Something is rotten in the pond of Donnelly.

Scores of non-native fish are dying in droves, stinking up Turlock’s largest park.

“If you’ve driven east or westbound on Hawkeye with the windows opened you might have noticed some odors near Donnelly Park,” said Turlock Municipal Services Director Dan Madden.

The 38 acre Donnelly Park is centered on a 20 acre pond, used as a storm water collection basin. Over the years, a large fish population – mainly carp – has developed in the pond.

“Through ‘donations’ of some nature,” Madden noted, as city staff did not place fish in the pond.

But recent weather and large resident geese populations have caused an algae bloom to occur in the pond.

Phosphates and nitrates in geese dung, added to high temperatures, have led native algae populations to grow rapidly. Those algae then die quickly, and blot out sunlight from underwater algae, causing it to die as well.

As dead algae decays, it consumes dissolved oxygen in the water, reducing the water available to animals and plants.

“Consequently, our large population of fish has been depleted, rather quickly,” Madden said.

Those dead fish float to the surface and begin to rot. Combined with the rotting algae, the smell is quite pungent.

City staff are visiting Donnelly Park daily to remove dead fish and scrape up debris from the pond, in hopes of reducing the stench. But the city’s options for dealing with the issue are limited, as Donnelly pond is a closed, artificial pond system.

The city would have to pump millions of gallons of drinking water into the system to stabilize levels – a process that could take a month or longer, while reducing available water pressure citywide. Or else the city could pump out some of the algae-filled water, in hopes of lowering the algae population.

“During peak summer months, I really don’t want to use drinking water to fill up a lake,” Madden said. “… It’s a balancing act.”

Madden said the city will continue to work to address the problem, but noted that algae blooms are a natural phenomenon.

“It’s the circle of life,” Councilwoman Amy Bublak said.