GREENSBURG – Many Decatur County residents near Lake McCoy have noticed a foul stench in the air and those that get close to the shore are met with a disturbing sight- hundreds of dead fish at the water’s edge.
As one approaches the shore, the stench of the lake grows stronger. When the actual shore comes into view, the grisly line of fish carcasses can be seen all along the edge of the water, complete with swarms of opportunistic insects. Mysterious and perhaps ominous bands of lighter-colored water and serpentine ribbons near the shore are visible. The water around the dead fish has an oily, iridescent sheen.
One might develop several theories for the cause of the massive fish kill- pollution, geological activity, or even global warming. However, the actual cause is completely natural. Both the Department of Natural Resources (DNR) and the Indiana Department of Environmental Management (IDEM) investigated reports of the fish kill.
According to Indiana Conservation Officer Corey Norrod, both the DNR and IDEM conducted a number of water tests to rule out pollution as the cause of the changes at Lake McCoy. IDEM officials discovered negligible levels of ammonia and tolerable levels of dissolved oxygen, proving that an unknown contaminant did not kill the fish.
DNR conservation officers investigating the report concluded that the light bands seen across the lake were actually ordinary pond scum, which is very common in stagnant water and forms a greenish film near or on the surface. The algae that comprises the pond scum is the base of the food web in the lake’s habitat. Conservation Officer Norrod also reported that Lake McCoy appears to have a longstanding problem with excessive nutrient and sediment runoff. The excess nutrients in the water can hamper plant life, which supplies some of the oxygen in the water.
Norrod said that based on the information gathered, Lake McCoy appears to be undergoing a process called “turnover.” His statement was based on the characteristics of the lake itself and the known signs of lake turnover, as well as a lack of evidence of contaminants. A combination of factors likely contributed to the current situation in Lake McCoy.
When there is a large difference in temperature between the water at the surface and the water at the depth of the lake, it is split into three separate and distinct layers in a process called stratification. The layers are differentiated by temperature and oxygen content. Warmer water is less dense and tends to float on top of the cooler water.
During the summer, the sun’s energy heats the topmost layer, the epilimnion, making it more buoyant and keeping it floating at the surface. Wind, storms and access to open air cause some mixing and add some oxygen to the water in that level. Oxygen is also added to the water by aquatic plants.
Thermocline, the middle layer, acts as an effective barrier to prevent the mixing of the deep and shallow water. Toward the end of the summer, the deep water becomes more and more depleted of oxygen because the thermocline prevents mixing.
The bottommost layer is called the hypolimnion and it stays close to 39 degrees. The sun’s radiation doesn’t reach this layer and the density of the cooler water keeps it at the bottom of the lake. Very little oxygen resides in this layer.
As the days grow shorter and cooler when summer turns to fall, mixing between the levels becomes easier. The water on the surface, with a higher oxygen content, begins to cool and then drops into and then through the thermocline, displacing the water from the bottom level and forcing it to the surface.
Eventually, the water mixes enough to erase the stratification levels built up over the summer. At some point, the water will achieve a basically uniform temperature. The deep water coming to the surface contains an abundance of decaying matter and sulphurous gases. The odor released upon reaching the surface is a telltale sign that the process has begun.
In time, the turnover will mix enough fresh oxygen into the entire lake mass that the deep waters are replenished with the life-sustaining element and fish are able to return to the depth where they will spend the winter months.
The fish in Lake McCoy were likely killed by the changing temperature as the lake’s layers shift, in addition to the excess sediment and lack of oxygen in the water. Sulphurous fumes released from the lower level of the lake were also a possible contributing factor.
There is no set amount of time for a lake’s turnover. The excessive nutrients and sediment in Lake McCoy, as well as a dry period followed by heavy rain are all factors that contributed to the turnover and will, in part, determine how long the process takes and whether it is successful.
The lake should stabilize, said Norrod, though the level of excess nutrients and sediment make the final result and timeline unclear. Lake McCoy is not managed by the DNR, so final decisions regarding what actions to take will rest with the owners of the lake.
Lake McCoy residents have long been prohibited from using the water for anything other than recreation and swimming has been banned for several years, as well as eating any of the fish caught there. Overall, they won’t be facing overwhelming life changes while the lake turns over, though the smell and sight of the dead fish are decidedly unpleasant.
The situation is likely to be closely monitored and further action determined as needed. Because the lake is privately owned, DNR and IDEM don’t have the authorization to make changes as of now. If the situation worsens or becomes a danger to people or the surrounding environment, that may change.
Contact: Amanda Browning 812-663-3111 x7004