Lakes naturally “age” over centuries, but human activity – and zebra mussels – can speed up that process, called eutrophication, in decades.

Steve Henry is a freshwater sciences project manager for RMB Environmental Laboratories Inc., based in Detroit Lakes. He spoke to the Hubbard County Coalition of Lake Associations last week.

“Not all lakes are born deep and clean, but some lakes are – and you guys are lucky that you have a lot of lakes in your area that were born deep and clean. We call that oligotrophic, low-nutrient lakes,” Henry said, noting this type of lake has a rocky, sandy bottom; specialized fish, like cisco and lake trout, that are sensitive to pollution and warmth; and few aquatic plants or algae.

“Your lakes are still relatively pristine,” he said. Conversely, lakes in the Alexandria area, where he grew up, have aged more rapidly since the glaciers receded.

In the middle range are mesotrophic lakes, Henry said, like Lake Belle Taine. These have some sediment; sandy, firm bottoms; more walleye, shiners and more plant beds.

High fertility, or eutrophic lakes, have high sediment build up and are a lot shallower. Bass and fathead minnows, which are pollution-tolerant, are more prevalent.

“Without that deep water, nutrients affect the lake a lot differently,” Henry said.

Where are nutrients coming from? Runoff from lawns, shallow groundwater flow, pollen, leaves, dirt in the snow – all these convey nutrients toward lakes, explained Henry. “Your lakes, that’s probably a third of the food it gets every year. It’s stuff falling from the sky.”

When talking about runoff, Henry said there is not one single source. If every landowner reduces runoff from their lot, it will make a big difference.

“I’m an ardent advocate for lakes. If you want to preserve a lake, reduce runoff,” he said. “Action you do really do help lakes for future generations.”

Deep lakes are naturally protected because they can lock away nutrients, Henry said.

A naturally shallow lake, like First Crow Wing, has nowhere to store nutrients, so they have more vegetation.

Zebra mussels consume algae and deposit their excrement in the shallow waters where they live, Henry explained, making those nutrients readily available for aquatic plants.

Researchers in Wisconsin found that, after a decade, there is twice as much muck and three times as much plants. “That is a sign of lake aging,” Henry said.

He pointed out that Lake Garfield, in Laporte, did not have any zebra mussel larvae, called veligers, this year. “Is it possible, if your water is clean enough, there isn’t enough for them to eat?” he wondered. “It’s possible clean lakes are self-protecting from AIS.”