Here's a couple of comments people have made on the bottom of the article:

Sounds like it could be organic phosphates - blasted chemicals which were the thing that killed a dog of ours many years ago
- Gill, Norfolk, 30/5/2011 14:03

There is also talk that it may be blue green algae poisoning. This algae is fatal to dogs so please keep them away from water it it contains it. There is also the cases fairly recently of dogs dying after being walked at Sandringham estate & Sherwood Forest.
- Dog owner, here, 30/5/2011 13:39