Researchers found Lake Erie's notorious oxygen-depleted "dead zones" are most likely linked to droughts that occur in the region.
The 2012 drought was found to have been linked to the largest dead zone seen since mid-1980s, the Carnegie Institute of Science reported. In the past data on factors associated with changes in Lake Erie's oxygenation have been limited, but this new research provides key insights into the phenomenon.
To make their findings the researchers used 28 years-worth of data collected from the lake, allowing the researchers to "measure" the size of each dead zone every summer. They found the drought, which caused low inflow from tributaries, was directly related to the record-breaking dead zones. Past studies have suggested phosphorous and agricultural runoff was the main contributor to the dead zones, but these findings show the inflow of water from tributaries is actually the most to blame.
"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," said Carnegie's Anna 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 researchers used the collected data to create a statistical model that included factors such as agricultural nutrient loading and meteorological conditions. These findings showed that while the low rate of tributary inflow was the largest contributor to the 2012 dead zones, it was also associated with a smaller algal bloom than what was seen the previous year.
"Interestingly, we found that increases in water inflow are associated with smaller dead zones, but greater algal blooms," Michalak said. "In fact, water inflow from rivers alone explains over a third of the year-to-year variability in the size of dead zone, and the mechanisms behind this require further investigation."
"When you add three other factors - phosphorus loading into the lake in May to July, northwesterly winds in June that push the nutrient-rich water from the western basin to the central basin, and wind speeds in July that affect the stratification during the hypoxic season, you can explain over 80 [percent] of the variability that the lake has experienced since 1985," she concluded.
The findings are important because they provide insight into how to better-manage the lake and in developing strategies that control the harmful dead zones.
The findings were published in a recent edition of the journal Environmental Science & Technology.
