These last few years haven't exactly been Lake Erie's best. It was recently revealed that the lake has become even more vulnerable to harmful algae blooms (HABs) than it once was. To top that, the lake's invasive zebra mussel problem has simply gotten worse, and now experts are saying that a 2012 drought has led to the worst dead zone the lake has seen in decades.

"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," researcher Anna Michalak recently explained in a statement. "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."

Michalak and her colleagues at Carnegie Mellon have long known that Lake Erie suffers from dead zones, which ultimately affect the distribution and prevalence of aquatic life in the massive lake. After all, most fish and other freshwater animals will only stay in parts of the lake where breathing is easiest. However, the size of these dead zones appeared to change each summer, and a concise explanation for why this occurs had remained elusive.

As described in a study recently published in the journal Environmental Science & Technology, Michalak and her colleagues used 28 years of data collected in and around Erie to measure the size of the dead zone each summer and identify factors that explain the year-to-year variability.

They found that extremely low water inflow from tributaries - the result of a 2012 drought - was associated with a record-breaking dead zone in the lake, the largest since at least the mid-1980s.

"Interestingly, we found that increases in water inflow are associated with smaller dead zones, but greater algal blooms," Michalak added. "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."

The researchers added that when the factors of prosperous concentration and wind speed and direction during various seasons (which push nutrient-rich water around), you can account for over 80 percent of the variability in annual dead zones.

And that's important information for management of Lake Erie and lakes just like it, they said.

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