We've had a small "nature pond" in our garden for -- ooh -- 25 years.
So I know a bit about looking after ponds. That said:

Today we were admiring the frog spawn, and looking for creatures, when I
decided to test the depth -- at its deepest it was about 2 feet when I
made this pond. I pushed a stick down into the accumulated muck to test
this; there seems to be about 9" of solid matter at the deep end, under
the lilies and weeds, which must be roots.

Anyway, this is my question: Bubbles rose -- quite natural of course,
but we noticed a smell, which set me thinking: so that's gas, and should
I stir the whole pond up a bit to get rid of it?

I did notice that there's quite a large accumulation, by now, of muck,
so also wondered if I ought to remove a certain amount of this stuff, in
order to "freshen" the water?

This particular pond (roughly 9' long, 5' wide, 2' deep at max) is
about 12 years old now, and I've never really removed muck. I do net it
in winter to prevent [most] leaves entering the water, and we have a
small pump which moves the water a bit from one end to the other, 12
hours a day.

Comments appreciated!
John

On 30/03/2018 13:25, Another John wrote:
We've had a small "nature pond" in our garden for -- ooh -- 25 years.
So I know a bit about looking after ponds. That said:

Today we were admiring the frog spawn, and looking for creatures, when I
decided to test the depth -- at its deepest it was about 2 feet when I
made this pond. I pushed a stick down into the accumulated muck to test
this; there seems to be about 9" of solid matter at the deep end, under
the lilies and weeds, which must be roots.

Anyway, this is my question: Bubbles rose -- quite natural of course,
but we noticed a smell, which set me thinking: so that's gas, and should
I stir the whole pond up a bit to get rid of it?

Not worth the effort and if you puncture the liner you will regret it.
It is natural anaerobic decay of plant material at the bottom of the
pond. So long as the pond is deep enough and the clear water is
oxygenated it doesn't really matter. Plenty of things live in mud.

I did notice that there's quite a large accumulation, by now, of muck,
so also wondered if I ought to remove a certain amount of this stuff, in
order to "freshen" the water?

If you have got frogs and toads then the water above must be OK.
Changing the water by adding anything other than rainwater can cause
problems for an established pond as the antibacterial agent in potable
tapwater is extremely toxic to aquatic animals.

It might be worth taking a bit out if you are feeling energetic going
into winter but certainly not worth doing anything at this time of year.
That or plant something that converts mud to plant material like reeds.

--
Regards,
Martin Brown

In article ,
Martin Brown wrote:

On 30/03/2018 13:25, Another John wrote:
We've had a small "nature pond" in our garden for -- ooh -- 25 years.
So I know a bit about looking after ponds. That said:

Today we were admiring the frog spawn, and looking for creatures, when I
decided to test the depth -- at its deepest it was about 2 feet when I
made this pond. I pushed a stick down into the accumulated muck to test
this; there seems to be about 9" of solid matter at the deep end, under
the lilies and weeds, which must be roots.

Anyway, this is my question: Bubbles rose -- quite natural of course,
but we noticed a smell, which set me thinking: so that's gas, and should
I stir the whole pond up a bit to get rid of it?

Not worth the effort and if you puncture the liner you will regret it.
It is natural anaerobic decay of plant material at the bottom of the
pond. So long as the pond is deep enough and the clear water is
oxygenated it doesn't really matter. Plenty of things live in mud.

I did notice that there's quite a large accumulation, by now, of muck,
so also wondered if I ought to remove a certain amount of this stuff, in
order to "freshen" the water?

If you have got frogs and toads then the water above must be OK.
Changing the water by adding anything other than rainwater can cause
problems for an established pond as the antibacterial agent in potable
tapwater is extremely toxic to aquatic animals.

It might be worth taking a bit out if you are feeling energetic going
into winter but certainly not worth doing anything at this time of year.
That or plant something that converts mud to plant material like reeds.

Thanks Martin, and also Chris -- clear and sensible reassurance!

Cheers
John