WHY DO WE CARE ABOUT DISSOLVED OXYGEN AND WHAT IT IS? About 20% of the air we breathe daily is made up of oxygen. Oxygen, which is dissolved in water, is also necessary for fish and other aquatic life. Every type of creature has different oxygen requirements. Coldwater species like salmon and trout require a lot of it, ideally 6.5 milligrams per liter.
Although they would prefer more, carp are resilient and could survive with only around 2 mg/l. Less than that is sufficient for bacteria to survive, and once it is all gone, they begin to feed on nitrate.
A portion of the Delaware River below Trenton was regarded as a “dead zone” until the Clean Water Act of 1972. There were no fish since the oxygen levels in the water dropped due to the release of organic compounds from slaughterhouses and untreated sewage. Because of the organic matter’s favorable microbiological conditions, all of the oxygen was eaten, leaving no oxygen for the fish to breathe.
The amount of dissolved oxygen in a river is one of the key indicators of its health. Since cities like Philadelphia started treating their wastewater, employing microbes and oxygen in a controlled atmosphere at the treatment plant, oxygen levels have been increasing, according to research done by the Delaware River Basin Commission for decades.
The number of fish caught, according to fishermen, increased in the 1980s.
Since cooler water can store more oxygen than warmer water, dissolved oxygen is naturally higher in the winter. Aeration in rivers is aided by both natural (from wind and waterfalls) and artificial (from dams) sources.
Less oxygen can be found in saltwater.
There are risks that the Clean Water Act does not address, despite the fact that it assisted in increasing the dissolved oxygen levels in the Delaware River to the point where fish like shad and sturgeon have returned.
Issues related to climate change are numerous. Longer summers and warmer rivers are projected as a result of rising temperatures. The salt line on the Delaware River may shift further upstream due to rising sea levels. Additionally, scientists have already seen a difference in the wind speed, which is falling.
Senior scientist and ecologist of the Partnership for the Delaware Estuary is Danielle Kreeger. These are all reasons for caution, according to Kreeger.
We still have work to do, notwithstanding advances made in the treatment of sewage and wastewater as a result of the Clean Water Act, according to Kreeger. And if we let our guard down, these animals, like Shad, which we’ve seen come back, could suffer oxygen losses in the future.
Read Part 3 of Reviving the River to find out more about the hidden dangers that still exist for the Delaware and its watershed.
Dana Bate and Catalina Jaramillo both contributed to this report.
This article was first published in January 2019 as a part of a series on the Delaware River’s restoration thanks to the Clean Water Act. It won a regional Edward R. Murrow Award for Best Series in 2020. It was written and produced by Susan Phillips and Dana Bate. The William Penn Foundation provided funding to assist this reporting.