AMES, Iowa – Iowa’s hot July and August days are notorious for producing pond water conditions that can lead to fish kills. The recent heat wave has increased evaporation tremendously and caused a low water table, and thus low water conditions in watershed and spring-fed ponds state-wide. “With increased evaporation and decreased influx of water, pond water has been concentrated down and has very high concentrations of nutrients,” said Allen Pattillo, aquaculture and fisheries specialist with Iowa State University Extension and Outreach. “This has led to increased aquatic plant issues in some ponds for 2012.”
Pattillo has received reports of high numbers of submerged plants like coontail, sago pondweed and American pondweed, as well as filamentous algae this year. Many pond owners are having issues with floating plants like duckweed and watermeal. The low water conditions have led to an expansion of habitat for emergent vegetation like cattails and smartweed. Concurrently, these aquatic plant problems have led to the use of aquatic herbicides by pond managers, which may lead to oxygen depletion issues that are problematic for fish.
“It is common in warm, nutrient-rich waters to have exponential increases in the algae biomass, or microscopic algae blooms,” Pattillo said. Plants produce oxygen during the day, and respire and consume oxygen all the time. Fish, insects, bacteria and other organisms constantly respire. The combined consumption of oxygen is very high at night during the summer because the warm weather increases the total number of organisms in the water as well as the metabolic rate of the organisms.
Fish kills occur when there is low dissolved oxygen available. Different fish species and sizes have different tolerances to low dissolved oxygen – larger fish are much less tolerant than small fish and will expire first. Fish kills can be devastating to fish populations, but rarely eliminate all fish in a pond.
“A good rule of thumb is that fish need at least 5 mg/L dissolved oxygen, which can be achieved by using an aerator,” he said. The main purpose of an aerator is to increase the surface area of the water in contact with the air so that an exchange of gases can be either incorporated into the water or toxicants released from the water.
Surface aerators and diffusers are two alternatives Pattillo recommends. Surface aerators provide adequate aeration and are the best choice for emergency aeration. Diffusers, which carry water from the bottom of the pond to the top, are used to break up stratification rather than oxygenate the water, but may be a cost effective alternative to surface aerators. They should be run from May through September to avoid stratification.
Aeration should begin before chemical treatment and should be continued for a week or more after treatment, especially during hot, windless days.