CLEVELAND, Ohio - Lake Erie's harmful algal blooms typically spend the summer in the Western Basin, but as the lake warms and winds from the west blow the blooms eastward, they merge with a separate, less toxic bloom of a different type of algae in the Central Basin.
The combination develops into tons of algae that from August to the end of September sinks to the bottom of the lake where it decomposes and consumes oxygen until the lowest 10 feet is nearly entirely depleted.
This "dead zone" in the deep, frigid water about 15 miles north of Cleveland, ranging as far east as Ashtabula, turns the water yellow, smelly and distasteful. It threatens the drinking water for millions of Northeast Ohioans and forces fish and other aquatic creatures to leave the contaminated area or die from lack of oxygen.
"The danger comes because there's no good easy way to get oxygen into the cold bottom layer," said Jeff Reutter, the retired director of Ohio State's Sea Grant and Stone Lab. "The demand for oxygen by microorganisms is so great that there's none left for the fish and critters."
Monitoring and tracking the dead zone as it develops is the responsibility of Scott Moegling, the Cleveland Water Department's Water Quality Manager.
"We have buoys in the lake and satellite imagery to help us understand the hypoxia," the scientific definition for oxygen deprivation in the water, Moegling said.
The centerpiece of the water department's three-buoy monitoring system is a deep-water buoy anchored 15 miles offshore. The high-tech buoy is placed in the lake in the spring and brought out in November.
Moegling receives data readings from the buoys for wind speed and direction, water temperature and oxygen levels from the lake bottom, 51 feet to 66 feet deep. The data is shared with the National Oceanic and Atmospheric Administration, which is conducting a five-year project to develop a hypoxia forecast system in the Central Basin.
Moegling serves as chairman of the research study group's Management Transitional Advisory Group, which is assisting NOAA with ways to transition from a research project to an operational product for use by public water systems, fisheries and power industries. One of its work products is a color-schemed hypoxia map that is updated daily and posted online.
"To the best of our knowledge, it's the only real-time hypoxia monitoring device being used by any water system on the Great Lakes," Moegling said. "We've learned how to treat" the dead zone water, "we just have to know when it's coming so we don't have to wait until it's on us to react."
Treatments include adding powdered activated carbon or sodium hydroxide to the raw water, which help to alleviate problems with taste, odor and color; adding sodium hydroxide if the pH drops too low; and adding aluminum sulfate, a coagulant that makes suspended particles bond to each other in the plant's flocculation tanks, then settle to the bottom during the next step in the water treatment process, Moegling said.
Dead zones have developed in Lake Erie's Central Basin for a hundred years due to phosphates and raw sewage that were dumped into the lake prior to passage of the Clean Water Act in 1972. The dead zones disappeared for awhile, but became a concern for water treatment plants again after they returned about a decade ago with the emergence of harmful algal blooms in the lake, said Reutter.
Earlier this month, a band of oxygen-depleted water formed along the lake's southern coast west of Cleveland. But it had had no impact on the city's drinking water, and NOAA color-coded maps show that oxygen levels have improved due to wind action and mixing with warmer water.
The Central Basin dead zone eventually dissipates during the fall when the warm, oxygenated upper levels of the lake water mixes with the cold, oxygen-depleted lower levels, returning the lake water to normal until the next year.
The only way to prevent the dead zones would be to reduce phosphorus runoff by at least 40 percent from agricultural fields in the Maumee River watershed, Reutter said. That is the goal of a partnership between Ohio, Michigan and Ontario, and a recent executive order signed by Gov. John Kasich. But those goals have not yet been met, and House Republicans have asked the governor to rescind his order.
"The dead zones definitely receive less attention than the harmful algal blooms because they don't pose the imminent danger that HABs do," Reutter said.
"They don't form the green scum that everybody sees. They're 50 feet under water, sort of out of sight, out of mind. But from a scientific standpoint, we've been focused on them for many years."
The Cleveland water department tests weekly for bloom-induced cyanobacteria genes and cyanotoxins at each of its four intake pipes. Since testing began in 2010, the Ohio EPA has categorized the department's gene counts as "very low." Cleveland's is the only public water system in Ohio producing Lake Erie water with zero cyanotoxin detections in any of its water treatment plants.
Moegling considers the algal bloom-produced dead zones an occupational hazard. They aren't as toxic as harmful algal blooms, but they pose an annual hazard he's entrusted to minimize.
"Algae is a normal part of the eco-system," Moegling said. "It's the bottom of the food chain. You gotta have it or you don't have walleye and perch."