Water quality

Initially I used a homemade filter. It was an Eheim pump put inside a 5lt water bottle with filter foam and a mesh in the bottom. The water was forced to pass the foam. It worked for some time until it started clogging, especially when I put duckweed in the pond. In addition the plant roots tangled with the mesh. The turtles made a refugee in that place and I didn't want to destroy it because of cleaning the filter. One day it clogged completely .Since then the plants do the filtration. And they do it well. The water looks crystalline and the regular ammonia, nitrate and nitrite checks are always correct.  Somewhere there are nitrifying bacteria that do their job. The low animal density as well as not leaving uneaten food in the pond also help.

Plants do the filtration and offer refugee to the turtles

Regarding the oxygen levels, I have the feeling that they are correct, but it has not been checked. I would like to do it  but I don't think it is easy at amateur level. 

In a pond the water oxygenates mainly by diffusion from atmosphere and by photosynthesis. If the water moves, especially if it hits a rock or forms bubbles, the pass from atmospheric oxygen to the water increases. In the pond there in no pump mowing the water. There are aquatic plants that make photosynthesis. When the filter stopped working I didn't see signs of lack of oxygen. I had doubts whether to connect it again or not, but it was summer and I wanted to know if the water stratified. A pump would avoid a thermal water stratification. I finally put an aerator. It makes a column of bubbles but doesn't move the water as a pump would.

Aerator bubbles

If there was no oxygen fish and nitrifying bacteria would die. The water would stink and the ammonia levels would rise up. There would be fermentations... nothing of this has happened.

My doubt is whether the oxygen levels are normal or low. I have never seen the fish gasping, not in winter nor in the summer. Neither in the early morning, when the oxygen levels in the water are the lowest because of plant consumption during the night.
Certain situations can deplete the oxygen levels in a pond. For instance when the pond surface freezes the atmospheric oxygen does not reach the water. This is not a problem in my pond. It freezes often in the winter nights, but it usually thaws, at least partially, during the day. Since I made the pond the maximum time that it has been frozen without thawing is four days. The aerator also creates a window in the ice.

Frozen pond

Another situation that depletes the oxygen in the pond is the organic matter decomposition. A plump tree drops the leaves over the pond in autumn. Although I pick up most of them, some go to the bottom.

Water stratification can also reduce the oxygen levels. When it happens, there are two different temperature (and density) water layers in the pond. Water circulates independently in each layer. The deepest one does not mix with the higher one, the oxygenated one. If there is also some decaying matter in the bottom, it is possible that there is anoxia in the deepest layer. I am measuring the temperatures to see what is happening in the winter, but in the summer there was not a permanent stratification, so I don't expect it in the winter neither.

If there were many green microscopic algae in the water, sometimes there is anoxia during the night or if, for any reason, the die suddenly. They are not a problem in my pond.

If there were too many fish they could deplete the oxygen levels, especially in the summer when the warm water has less oxygen and the fish use more. I have few fish and belonging to small sized species.

In the event that there were punctual problems with the oxygen levels, would it affect the turtles? I don't think so.

If there was less oxygen than normal in the water in the summer, it would not affect them because they have no limitation to pulmonary respiration and the extrapulmonary respiration in this season is negligible. Extrapulmonary respiration is the one that takes place when the exchange of oxygen/CO2 is between the skin/mucous membranes and water instead of lungs and air.

If there was less oxygen than normal in the water during the winter, it would cause more problems. Clemmys guttata usually hibernates underwater and can go without breathing air for several months (they don't do this  in my pond). It uses extrapulmonary respiration, probably cloacal in this species (other species use the skin or the pharynx mucose membranes).  Their metabolism is very depressed at low temperatures and the oxygen needs are so minimal that the extrapulmonary respiration provides enough. But the oxygen levels in the water must be normal. What happens if the water has little oxygen?
Clemmys guttata uses to hibernate in shallow ponds, without flowing water, that frost for several months and rich in decaying  organic matter. Precisely because they hibernate in such ponds, that tend to run out of oxygen, this species is  thought to belong to the anoxia tolerant group of turtles.When there is not enough oxygen in the water to meet the needs of the turtle by cloacal respiration, or when the oxygen is completely depleted,  the turtle switches to anaerobic metabolism. How does a turtle survive without oxygen (some species up to 5 months) is fascinating. Some links about it:

Hibernation: Poikilotherms (Storey) http://http-server.carleton.ca/~kbstorey/417-r-els.pdf

Physiology of hibernation under the ice by turtles and frogs (Jackson and Ultsch) http://www.mnf.uni-greifswald.de/fileadmin/Zoologisches_Museum/Hildebrandt/Dokumente/Hibernation_under_Ice_Frogs_Turtles_Jackson_2010.pdf

Life in a shell: A Physiologist's View of a Turtle (Donald C. Jackson)

So just now I keep the pond with an aerator and the plants. I will try to put a tube with more air exits  to increase oxygenation. If possible I will avoid using a pump because I don't want to loose the temperature gradient that forms in the water during the summer in the hottest hours.

Yesterday I saw the female. I had not seen her since early November. She was between the plants, with the head out of the water. I have discovered that she now hides in shallower water, about 10-15cm. I don't think the ice can reach her there, but I would be more assured if she were in a deeper place.



The female in her new refugee