It’s hot, it’s steamy, and no, it’s not a romance novel. It’s the summer weather in most of the country as we move into June and July. This is a perfect time to talk about a phenomenon that often occurs during this time of year and it’s not a pleasant development if you have fish.
The incident is called “inversion” and it’s one of the various things that can deplete a pond of oxygen…quickly. Inversion usually won’t occur in smaller ponds, however larger ponds with a fair depth of 10 feet or more may be susceptible.
As the weather warms up a pond can experience the development of layers that vary in temperature and oxygen density. The upper level of the pond will develop rapid warming which creates water with lower density. Along with this reduction in density, the water will have a decreased ability to hold oxygen. Simply put, warm water holds less oxygen than cool water.
Usually though, with the help of photosynthesis and oxygen exchange there is enough oxygen to sustain fish populations.
As you go lower in the pond, the water begins to cool and while cooler water will hold more oxygen normally, due to the lack of photosynthesis and decaying organic material at the bottom of the pond, you’ll actually find less dissolved oxygen in the lower layers of a pond compared to the surface or top layers.
When environmental factors get rolling like really strong winds, or extremely heavy rains, then an inversion can take place. This represents a complete turnover or flipping of the pond from top to bottom. In other words the cooler, lower layer goes to the top and the warmer upper layer goes to the bottom.
Fish tend to like to stay near the surface or mid-layers where more oxygen is present in normal conditions but when changes can happen so quickly, they can experience oxygen depletion and suffocation.
Visually you may notice that the pond becomes almost black or very dark, where there was once relatively clear water. It may smell badly since the decaying matter has now been displaced and moved towards the top of the pond.
Often what’s most surprising to pond owners is how quickly this can take place and it can take some time for the pond to return to a normal state or condition. Nevertheless, in a matter of minutes the damage can be done to fish populations and certain sensitive species such as trout can be affected with even moderate oxygen depletion.
So how can you guard against inversion in your pond?
While there are no absolute guarantees of protection, one of the very best additions you can place in your pond is submerged aeration. We talk about this quite a bit but it can’t be stressed enough. Aeration adds consistent and reliable oxygen levels in a pond and it does so in the lower levels where oxygen content is inherently low. This simple act can provide a level of insurance against complete oxygen depletion in a pond and it also improves many other factors in the overall health of a pond environment.
Secondly, one of the elements that adds to the negative affects of inversion is heavy organic build up at the bottom of the pond. This gunk is literally like rotten compost, and this is often the material that turns pond water so black and disgusting when a pond turns over. So, reducing this element in a pond is a good thing to do.
For large ponds there are really only a few options that you have to reduce this organic buildup. Dredging or the manual scraping and cleaning of the pond bottom or the through the ongoing use of beneficial bacteria.
Bacteria plays an important role in controlling algae growth but one of it’s other great qualities is the enzyme activity that helps maintain the natural breakdown of organic solids in a pond. Studies show that proper bacterial applications with adequate oxygen can “eat away” or reduce bottom organic sludge by up to 3 to 4 inches in a season. By our definition a season would include spring, summer and fall. In wintertime most bacteria and algae become dormant.
So the moral of this story is that if you have a larger pond with adequate depth and you have fish, be on the lookout for the early elements and conditions that can lead to pond inversion. Do your best to keep adequate oxygen levels in your pond and work to keep the bottom free of excessive organic build up. By doing these two things, you may very well be able to avoid an inversion in your pond even when mother nature throws a variety of conditions your way.