Wednesday, January 7, 2015

Drought led to massive “dead zone” in Lake Erie

A press release from the Carnegie Institution: Lake Erie just can’t catch a break. The lake has experienced harmful algal blooms and severe oxygen-depleted “dead zones” for years, but now a team of researchers led by Carnegie’s Anna Michalak and Yuntao Zhou has shown that the widespread drought in 2012 was associated with the largest dead zone since at least the mid-1980s.

Until now, the size of the dead zone each summer and the factors explaining the variability from year to year have been elusive. Using 28 years of data collected in and around the lake, the team
was able to “measure” the size of the dead zone each summer and identify factors that explain the year-to-year variability for the first time. They found that the 2012 drought, with extremely low water inflow from tributaries, was associated with a record-breaking dead zone in the lake, and that meteorological factors together with agricultural practices explain why these events vary annually. Previous studies have focused on phosphorous from agricultural runoff as the primary driver of the lake’s dead zones, but this analysis shows that the inflow of water from tributaries is actually the largest explanatory factor. The results are published in Environmental Science & Technology.

“Fresh water dead zones—areas depleted of oxygen—result when massive amounts of phosphorus and nitrogen are added to the water, often from fertilizer runoff from agriculture,” explained Michalak. “The excessive nutrients promote excessive growth of algae. When the algae die and decompose, the oxygen in the water gets used up and can drop to levels too low for aquatic life to survive. This happens especially when the water is stratified, with warm water layered on top of cold water, keeping new oxygen from reaching the bottom of the lake.”

The revelation about the size of the dead zone in 2012 comes on the heels of the record-setting harmful algal bloom that occurred in 2011, and the closure of the Toledo water supply in August 2014 due to high concentrations of Cyanobacteria-produced toxins at the city’s water intake. Whereas the record-setting dead-zone event occurred during the 2012 drought, the 2011 record-setting algal bloom was attributed to intense spring storms and various other confounding factors....

NASA image of a 2011 toxic algae bloom in Lake Erie

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