Fine sediment run-off in rivers could promote toxic algae blooms a Nelson scientist says.
Cawthron research scientist Dr Susie Wood recently observed from working in rivers that fine sediment that came from the land encouraged toxic algae growth in rivers.
“We were mimicking what might happen when we have a rainfall event and it flushes sediment into the river off the land, particularly where land has just been forested for example.”
She said the experiment ran for five weeks and the first three weeks clearly showed a growth in toxic blooms due to an increase in fine sediment.
The experiment set-up included four different “streams” made out of plastic and filled with running water and rocks, mimicking a flowing river.
Wood said they added a type of toxic algae, Phormidium, to every rock in all four channels and different amounts of fine sediment to three channels, leaving one channel the control group.
She said the increase in sediment was one of the reasons toxic algae was growing in rivers.
The growth was also influenced by the right balance of nitrogen and phosphorus in the water.
Wood said the the experiment showed the use of land around rivers needed to be carefully considered.
She said corridors alongside rivers that were not logged or farmed on would be beneficial to rivers.
“I mean that’s just not something that’s going to help reduce Phormidium, it’s also going to improve water quality for a variety of other reasons.”
Wood said the amount of fine sediment on the river bottom increased at the lower end of a river.
“We can see in the Maitai River that up above the reservoir there’s very little sediment deposited and what’s deposited doesn’t actually have much nutrients bound to it.
“But down here on Avon Tce there’s a lot more fine sediment and it got a lot more biologically available nutrients.
“You can associate places with blooms with large amounts of sediment and the settlement with high nutrients.”
She said planting the river banks with native bush,and keeping livestock away from rivers would help reduce the amount of sediment running off.
“Wetlands are another great way to capture sediment that’s coming off the land before it enters the river.”
Wood said the next step in the research included looking at the effect of fine sediment on Phormidium blooms in comparison to nitrogen and phosphorus.
She said this time of year, with warmer weather and less rain, means toxic algae blooms can be expected in the Maitai River.
The Nelson City Council monitors the river weekly throughout summer.
The council and community group Project Maitai are also working on planting more native plants along waterways to reduce erosion and sediment run off, provide shade to cool the water, and increase habitat for other native plants and animals.
For more information on toxic algae, visit nelson.govt.nz/environment/water-3/toxic-algae.
Article written by SARA MEIJ
– Stuff