- Author: Dustin Blakey
As you no doubt have noticed, many backyard ponds are different in many ways because of the drought. A lack of continual flow may have caused weeds and algae to build up in these ponds. If your pond is just a water feature in the yard then an algal bloom is nothing more than an unsightly mess.
For people who may be keeping fish, there is a risk of a fish kill when too much plant life accumulates in a pond. What happens is this: first, nutrients like fertilizers, fish food, and feces wash into ponds. Our water tends to be clean but if allowed to sit in one place, being the lowest place in your yard, nutrients will accumulate in the pond. If there are excessive nutrients in water then something is going to use them. That usually means algae.
Algae is beneficial to an aquatic ecosystem; however, when levels get too high there can be problems. Some algae can release toxic compounds, but the most common source of fish kills related to algae is oxygen depletion.
If you remember your elementary school science class, you'll recall that plants (including green algae) take carbon dioxide (CO2), water and sunlight to make sugars and oxygen. Oxygen is good! But at night plants respire just like we do: they burn energy sources like sugar and fat with oxygen and release CO2. It is the extraction of oxygen for respiration in water at night that causes most fish kills.
In the short run, the best solution is to aerate the pond at night. A pump that draws water up and splashes it over rocks will create an oxygen-rich zone that can keep fish alive overnight.
We are lucky that we have relatively cool nights all summer. Respiration is temperature driven so it happens more slowly. Remember that it is the water temperature that drives respiration of aquatic plants, not air temperature. If we have a really warm day with cloud cover at night, then the risk would be higher of a fish kill. Smaller ponds are more affected by oxygen depletion.
When should you be worried about oxygen-related fish kills? These are the conditions that all need to be met to be at risk:
- Warm water temperatures at night (summer)
- No flow in pond (stagnation)
- Green water from algae where you can't see more than a couple inches deep, or very thick growth of submersed aquatic plants and filamentous algae
- Fish in pond, obviously
In the long run the solution is to have cool, clear water. When the ditch system is working as it should, the water will be kept this way on its own. Without flow, limit the amount of nutrients that can enter the pond. You can do this by reducing fertilizer applications and if you feed fish, by limiting their feed. Also allowing taller, more aggressive plants to collect these nutrients before they make it into the pond can help. In fact many properties are having problems with bullrushes, tules, and other stream-side plants as the bank, enriched with nutrients, is exposed. Simply allowing your turf to grow a little taller along the edge might help.
If you've been filling your pond with well water to keep it full, it wouldn't hurt to aerate that water while you add it. Let it splash on a rock or hard object above the water and it will dissolve oxygen into it.
In all likelihood, you won't have a fish kill in Owens Valley, but because the drought has affected yard ponds and water features so dramatically, it is definitely more of a possibility than in the past. Keep an eye on your ponds and it should be evident whether you are at risk of a fish kill.