Fish kill, mystery solved

Fatally low levels of oxygen killed the fish found two weekends ago at Ridley Park Lake, according to the Pennsylvania Department of Environmental Protection, and the weather almost certainly was a factor.

The carcasses were reported on Saturday night, June 11, and that was right after that short but intense heat wave that saw the temperature hit 99.

In addition, that was at the end of a long dry spell. No rain was measured in Philadelphia from May 23 through June 8.

Fish rely on the small amounts of oxygen that dissolve in the water -- so-called dissolved oxygen -- to survive. Sampling by DEP biologist Mike McAdams on Sunday morning, June 12, found dangerously sparse oxygen levels.

Oxygen thrives in active, cool waters, but levels drop in hot weather, especially along the more-stagnant bodies of water, such as Ridley Park Lake.

An algal sheen was sighted on the water, and that algae probably consumed some of the precious available oxygen.

Fish-kills aren't all that unusual, and given the inevitable hot spells to come, expect more later in the summer, said DEP spokeswoman Deborah Fries.