Why rivers need low nitrogen trigger levels

Last week’s column, addressed many significant points and drew several calls from readers seeking to better understand why nitrogen limits are at the forefront of pollution concerns…

The following technical explanation provided by panellist Dr Mike Joy (senior lecturer; environmental science/ecology, ecology group – Institute of Agriculture and Environment) clarifies the situation.

“I suspect the confusion comes in part from industry commentators fueling fears of economic impacts that would result from the protection of waterways and limit their ability to expand and intensify,” he said.

“Many studies have been done in New Zealand over the last few decades to find what levels of nutrients in rivers would trigger impacts on the ecosystem health of our rivers lakes and streams through nuisance algal growth.

“The result of these and other studies in Australia led to the acceptance of numerical trigger values in the Australia and New Zealand guidelines for Marine and Freshwater quality known as the ANZECC (2000) guidelines.”

He says those guidelines have been the basis of river health protection by regional councils ever since and for the various forms of nitrogen the trigger levels are around 0.5 mg/l or 0.5 parts per million.

“Nutrients (mainly nitrogen and phosphate) will negatively impact our recreational values and the health of our waterways at levels well below that which will actually kill fish and aquatic insects.

“Often, as nitrogen begins to build up, the first sign is nuisance algae begin to grow. This covers the bed of the stream in slime which reduces the habitat for fish and their insect food as well as making it unpleasant for us to swim.

“But that’s just the beginning, as the algae continue to grow and photosynthesise as the nutrients increase, the oxygen levels in the water start to fluctuate making it hard or even impossible for fish and insects to survive.

“During the day the water might become supersaturated with oxygen and then get down to very low levels at nighttime.”

He adds, “in a healthy river oxygen is constant like it is in our atmosphere but as the nutrients and algae build up, the fish and insects can’t handle the low levels and die or move away if they can.

“From a human-use point of view too many nutrients and algae make our favorite swimming holes and fishing spots smelly, unattractive and devoid of fish.

“But worse than that, too many nutrients can sometimes lead to the growth of toxic bacteria mats, known as cyanobacteria. They can and have killed dogs and can make people sick or die .

“This is why the ANZECC trigger values are set at around 0.5 mg/l of nitrogen – it’s the point where nuisance algae growth starts to impinge on stream health and our recreational opportunities.

“If the nitrogen levels continue to increase to more than about five times higher than that – say around 2.5 mg/l then you reach the level where fish and insects would be killed by the nitrogen itself (if they hadn’t already had their habitat destroyed or been killed from oxygen fluctuations).

“The lethal toxicity tests are done in the laboratory and only nitrogen is increased. Nothing else is changed so [for example] oxygen levels are healthy and constant. The reality is that the ecosystem level impact on fish and stream life happens way before toxic levels are reached in the same way that the impacts on humans of alcohol occur at a much lower level than the level at which alcohol will kill us.

“If the nitrogen levels were allowed to keep rising another 20 times the 0.5 mg/l trigger level to around 10 mg/l then you get close to the drinking human World Health Organisation water standard for nitrogen (11.3 mg/l).

“Most people unsurprisingly are shocked that the drinking water limit is so much higher than the river health limit. But the reality is that humans don’t live in the stream like fish and insects do, so it’s logical that we could tolerate a higher toxicity.

“However, we would not want to drink water from a river that was anywhere near that level because it would taste and smell horrible because of the algal build-up.

“Most drinking water that is at any risk of getting to these levels comes from groundwater and because it doesn’t see the light of day it could not have any algal growth.

“To put things in perspective the trigger for nuisance growth starting to affect oxygen and habitat is around half a milligram (0.5 mg/l) and the level decided by the board of inquiry in Hawke’s Bay [refer lake Ruataniwha hearing] is a bit higher at 0.8 mg/ l. Around 5 to 10 times that level (2.5 -6.9 mg/l) the nitrate itself can kill aquatic life that is left in the stream, and another 10 to 20 times the trigger level is where it may affect humans (11.3 mg/l).

“The background levels of nitrogen in New Zealand lowland rivers that are not surrounded in intensive farming catchments is around one fifth of the trigger levels for nuisance growth (0.01mg/l), and according to the report just out from Niwa (commissioned by Irrigation New Zealand), around 10 per cent of waterways in some regions exceed the 0.8 mg/l and are likely to be suffering from nuisance algae.

The fact that our rivers are already in trouble is not news to freshwater ecologists. Our freshwater miners canaries (our native fish) have been telling us this. We have the highest proportion of endangered freshwater fish in the world. The fact that our rivers are polluted is not a reason to lift the levels even further.”

I expect it was their understanding of this situation that encouraged panellists Willy Leferink (Federated Farmers dairy chairman), and Irrigation NZ chairman John Donkers to be adamant that things have to change. “It can’t continue,” they intimated.


Peter Shutt, The Timaru Herald

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