Dissolved Oxygen Testing For Ponds

There are a few things we know for sure. 

Hot weather is brutal on ponds and fish, and we’ve had a lot of that kind of weather lately.

Just this week, MSN of all places profiled this news article on the homepage.

And, other than watching your fish gasp at the surface of the water for air, or float belly up, you don’t really have any idea how much dissolved oxygen is in your pond right now.  There’s simply no way to tell that kind of thing with the naked eye.

Yet this thing called DO for short, is vital to the life and well-being of any pond and if you have fish, it’s critical.  Fish, as you probably guessed, are not air breathers like those of us of the mammalian persuasion.  Fish have gills which work to extract dissolved oxygen from the water and they’re generally pretty efficient at this work if a fish is healthy.

But major problems can arise at various times of the year, or even certain times of day, when DO can fluctuate to very low levels.  It’s important to remember that ponds, no matter how stable and unchanging they may appear, are not static environments.  Elements in them are always experiencing change.  pH levels will rise and fall during the day, and temperature of course, and along with that, dissolved oxygen can change too.

For a pond owner with fish, there are two critical times when you need to watch out for low DO problems.  One is during really hot and still or stagnant weather.  With no wind or agitation of the water’s surface, there’s very little transfer of oxygen into the water, and as we’ve talked about before, when water temps get above 78 degrees F. the water simply can’t hold or retain as much dissolved oxygen. In terms of how much dissolved oxygen might be present, very seldom will a pond have more than 10 ppm oxygen dissolved in its’ water. Critical conditions can arise when dissolved oxygen concentrations go below 3 ppm as this will stress most warmwater species of fish and concentrations below 2 ppm will kill some species. Often fish that have been stressed by dissolved oxygen concentrations in the range of 2 or 3 ppm will become susceptible to disease.

Ponds go through a daily cycle or routine too, and where plants or even algae is present, they can take or provide oxygen at various times.  When a plant is growing well and photosynthesis is occurring, meaning they are getting energy from sunlight, a plant will produce DO.  During the night, or when a plant is dying off, and no photosynthesis is happening, plants can pull oxygen from the water.

So it’s in the early morning, before the sun has really kicked in it’s helpful rays that oxygen can reach it’s lowest point in a pond. 

You can probably imagine what can happen when plants, high heat, and this daily cycle comes into play.

With all this willy-nilly change going on in a pond every single day, there’s just no way to tell for sure where you stand on DO levels.  That’s where a testing meter can come in handy.  Actually there are several types of testing equpiment you can use for dissolved oxygen.  Test kits are available and while they are less expensive than a digital meter, they take some time to run an analysis and some also contain hydrochloric acid which may not be preferred by some folks.  All in all, if DO is something you intend to keep an eye on regularly, a meter makes a lot more sense.

Your typical DO meter (the YSI Economy Oxygen Meter is pictured above) will usually measure dissolved oxygen and temperature and will have three components to it.  The handheld meter, cable, and the probe make up the entire system.  The real benefit to this set up is that the cables come in several different lengths (some up to 50 feet long) which allows the probe to be lowered down to the deepest parts of a pond to get accurate DO readings.

Since every pond will have most of the dissovled oxygen near the surface, the real key to getting an overall idea of the true health of a pond is to take DO readings at depth and see how the levels are throughout the pond.  Now you may wonder why oxygen is so important down deep?  Mainly it’s because most of the beneficial cleaning bacteria (which are aerobic) will do their work at the bottom of the pond as they help to breakdown and reduce organic muck and sedimentation.  They need some oxygen in order to thrive and remain vital so if a pond is deficient in DO, all this important cleaning work slows down or comes to a halt.

When we’ve discussed pond aeration systems in the past, you may recall we also stress the importance of using a bottom based diffuser array, rather than a surface fountain in mosts cases for the same reason…the work and improvements in DO at the bottom will usually help to improve the pond at the top too. 

Because DO meters cost a bit of money, I generally only recommend them to those who are serious about pond management.  Koi pond owners may find them to be quite helpful at maintaining happy fish, and large pond owners who have a strong interest in stocking trout and other fish will have a strong interest in monitoring dissolved oxygen levels in their pond.

Now you could shop around and look for the cheapest, most inexpensive meter and I’d encourage you to do your research on this.  From what I’ve found, many of the cheap one’s can take a lot of tweaking to get the calibration right and you kind of hope it stays there.  When all is said and done, YSI meters have proven themselves to be on of the best meters on the market and they do have an economy unit that does a very good job.

Check out the YSI Economy Oxygen Meter here…

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