Pond algae and minibeasts

[(R)ick Neumon]

My neighbour has just asked me what's turning his pond green. I don't know much about ponds, but I'm sure somebody here can tell me what to look for.

The pond water has turned green, it doesn't smell and isn't particularly cloudy. There has been duckweed on the top which he's been removing. There are some oxygenating plants (maybe not enough?). Is it best to let the pond recover naturally or should he help it along with an algicide? The pond does not contain fish. It does however have some tadpoles.

There are some minibeasts in the pond which I presume to be the larvae of some sort, possibly mosquito? There are also some strange critters which are curled, almost snail-like, with antennae, but they move in a very peculiar fashion (up and down).

[Red Robin]

How green? Could you fill a glass jam jar with the water and see how green that looks. Photos of the pond and size info might also help.

With the minibeast life you describe the water might not be as unhealthy as you perhaps think.

Personally I would always recommend natural methods if recovery is needed rather than introduce chemical or man-made substances.

Someone else with more knowledge than myself will doubtless be along to help soon.

[(R)ick Neumon]

Hi Red Robin,

Thanks for your reply. I had trouble uploading my pic, I think it's on the Gallery. I will try again now.

The pond is the size of an old bath(which is what it is). The water is a lime green colour.

The birds are drinking and bathing in it too, would they automatically avoid it if there was a major problem with it?

I have been looking at the beasties through a magnifying glass. They're not so easy to photograph, but great fun to watch. The straighter looking bugs are mosquito larvae, but I have really struggled to identify the other curled up bugs. They seem to have a carapace and when they swim, they temporarily straighten out, they swim up, down and sideways, but then settle at the top.

[triops]

Your bugs are probably mosquito pupae. The water in the pond has probably turned green because it is getting lots of sunshine, because the duckweed has been cleared!

[King Edward]

How long has the pond been established? The prevalence of first duckweed then algae implies an excess of nutrients in the water. These could be from the water used to fill the pond originally (clean rainwater is best), or from soil in the water (perhaps that used for plants). Or perhaps from tadpole waste, especially if they have been given any supplementary food.

In a small pond, a little soil or other nutrient input could have a big effect.

What plants are in the pond, and how are they planted?

It might be just a temporary algal bloom which will subside as the available nutrients get used up and small crustaceans multiple and eat it, or it could be a sympton of an underlying nutrient excess. If the latter, then even if the water clears there might well be recurring problems with excess duckweed and algae.

[dampflippers]

Is the pond raised up like a bath? If so the water will be warmer than if sunk in the ground and be the cause of excess algal growth.

If the bath is sunk in the ground, nutrients may get washed in when it rains, causing the growth.

Eventually it will suddenly clear, but more oxygenators or marginals to use up nutrients will help. Also the duck weed will have shaded the water to some extent, so when removed the green water will have increased.

I would avoid chemicals, but increase the number of plants, remove dead leaves in the autumn as these will rot down and release nutrients.
If he has fish in it, does he feed them? This too will make green water worse.