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A new study in Knowledge and Management of Aquatic Ecosystems showcases how untreated wastewater released into the Chicago River during extreme rain events significantly altered the freshwater ecosystem.

Shedd Aquarium researchers found that some zooplankton populations disappeared while others dramatically increased, which could have cascading effects on the entire food web.

Chicago, like many cities across the country, utilizes a combined sewer system where both wastewater and stormwater flow through the same pipes. When heavy rainfall overwhelms the system, pumping stations might redirect the untreated wastewater and stormwater directly into the Chicago River to prevent flooding—an event called a combined sewage overflow (CSO).

During a few days of heavy rainfall in July 2023, 12.3 billion liters of untreated wastewater and stormwater was pumped into the Chicago River. Bubbly Creek, in the South Branch of the Chicago River, received the highest concentration of untreated water with 9.1 billion liters during the CSO event.

Throughout the summer months, Shedd Aquarium researchers collected water samples from different sites along the river to monitor zooplankton populations and other variables. This data enables Shedd to analyze how certain events, like the July 2023 CSO in particular, affect the aquatic ecosystem.

Using this data, Shedd researchers found that the CSO event caused one type of zooplankton, Chydoridae, to nearly disappear from the Bubbly Creek area. Two other types of zooplankton, Cyclopoida and Moinidae, significantly increased in Bubbly Creek after the CSO event.

"It's alarming any time a human-caused disturbance alters the natural ecosystem," said Dr. Austin Happel, research biologist at Shedd Aquarium and lead author of the paper. "This study reveals another piece of the puzzle in understanding how CSOs affect the Chicago River, which provides a home to hundreds of different animal and plant species."

Zooplankton are an often-overlooked indicator of an aquatic ecosystem's health and resilience. These tiny animals drift in the water currents and feed on tiny plants and algae called phytoplankton. Both phytoplankton and zooplankton form the base of most aquatic food webs, providing food to many fish species. Thus, changes in zooplankton populations can indicate changes in the health of an ecosystem as a whole. In the case of CSOs, sudden zooplankton blooms have the potential to infuse harmful chemicals into the food web and shift the balance of nutrients in the water.

Shedd researchers also noted that the July 2023 CSO event altered the water quality of Bubbly Creek for weeks after the untreated wastewater initially entered the river. Oxygen levels dropped to nearly zero and nitrogen levels increased. As a result, fish fled the area and stayed away for weeks before returning, as observed by Shedd's ongoing acoustic telemetry project to track fish movements.

"In the midst of an urban environment, the Chicago River is a biodiversity hotspot teeming with life, and it's everyone's job to protect this critical habitat," said Dr. Happel. "As climate change increases the frequency and severity of extreme weather, this research on CSOs helps us understand and prepare for the threats facing our freshwater ecosystem."

Chicago residents can sign up for sent out by the Metropolitan Water Reclamation District of Greater Chicago to help conserve water on days of heavy rainfall by limiting showers, delaying running the dishwasher and avoiding doing laundry. The City of Chicago also encourages homeowners to disconnect their downspouts from the sewer system and direct the water instead to their yards or gardens.

Shedd Aquarium is also committed to working with partner organizations to help rewild and reimagine the South Branch of the Chicago River. This includes expanding environmental education programs, encouraging community members to recreate on the river and building floating wetlands to provide habitat for wildlife.