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August 8, 2011 | by  | in Opinion | [ssba]

Failure To Communicate – A Policeman on Every Corner, an Algal Bloom in Every Lake!

The subject of today’s column is the science in a speech given at the end of June by the outgoing head of New Zealand Federated Farmers, Lachlan McKenzie.

Obviously upset at the bad reputation farmers are getting for the condition of our waterways, he decided to tell us a few things. He raises a few interesting points, but there are some definite flaws in the conclusions we are being asked to draw from the facts that he presents. Let’s see what they are!

Lachlan seeks to show us, presumably, that the condition of New Zealand waterways is not the fault of intensive agriculture—or at least that agriculture’s part in it is smaller than we thought. He has two main pillars on which he bases his argument. The first is that trout rather than dairy cows are the agents causing our rivers to become dirtier. The second is that the impact of nitrates on water is not as great as was first thought.

Lachlan makes a pretty convincing case with his first point. He quotes research which compared waterways with trout to those without, and found that algal growth was six times higher in streams where trout had been introduced. There’s no denying that that is a significant difference. However, and this is the main problem I have with most of the facts raised by Mr McKenzie, the amount of algal growth is the only indicator of waterway health that he has presented. As is usual with biological systems, there are a huge number of factors that can contribute to the health of a waterway. We want to know, for example, how much biodiversity is present in the waterway, whether the plants and animals in it grow healthy, and if it is safe for humans to go swimming in. Six times more algal growth is all well and good, but it’s not a great answer to the question: is this waterway healthy? Nevertheless, that doesn’t invalidate the facts presented; they are certainly something to keep in mind.

Point number two is that the impact of nitrates on our waterways is not as big as we thought, and to convince us of this he presents us with a long-term study of over a thousand water sites (large sample size with a long duration—sounds good to me!). The finding he highlights from this study is that in 66 per cent of the studied sites, the amount of nitrogen in the waterways is not the limiting factor for algal growth. Take note—once again, it’s only algal growth data that has been presented, but here I see a bigger flaw as well. If in a waterway the amount of nitrates is not a limiting factor for algal growth, this must mean that there is already an overabundance of nitrates present. In effect he is trumpeting the fact that 66% of the sites are already flooded with nitrates—hardly a convincing reason to allow continued, let alone intensified leaching of nitrates from farms into our waterways!

Mr McKenzie raises a few interesting points in his speech, but I fail to see how any of them constitute an argument that dairy farming is not detrimental to waterway quality.


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