Hypoxic dead zones, which occur when dissolved oxygen levels in water drop so low that fish and other aquatic animals living there suffocate, are well-documented problems in many coastal waters. Now, a Duke University-led study reveals that they also occur in freshwater urban streams.

The study measured dissolved oxygen concentrations, light levels, water chemistry and stream flow in six streams draining urban watersheds in Durham and Raleigh, N.C., from 2015 to 2017.  Researchers used the data to model the growth of algae and oxygen-consuming bacteria in the streams and examine the frequency at which dissolved oxygen concentrations dropped below two milligrams per liter—the danger point for fish and other aquatic organisms. 

The researchers found that stream channels that stop flowing in late summer become pools containing high levels of runoff and organic matter, like nitrogen from sewer pipes, fertilizer and pet waste. The elevated nutrient levels spur greater consumption of dissolved oxygen by bacteria in the water, causing the pools to become hypoxic until the next storm flushes them out. 

Some streams were more vulnerable than others,depending on their underlying geology. While the study was conducted only in small streams draining urban watersheds, its findings are broadly applicable, because pools are ubiquitous features of rivers, made even more so by the long-term legacies of dam building and dam removal.

“Hypoxia is not commonly assumed to occur in streams and rivers because of stream flow, which typically moves water fast enough to prevent the draw down of dissolved oxygen by bacteria to hypoxic levels,”
study leader Joanna Blaszczak said. “However, dam building and other human alterations that stop the flow of water make these freshwater ecosystems particularly vulnerable to hypoxia with negative implications for biodiversity, especially in rivers already burdened with high nutrient pollution.” —DukeUniversity

The study was published Dec. 3, 2018 in the journal Limnology & Oceanography. Read the full paper: “Scoureror Suffocated: Urban Stream Ecosystems Oscillate Between Hydrologic andDissolved Oxygen Extremes.”