Mystified about temperatures

Jkm

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I am really curious about what’s happening in my pond right now. The Pond of Death is devoid of fish, so this winter I am setting up a nitrogen cycle and experimenting with equipment.

A week or so ago I fed the pond with ammonia to start a good nitrogen cycle. I fed it according to instructions from MitchM (thank you), and as expected when I tested it a few days later, when it had been well-circulated, the ammonia level was right about 3 ppm. So far, so good.

Yesterday I tested it again, this time from both surface and drain level (I made a long tube-and-syringe that can draw water in from the 5’ deep drain). The drain had been closed, with the pond equipment drawing water from the skimmer, deep underground and then up to a pump and large biomechanical filter that are located in a heated, insulated pond shed (~50F), and sending it back out underground and then up to a surface-level waterfall pipe and a shallow submerged outlet. I found the surface water ammonia was back to zero (0 nitrites, 0 nitrates), and the deep water was at slightly less than 3 ppm ammonia (0 nitrite and nitrate).

So I opened the drain, and after several hours both the surface and deep waters tested about 2-3 ppm ammonia, definitely less than the deep water when the drain had been closed. This tells me the water is mostly circulating in the upper level of the pond when the drain is closed. So far, so good - I’ve left the drain open for two days to get all the water into the nitrogen cycle.

My question concerns the temperatures. It’s been about 20F for the last several nights, and up to 40F during the day. The pond has not iced up because of the movement. The surface temp is consistently 37F. Before I opened the drain, the deep temp was 32F. This morning, after the drain has been open for full circulation, the deep temp was 30F.

Why would the temp drop to 30F when I start pulling water out through the drain? Shouldn’t it warm up because the cold water is being pulled out and warmer water being pulled down to replace it? And what happened to the 40F water that is supposed to be heavier than both warmer and colder water? The whole pond is colder than 40F.

Mystified.
 
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The biggest single factor that affects pond water temperature change is humidity at the pond surface.
High humidity = less heat loss
Low humidity = more heat loss

That's why pond covers are so effective at retaining heat within a pond - they help keep a high humidity level just above the water surface.

Because you are moving water through a heated shed won't affect the pond water very much.
Water's high density helps it keep a stable temperature.
Also, a difference of 2 degrees Fahrenheit could be accounted for by the accuracy of your temperature probe.
Wind and humidity at your pond surface is something you should be aware of if you are interested in water temperature.

At the end of the day though, for koi and goldfish, you should only concern yourself with high temperatures - above 90F. Goldfish and koi can easily withstand low temperatures. Just make sure you keep your water quality high, well circulated and well aerated.

...

Why would the temp drop to 30F when I start pulling water out through the drain? Shouldn’t it warm up because the cold water is being pulled out and warmer water being pulled down to replace it? And what happened to the 40F water that is supposed to be heavier than both warmer and colder water? The whole pond is colder than 40F.

Mystified.
Temperature thermoclines only come into play in ponds deeper than 5 feet with mostly still water.
I wouldn't worry about it, but if you're interested further, here is some reading:
The pond heat loss information is in section 4
https://www.oit.edu/docs/default-source/geoheat-center-documents/publications/aquaculture/aqua.pdf?sfvrsn=2
 
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Jkm

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Fascinating! Thank you.

My husband says I’m over-thinking this. I appreciate all your guidance.
 
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You're welcome!
There is a lot going on in our ponds and it's always interesting to learn about.
There are also ways to tinker if one is so inclined.
Usually it's best to let nature take it's course.
 
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