What’s this / what can I do about it? (Pond-related)

could be an orange / red  algae or possibly a bacteria that is  out-competing the neighbouring plants, rather than anything leaking / leaching into the pond

It’s definitely something like that but it doesn’t appear anywhere else and if I dig out the area it still comes back.

but it doesn’t appear anywhere else

Presumably that is because it is where the ideal conditions for the red algae are - warm shallow water.

The easiest solution to getting rid of it is probably to reintroduce some Canadian pond weed. A very fast growing plant like that will suck the nutrients out of the water - it will starve your red algae!

Just carry out occasional harvesting to keep the Canadian pond weed under control.

Good plan but no thanks! There’s plenty of native stuff growing so perhaps it will sort out naturally. It’s not the shallowest part of the pond so don’t think it’s to do with that. Might dig a bit more out and see what happens!

Any old mines in the area?

Or abandoned Indian burial grounds?

It’s not the shallowest part of the pond

It certainly looks shallow! Can you move some floating plants over that area, maybe have a length of bamboo along the surface to stop them floating away? The reduce light will surpress algae.

And yeah when heavily stocked with native species it might deny sufficient nutrients for the algae. By removing a fast growing plant like Canadian pond weed you have probably shifted things in the algae's favour.

The root problem might be that the water draining from your garden into the pond is particularly nutrient-rich, especially if you have been feeding the lawn and flowerbeds.

That I think is bacteria that are oxidizing iron that is naturally in the water. It tends to indicate where groundwater is seeping into the pond

Beyond that I can't add much. In the old days people might use the colour in limewash to pigment it.

If you Google iron oxidizing bacteria there are lots of articles on it although mostly American it seems

One of the stones at the edge of the water looks “rusty”. Perhaps there are a few mote iron-rich stones in the water that are providing a source for the bacteria?

Sauce for the bacteria?

Yep I think it’s something to do with water coming into the pond.
Yes it is fairly shallow but a lot deeper than it appears because the sediment at the bottoms is so deep. I’ve tried digging out the sediment but it’s basically impossible! It’s a lot shallower at the bottom of the photo where the weed is growing really nicely with no algae.
Definitely shifted it in the favour of algae when I first cleared it out but that was green algae which is now clearing up.

I’ve dug it out some more and it’s definitely related to contamination - all of the rocks coming out are discoloured and for want of a better term smell a bit like a drain. I’m assuming that some underground source of water is taking in some contamination from underground sewerage or something like that.
Might try and put some activated carbon in there and see if that helps.

That looks like either acid mine drainage, or, more likely given it smells a bit of drains, septic tank soak away discharge coming to the surface. Discharges for the supernatant are often piped to ditches or other wet spots were they join the greater flow of surface water from springs, seasonal streams, etc...

You could get a sample cultured for faecal coliforms pretty cheaply. Then you'd know.

Weird thing is there are no other houses within about half a kilometre other than ours and a few others converted from an old farm. We have an anaerobic reactor which discharges in a completely different place and is regularly tested. So if it is that god knows where it’s coming from.

How long has this rusty stain been there? All the weeds, in fine singletrack tradition, look very healthy! it doesn't appear to affecting them, yet...

Is the rusty looking stone the cause, or is it looking rusty because of its position?

The pond is about 12 metres long and from 20cm - 60cm deep. I never noticed this until I cleared it out and removed all of the Canadian pond weed. Everywhere else the main problem is rampant growth.
This one area is the only bit affected. I lifted out many discoloured stones last night, so something down there is coming in and causing it. Possibly will resolve when the weed grows more.
For now I think I’m going to get a kilo of activated carbon in a mesh bag and lodge it down there, then plant some of the more vigorous non-Canadian weed on top.
I wonder whether it might have come in during the period of strong rain somehow, and therefore might not continue…

So if it is that god knows where it’s coming from.

If it wasn't a problem before and it is now after you have changed the plants in the pond then that is more likely cause imo, rather than a new outside source which wasn't there before.

Barley hay is a natural suppressant of algae, although that looks a slightly different issue.

Is it growing , changing after heavy rainfall?  My assumption is that it's a type of cyanobacteria, a very primitive algae.  I'd also assume similarly to you that there's a seep of groundwater in there, and that is nutrient, and particularly phosphate rich, and the cyanobacteria like that very much.

I'd just leave it and keep an eye on it.  When the pond establishes a full balance again (you've removed the canadian pondweed) it should disappear, or at least not grow.

## Cyanobacteria are very primitive, and normally not successful, but have an advantage over many other algaes and plants.  Other species need nitrate and phospate to grow (8:1 ratio I think), cyano can survive on phsopahate alone and 'fix' atmospheric nitrogen, thus locally surviving where others can't)

You can buy kits for testing water to see what metals etc are in it.

Whats the stuff that looks like an oil slick in the centre left?

The oil slick will be a thin bacterial layer, you see it quite a lot at the foot of wooded hillside, along with the ochreous sludge you see in the pond.

in wooded areas it's due to decaying leaf litter lowering redox of groundwater (or even damp ground) and leaching out iron from the underlying soils and rocks. this then stays dissolved until it meets either another water body with different redox, or pools at ground surface, where oxygen can easily diffuse into the water, oxidising the dissolved iron and precipitating out these iron sludges. As previously mentioned, iron oxidising bacteria take advantage of this, increasing the rate of production of sludge by speeding up the process, and becoming part of the sludge.

my guess is you've got groundwater or near surface water coming in to the pond, or a leaky something with a high BOD/COD doing the same thing.

stinky rocks may be naturally stinky, if anaerobic, sulphury.

Sorry should have mentioned that happens if I disturb the manky bit. Not there all the time but if I disturb it / dig into the sediment that happens. It is in woodland with loads of big trees and there has been a load of leaves in there which I have dig out as best I could and put a net over it now.

chances are, that the plants that were at that place previoulsy had helped keep the iron sludge in it's place. root systems promote precipitation by oxygen diffusion and providing a decent surface area to precipitate onto. Put some more pond plants/reeds there and it'll not spread (but they might)

Ochre is a particular problem as it removes oxygen, and in some more extreme circumstances can lower pH, in addition to the precipitate itself coating leaves, preventing photosynthesis and impacting gill funcion of pond life.

if it becomes a particular problem you'll need to seal your pond to stop the influx of high iron containing water.