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Getting to the bottom of lake pollution

The floor of Green Bay is the base of a vital food chain.

Getting to the bottom of lake pollution by Ed Legge

Before industrial pollution nearly wiped out the aquatic mayfly in the 1950s, Green Bay was, as Jerry Kaster calls it, a bustling “walleye factory.”

But no mayflies means no walleye.

That’s why, in researching how to rebuild the food chain, Kaster and graduate student Christopher Groff are focused not on the fish that inhabit the water column, but on the insects living in the relatively overlooked benthic, or lake bottom, ecosystem.

A large mayfly hatch can release around 350,000 tons of protein into the food chain.

Pollution has stripped many invertebrates from the sediment and, as a result, the consistency of the lake bottom has become soupy. In a healthy ecosystem, mayflies spend all but about two days of their lives burrowing in the bottom mud. In those 48 hours as adults they congregate over the water, forming a swarm so large and thick that Doppler weather radars in the vicinity can mistake the hatch for a thunderstorm.


The mayflies, also known as Hexagenia, conclude their terrestrial existence by mating over the water, dropping fertilized eggs in the process. Kaster, an associate professor in UWM’s School of Freshwater Sciences, says a large mayfly hatch can release around 350,000 tons of protein into the food chain.

“Limnoforming” is the word he coined to describe the process he is using to recondition the mud at the bottom of Green Bay, returning it to a thickness that can physically support the burrows of mayfly nymphs.

“Invertebrate ecology is a largely underrepresented part of the ecosystem, and it’s extremely important to the healthy functioning of the rest of the system,” says Groff, who grew up in Fond du Lac and earned his undergraduate degree in biological sciences from UWM.

Testing his theory involves not only the still-absent population of mayflies, but also another creature that should be inhabiting the bottom of the bay in greater numbers – tiny threadlike worms (aquatic cousins of earthworms) whose life cycles help thicken the mud.

In their research, funded by the National Oceanic and Atmospheric Administration, Kaster and Groff are using thousands of Tubificidae worms harvested from the Kinnickinnic River in Milwaukee.

The researchers deploy custom-made “worm condos” – enclosed microcosms containing the thin Green Bay mud infused with worms – to test whether the mud can be rehabilitated.

“We’re also hoping the worms increase the available oxygen content in the sediment, especially given the hypoxia [low oxygen] in Green Bay,” says Groff.