Blanket weed, string algae or blanketweed. possible new approaches

The information below was sent to me by John Kinross of the School Of Life Sciences, Napier University, Edinburgh and John has given me permission to publish it. John is an authority on pond algae as you will see by visiting his site.

Blanket weed is a real evil and nuisance for many pond keepers

It was in response to a question I posed asking for the latest information on possible blanket weed or blanket weed solutions for ponds. John is an authority on algae.

This is John Kinross's reply ..

I don't have problems in my own pond, but that's partly a question of perception: I'm quite happy to have algae to look at, which is probably not a feeling shared by most! Also, I don't have fish, and I think that may be where much of the problem lies. (Tony comments ... no doubt about this since nutrient levels favouring formation of blanket weed are cretaed by feeding pond fish)

Two of my colleagues here have had problems with blanket weed in their fish ponds this last summer (which has been exceptionally hot and dry for us), and we've also had a lot of algal growth accumulating in rivers, which is quite unusual in our climate. I think in both cases the cause can be put down to an excess (whatever that may be) of nutrients, either excreted by the fish, or in the case of the rivers, from agricultural sources and sewage treatment plants.

I was interested to read about your use of the UV 'filter' for clearing the algae in ponds: My sister has recently had one of these installed in a newly refurbished pond, and it seems to be doing the trick. However my feeling is

that this may not stop the build-up of blanket weed, since you are not removing nutrients from the system.(in addition blanket weed does not pass through the UV so cannot be impacted upon by the UV radiation)

The question is, how can this be achieved?

The major nutrients favouring blanket weed or algae growth are nitrogen, as ammonium or nitrate, and phosphorus, as orthophosphate, and plants require these in the molar ratio of 16:1  N:P.

Typically, the cause of eutrophication in fresh waters is put down to an excess of phosphorus due to human activities (sewage discharge, use of detergents, etc.), since there is usually an ample supply of nitrogen. (This may not always be the case in shallow waters, however). The treatment of eutrophication in lakes is seen therefore as controlling the inputs of phosphorus, and this may also involve trying to control the release of phosphate from the sediments, eg by dredging, or by chemical treatment to precipitate it.

In a pond, dredging out sediments which have accumulated over several years may be possible, but unless inputs are controlled also it is unlikely to do a lot of good. If you are feeding fish in the pond, there will be a constant input of nutrients. The only chemical process I can think of which would not harm fish or other aquatic life is to pass the water over some kind of material which will adsorb it:

A student here did a trial of the use of maerl ('calcified seaweed') in sewage treatment and found it very successful, but any form of mineral, especially if it contains limestone, which offers up a large surface area would do the job. Shells might be a good source.

My own preference would be to try to make use of the plants themselves to remove both nitrogen and phosphorus, by continuously harvesting them. (this is an approach favoured by many water gardeners and makes sense in the fight to reduce blanket weed infestation)

This approach is being tried by a number of people around the world for polishing of treated sewage effluents, eg in the US and Australia, and is usually referred to as 'Algal scrubbing'. I've no direct experience of this myself, other than some experiments I did using artificial streams some years ago, which showed me that a good lawn of blanket weed could strip the nutrients (especially nitrogen) from a growth medium in a few days.

The relevance to a real pond situation would depend on the size of the pond, nutrient levels, and the area colonized by the algae, but I could envisage a series of small troughs with a good flow of water through them, and some rigid plastic netting suspended in the flow. A good turbulent flow, with air injected in at the head of each trough would give the best results. The nets could be removed in rotation and scraped down to remove the growth. Such a system could be put downstream of your UV filter, if there was enough flow.

It might be necessary then to have the nets colonised in the pond first for a week or so before placing them in the UV sterilized water. Of course, a shell filter could be used simultaneously.

These are my sugestions; if we have more hot summers here I may get a chance to try them out if I can talk my colleagues around to it, but if you think it worth trying, I'd love to hear what success you have.

Best wishes,

John

PS ... comments by Tony Roocroft

The 2 suggestions by John seem well worth trying to me since they make very good technical sense and should therefore improve if not resolve most blanket weed or blanket weed problems. Should anyone try either of the two (or both) please let me know of your results so that others can benefit.