We’ve often mentioned, online and in the magazine, the dead zones in various places around the world, especially the expanding one in the Gulf of Mexico. Almost always, though, they’ve been in the ocean, or in large freshwater lakes like Lake Erie. A new study shows that algae blooms, hypoxia, and the resulting dead zones—the low-oxygen conditions that are deadly to fish and other marine life—are occurring more and more in small freshwater streams.
Researchers at Duke University studied and modeled the growth of algae and bacteria in different urban North Carolina watersheds. “We were surprised to find these dead zones are happening in our own backyards, not just in rivers and coastal waters downstream of major point sources of nutrient pollution,” says the study leader.
Although urban runoff is one contributor to the dead zones in the Gulf of Mexico and other coastal waters, the bigger culprit is usually considered to be agricultural runoff. High levels of nutrients washing off agricultural fields contribute to rapid algae growth.
In the freshwater streams this study examined, however, the main cause seems to be intense runoff from developed areas. Large volumes of rapidly moving water are eroding the stream banks. “We found that erosion caused by these intense flows changed the shape of some stream channels to such an extent that water essentially stopped flowing in them during late summer,” says one researcher. “They became a series of pools containing high levels of nutrient runoff and organic matter, including nitrogen from leaking sewer pipes, fertilizer and pet waste.” In most cases it required another storm to remedy the situation by washing out the water in the hypoxic pools.
The researchers speculate that alteration of streams and rivers by building dams—and sometimes by removing them—can exacerbate the situation.
The study was published in the journal Limnology and Oceanography.