Plant watercress problems in garden ponds

I have a small pond and recently all the plant watercress in it died.

Please can you tell me what tests I can do to see why it might have died. Would

it have died due to high nitrate levels?

Andrea Fletcher, Leeds

Peter May says:

Most people, in fact whole continents have the problem of watercress

taking over, filling ponds and blocking waterways. It is usually quite

difficult to kill once it is established and excess nitrates will make it

flourish even more. However it is delicious stuff and it is not just us

human beings that find it nice to eat. If you have any snails in the pool or

a slug and snail problem on dry land, I would look to these as the culprits.

It is very sensitive to cool air which slows down its growth, and if it is

being grown like a marginal plant, it will tend to duck its foliage below

the water level where it would be particularly at the mercy of water snails

and even fish.

Thinking of 'duck', ducks love it too. There haven't been any itinerant

quackers visiting, have there?