is likely to be expensive. One method is to dose it with an aluminium compound, which locks up the phosphorous in the sediment. It can work well, but some hapū find it culturally offensive.

"You are usually not talking about cheap fixes," Brown said.