As vertebrates, fish have these same needs for oxygen. Only instead of breathing it in from the air, they extract it from the water with their gills. This oxygen they are extracting isn't being torn from the water molecules themselves. They aren't pulling the the O from the H2O. The oxygen they absorb is 'dissolved oxygen', stable O2 molecules that become dissolved into the water through aeration and agitation, the same O2 that we breathe from the air.
If you're the scientific type and would like to see a detailed explanation of dissolved oxygen saturation in water, click here.
Aside from osmosis, there is another way for oxygen to get into your pond: Plants! As you may know, plants convert carbon dioxide into oxygen through the process of photosynthesis. This oxygen is released via its foliage, and any foliage that is underwater will be releasing this oxygen directly into the water. Reading this, you may be thinking "Great! I'll introduce a bunch of plants and they'll keep my pond flush with oxygen!" Unfortunately, it is not quite that simple. In addition to photosynthesis, plants also perform respiration, a process where they absorb oxygen and use it to convert sugars into energy, releasing carbon dioxide as a by-product. This act of plant respiration occurs around the clock, whereas the process of photosynthesis only takes place when the sun is out. Plants will constantly be using up some dissolved oxygen, but are only producing additional oxygen during the daytime. What this means for you and your pond is that adding plants will create an increase in dissolved oxygen during the daytime and a reduction in dissolved oxygen during the night.
We've covered how oxygen gets introduced to water via osmosis, how to increase osmosis via agitation and aeration, and how plants, introduce oxygen via photosynthesis. What about other factors that may affect the amount or rate of oxygen absorbtion?
Temperature and elevation both play a role in the ability of water to dissolve oxygen. The rule here is that hotter water holds less oxygen, and cooler water holds more oxygen. The biggest differences occur when water drops below 60 degrees, which here in NJ we'd expect to occur around mid-autumn. Altitude affects the rate of oxygen absorbtion as well. As you increase in elevation, the atmospheric pressure decreases, meaning there is less pressure on the water, and thus a slower rate of absorbtion. Here in South Jersey, however, the elevation remains pretty constant at sea-level across, so we don't need to be concerned with how elevation impacts our pond's oxygen levels. For those of you reading this who do not live in the SJ area, the following link will connect you to one of our associates in your area: Find a pond specialist near you.
One more thing regarding water temperature and oxygen levels: Whenever you are doing a cleanout and removing the fish, be sure to aerate the holding pool. Due to their smaller size and being above-ground, temporary fish pools can heat up much quicker than the pond you just emptied. Running an aerator or even just circulating the water with a pump will help counteract any temperature-related decreases in oxygen. Another helpful trick is to set up the fish pool in a shaded area so it doesn't get too hot, the fish will be happier and healthier in the shade
There are a few ways ponds become anaerobic, but they all come down to the same thing: an organic process that removes more oxygen than is being introduced. A common example of this is when there are too many fish and too many plants in a pond. There won't be enough oxygen to go around and levels will continue to drop until things start to die. Then the decomposing matter will continue to use oxygen as it breaks down, and the cycle continues. In cases where it gets that bad, the solution is to drain and clean the pond, fill with fresh water, and re-establish a balanced ecosystem. To prevent a pond from getting out of hand, the guideline is to limit the fish load to 1 inch of fish per 1 square foot of pond. For example, if your pond is 8'x11', that is 88sq.ft. of pond, so you could safely keep up to eleven 8" fish, or four 22" fish, or any combination that doesn't exceed 88". Going a little bit over isn't terrible, but you wouldn't want two or three times that amount.
Another cause of low oxygen is bacteria. Yes, those little organisms that keep your pond from being too high in ammonia and nitrites are also using large amounts of oxygen. Aerobic bacteria performs respiration in the same way as plants, and in a pond with thriving bacteria cultures, they can use even more than the plants and fish combined. The impact of aerobic bacteria on oxygen levels can become compounded due to the fact that bacteria lives mostly at the bottom of the pond on the rocks, and oxygen levels are always lowest at the bottom where the water is furthest from the air. In a 2-3' deep pond the levels won't be significantly different, but beyond that, especially at depths greater than 5', the oxygen levels will be noticeably lower at the bottom. If your pond is that deep and you're concerned about oxygen levels, a submerged water jet to push the water off the bottom can serve to better balance the oxygenation of your pond. To remove any doubt about whether your levels are good, you can pick up a dissolved oxygen test kit for about $25. Click here to purchase a dissolved oxygen test kit.
One of the most common, yet entirely avoidable, causes of low oxygen is algaecide. Decaying algae takes oxygen; in a healthy pond this is occurring at a steady rate, in harmony with the rest of the ecosystem. When you use algaecide, however, all that excessive algae is killed, causing a massive increase of organic debris that begins decomposing all at once. This ruins the harmony of your pond, causing drastic drops in oxygen that can be too sudden and extreme for fish to survive. This is our reasoning when we discourage the use of algaecide. We've seen too many occasions of a pond-owner inadvertently killing all their fish after using algaecide without understanding the side-effects it can have on their pond's ecosystem. It is an entirely avoidable tragedy. There are other, more elegant solutions to controlling algae, but if you are intent on using algaecide, at least do it safely. Break down the application into four or five treatments. Treat no more than one-fourth of the pond at one time. Wait two weeks between treating the next section. Ensure the water is being thoroughly oxygenated via aeration and agitation; if you're not sure, use a temporary aerator like the kind you would use during the winter.
Now that you're educated on how oxygen plays a vital role in your pond, you're better equipped to maintain your pond in thriving condition. A healthy, vibrant pond is always more fun, and requires less effort to maintain. Fish and plants are quite capable of adapting to their environment, so if the oxygen levels are already good, you don't need to change a thing. And with the help of this blog, whenever you do notice something looking off, you will have the confidence of knowing how to assess the situation and make the proper adjustments!
That's all folks! You've spent enough time looking at the computer screen, go look at your pond instead! If you're really enjoying what you see when you view your pond, we'd love to see it too! Click here and share a photo of your pond to our facebook page!