Winter aeration interrupted, fish kill likely at Powderhorn Lake

BY DAWN SOMMERS,
MINNEAPOLIS PARK BOARD

Low water levels, extreme cold and persistent snow cover are taking their toll on Powderhorn Lake and a winter fish kill is likely, reports the Minneapolis Park and Recreation Board (MPRB).

“The water level of Powder-horn Lake has continued its slow decline and is now below the level that can supply the winter aeration well, a large concrete manhole connected by pipe to the lake,” explained Tim Brown, environmental operations manager with the MPRB. “We pumped the full permitted allocation of groundwater into the lake, but dry subsurface conditions in the surrounding soils and sediments soaked up the water and resulted in the unrelenting water loss from the lake.”

The MPRB’s efforts raised the lake enough to allow skating, but the low water level means there is not enough water for the aeration pump to continue operation, even at the lowest setting. As a result the MPRB had to turn off the aeration system, and with the season’s extended cold temperatures and snow cover, the oxygen levels in the lake have rapidly dropped and the MPRB is expecting a winter fish kill.

The Minnesota Department of Natural Resources (Mn-DNR) will stock Powderhorn Lake with fish this spring.

Fish kills are natural occurrences during Minnesota winters. The MnDNR expects widespread winter fish kills throughout the state due to the extreme cold and persistent snow cover this year.

“It is difficult to predict the impacts to the fish populations that are due to oxygen depletion caused by thick ice and snow cover,” explained Brown. “The winter aeration system forms a relatively small area of higher oxygen near the aerator diffuser that acts as a small scale refuge for fish with high oxygen needs. However, some fish and many aquatic creatures survive with almost no available oxygen.” 

Brown stresses that the current problem with the low water levels is not related to last year’s winter kill. According to Brown, the 2007-2008 winter aeration system at Powderhorn was not run, at the request of the MnDNR, in order to help minimize the spread of the invasive Brazilian Elodea (Egeria densa) within the lake.  The MnDNR was not concerned with the operation of the aerator this season.  

When spring returns, the MPRB will review the intake situation at the aerator and develop a plan to insure that winter aeration can occur during low water conditions in the future.