Oxygen: The most important element in a pond

Oxygen is essential for life.  It forms the backbone of water.  It's the most crucial element for vertebrates; we need it constantly.  Just a few minutes is the maximum time we can be without it; no other element is as vital to supporting life as we know it.    

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.  ​

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As a solvent, water is extremely effective at dissolving gasses from the air, and since water has less oxygen than air, it is constantly absorbing oxygen wherever the two are in contact.  In a perfectly still body of water, the surface soon reaches stasis with the air in regards to oxygen content, so the oxygen content of deeper water never raises.  This is why we stress the importance of circulating the water in your pond.  As water circulates in a pond, it breaks the surface tension, allowing the oxygen-rich water at the surface to travel to the depths, and bringing oxygen-starved water to the surface to be replenished.  In addition, as the water rolls over rocks and splashes down the falls, it is capturing little bubbles of air and bringing them down into the pond.  When you see a 'whitewater' effect at the base of a waterfall, that is from all the oxygen being captured and dissolved; whitewater is extremely oxygenated water.  
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.  ​

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How much oxygen do fish actually need? This graph will help explain.

This parrotfeather plant is a good underwater oxygenator.

The best oxygenators are plants that have their foliage beneath the water.  For ponds in South Jersey, we recommend utilizing the following plants as oxygenators: Anacharis, Hornwort, Cabomba, Bacopa, and Mermaid Marsh.  Another efficient oxygenator, albeit one that has a bad reputation in backyard ponds, is algae.  Algae does serve an important role in the ecosystem.  In addition to providing oxygen, it is food for koi and other creatures.  The recommended approach regarding algae in your pond is to keep it in balance and prevent it from taking over.  This is best accomplished by supplementing with beneficial bacteria and having other aquatic plants that will compete with the algae for nutrients and limit its growth potential.   It is true that plants are an integral part of the pond ecosystem, but the net result of adding plants alone will not be adequate to rescue an oxygen-deprived pond; additional aeration should be your primary course of action in that scenario.

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 across South Jersey, however, the elevation barely reaches above sea-level , 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, click here to find a pond expert 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.

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How did an oxygen-starved pond get that way?  
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. 

One of the most common -yet entirely avoidable- causes of low oxygen is algaecide.  Decaying algae uses 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 looks off balance, 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!

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Landvista Aquascapes is a team of water feature experts that designs, builds, services and repairs ponds in the South Jersey area. Landvista Aquascapes aims to be the premiere pond contractor in the zip codes of 08004, 08089, 08055, 08053, 08088, 08037, 08009, 08081, 08021, 08091, 08043, 08026, 08084, 08083, 08012, 08003, 08034, 08002, 08033, 08049, 08054, 08048, 08057, 08036, 08096, 08108, 08106, 08035, 08007, 08045, 08080, 08071, 08094, 08215, 08330, 08028, 08312, 08322, 08344, 08052, 08060, 08068, 08015, 08019, 08343, 08062, 08090, 08097, 08051, 08061, 08078, 08029, 08031, 08107, 08109, 08059, 08086, 08066, 08310, 08326.