Pennsylvania environmental officials say a massive fish kill in a Delaware County lake was largely due to hot weather.
Water quality tests showed levels of pollutants were under the allowable limit at Ridley Park Lake. It was the low level of dissolved oxygen in the water that killed an estimated 1,000 fish on June 11.
“Fish have oxygen requirements just like we would,” said Alan Everett, with the state Department of Environmental Protection. “They do not have enough oxygen to meet their metabolic demands and they die. They basically, in a sense, are asphyxiated.”
In hot, sunny weather, algae blooms thrive in lakes and ponds. During the day, they release oxygen as part of photosynthesis, but at night they begin to consume oxygen.
“There’s this big competition now between the plants at night, the fish, the macro-invertebrates and the decomposing organic material for this limited amount of oxygen,” said Rex Miller of the DEP.
As water gets hotter, it is less able to hold dissolved oxygen. And as more algae grow, they die off and use even more oxygen in decomposition.
The water in Ridley Park Lake reached 77 degrees in the first weeks of June, Miller said. He called the high temperature, as well as sediment buildup in the stagnant lake, the “perfect storm” for the fish deaths. Hundreds of fish died in other locations throughout the area in the same week.
Fertilizer and nutrient runoffs from lawns also help feed algae blooms and lead to low oxygen levels.