Wee-vil get to zee truth

A typical scary picture of water fern

I notice silly season coverage of British Waterways kill-the-water-fern-using-weevils story:

The BBC and Independent both covered it. And it has propagated around the world too.

This is a case where the writers involved would do well to follow up the press release from the productive British Waterways PR team and dig a bit more deeply about the cause of plagues of water fern (Azolla filiculoides).

In fact, dense growths of water ferns usually occur only where there are exceptional levels of phosphorus pollution.

This is because water fern has a symbiotic relationship with an alga that can fix nitrogen from the air. So when there is a plentiful supply of the plant’s other main nutrient – phosphrous – it’s growth is effectively unlimited, and it can go wild. So in fact, this is another story about water pollution. Dense growths of Azolla are caused by pollution.

The problem is: it’s much easier to control the symptoms (the growth of water fern), than the cause (the pollution).

So is Azolla, and I quote from the BW press release, ‘a serious threat to waterway wildlife in the UK‘, or indeed any other, freshwater habitat? Not much. A far bigger impact on the canals is their generally poor water quality, and the constant churning up and down of boats which, together, virtually eliminates all underwater plants. But boaters can justifiably argue that this is what the canals are for.

In fact, water fern is petty rare and phosphorus pollution, although all pervasive and damaging, is still usually below the exceptional levels that allow water fern to grow explosively.

In ponds, where you might expect water fern to be most prevalent, we estimate it’s in about 1% of ponds nationally. Or put it another way, its absent from 99% of ponds – which is most of them! Although there are no equivalent statistics from canals, I would guess it’s equally infrequent there too.

But Azolla makes a a dramatic and deeply disturbing looking picture so it’s easy to get people exited about it.

(And for the headline, apologies to my German friends. I was unable to escape from my own cultural stereotype of constantly resorting to ancient World War II jokes – but I couldn’t resist the temptation).

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One Response to “Wee-vil get to zee truth”

  1. The Mr. Biggs of Ponds | Habitat Aid's Blog Says:

    […] point, together with informed comment on relevant stories. For example in August he wrote about water fern, or Azolla filiculoides. Water fern is one of those non-native imports which folk like me have been […]

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