Tech Talk 114
Trout, salmon, arctic char and other freshwater, coldwater fish can be severely stressed or killed when the water temperature exceeds 65°F (18°C). Lakes that traditionally contain these coldwater species are typically clean and clear; that is, they have a low trophic index, which allows them to maintain an oxygen-rich hypolimnion (bottom zone) during the summertime.
As the lake gets more fertile, the water below the thermocline has a greater likelihood of running out of oxygen, forcing the fish to move above the thermocline where they are subject to warmer water than they can tolerate, and a fishkill results.
In the summer most lakes will stratify, wherein the colder water is on the bottom (hypolimnion) and warmer water "floats" on the surface (epilimnion) with a thermocline* in between. Typically, oxygen levels in the hypolimnion will be low or non-existent during the warmer months, making all but the cleanest lakes incapable of supporting coldwater fisheries such as trout.
A hypolimnetic aeration system can be installed to support these fisheries by adding oxygen to the hypolimnion without disrupting the thermocline. There are several different designs available, each with its respective benefits and drawbacks. Some techniques inject pure oxygen, others use air and still others pump water to the surface and back to the hypolimnion.
The single most important factor in sizing a hypolimnetic aeration system is to have a thorough grasp of the hypolimnion's oxygen demand to ensure that the design will satisfy the demand. It is best to have several years of biological oxygen demand (BOD) and sediment oxygen demand (SOD) data to calculate the need.
Hypolimnetic installations should be planned well in advance and installed prior to thermal stratification if possible. We recommend contacting Pentair Aquatic Eco-Systems' Lake Department for assistance.
*The thermocline is the line of significant temperature difference.
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