From the Fort Collins Coloradoan (Bobby Magill):
The problem at Center Lake is complex. Colorado State University civil and environmental engineering professor Larry Roesner, along with city officials, say they’re working hard to rid the lake of both the stench and the dead fish while rehabilitating it, so its ecology and fishery can be healthy again. More than a century’s worth of accumulated organic matter and sediment sit at the bottom of the lake, the largest of three lakes the city created along with Fossil Creek Park in 1996 at the old Portner Reservoir site. As the organic matter decomposes, it produces hydrogen sulfide, which is toxic to fish, Roesner said. Oxygen in the lake is stratified because there is too little circulation for the oxygen to be distributed throughout. The hydrogen sulfide stays near the bottom, and the fish normally avoid it. Since the park was built, he said, some of that hydrogen sulfide mixed into the upper parts of the lake when the wind blew, and the rotten-egg stench would waft through the park and the neighborhood.
So, to get rid of the smell and to rehabilitate the lake, the city and Roesner’s team installed aerators, which are designed to provide oxygen to all depths of the lake. Signs at the lake explain the aeration process and warn visitors some fish may die because of it. But the concentrations of hydrogen sulfide at the bottom of the lake were higher than Roesner expected. When Roesner’s team started up six aerators last Thursday, the stench became worse than normal and fish began to die, but not just because the hydrogen sulfide was rising to the surface. Organic matter circulating through the lake because of the aeration process sucks up oxygen in the upper depths of the lake. So, even more fish suffocated.
To solve the problem, the team has a new solution: A private company is going to bring in microbes that eat hydrogen sulfide and improve aeration in the lake, Roesner said. The goal is to eliminate the oxygen-impoverished layer in the lake water, which should get rid of the odor and improve the fishery, said Craig Foreman, Fort Collins director of Park Planning and Development.
More restoration coverage here.