Brian Rudman: Slugs, cows, fish - they're all in it

As the battle over who killed the dogs and the dolphins on the shores of the Waitemata in recent weeks heats up, Auckland Regional Council chairman Mike Lee is suggesting the ultimate culprits are hiding on the Hauraki Plains, happily chewing their cuds while innocent pilchards and sea slugs get the blame.

Mr Lee is pinning it on the cows. He says "aberrational responses such as algal blooms in the Firth of Thames are likely if the current rate of nutrients being poured into the Gulf continues, and every indication is that they will".

It's true that Mr Lee is a tad grumpy with the attempt by Waikato politicians to annex the southern portions of the Auckland region, including several of his Hunua water supply dams, into their empire.

But his concerns about the consequences of excess nitrogen run-off into the Gulf are only echoing those of Environment Waikato soil scientist Dr Peter Singleton.

Last year, Dr Singleton published a damning report on the parlous state of rural water and soil in the Waikato region, and the effect of intensive farming on the related waterways.

He estimated that run-off caused by excess fertiliser use and animal defecation on farms, was resulting in the equivalent of 97 truckloads of nitrogen-rich urea fertiliser entering the sea at Port Waikato every week and the equivalent of 32 truckloads a week entering the Firth of Thames - the southern reaches of the Hauraki Gulf.

He said the region's estuaries and harbours did not yet show excessive nutrient levels, but it would only take a five-fold increase in nitrogen into the Firth to make algal blooms extending 5km to 10km into the firth very likely.

"While this may seem a large rise in nitrogen ... there is a lag in groundwater nitrate reaching rivers and then the sea. Therefore, current levels of nitrogen reaching the sea do not yet reflect the large increases in nitrogen use of recent years."

Since 1990 there has been a seven-fold increase in the application of nitrogen fertilisers on Waikato dairy farms.

In the past five years alone, nitrogen use in Waikato had increased 84 per cent.

Dairy farming is also intensifying in the region, meaning more urine and more nitrogen-based run-off.

To underline the scale of the problem, he pointed out that the region's dairy cattle produce the same amount of faecal bacteria as 15 million people - 117 cities the size of Hamilton.

This March, in a presentation to the Hauraki Gulf Forum - a talkfest of interested parties - Dr Singleton repeated his concerns. underlining the interrelationship between the Gulf and its catchments. He warned there was a 20- to 30-year delay in nitrates leaching through the ground to the sea and that in the past five years, nitrogen levels in leachate had increased 25 per cent.

Unless the situation improved, he warned, a 100sq km oxygen-starved, lifeless anoxic zone could one day stretch out into the Gulf.

Yesterday he told me he didn't want "to draw the link between the Waihou River [which drains the Hauraki Plains] and what going on at the moment in the Gulf". But "it's definitely something that has to be thought about".

He says that on the plus side, extra nutrients flowing from the river encourage algal growth which is added food for mussel farmers.

The problem is getting the balance right. The nutrients could encourage the growth of toxic algae as well, or lead to explosive levels of algal growth, which, when it dies, creates large anoxic zones.

"New Zealand is not unique. The scenario of intensification we have with our agriculture and this catchment is parallel to types of scenarios overseas which result in anoxic zones."

The lesson, he says, is "it's prudent to take precautionary measures".

So whether or not the cows can be blamed this time round, this week's kerfuffle is a timely reminder of the interconnectedness of the Gulf and its creatures, whether they be designer dogs from the North Shore, dairy cows on the Hauraki Plains or humble sea slugs.