It has the best of lakes, it has the worst of lakes.

Carver County scored four A grades and six F grades this spring in the Metropolitan Council’s annual report on water quality in metro-area lakes.

In the Met Council’s findings, Carver stands out because it had more F-graded lakes than any other county, but also had more As than most. However, because the study measured only 180 of the metro area’s 950 lakes, comparisons among counties may be misleading. The Met Coucil evaluated relatively few lakes in some areas, such as Anoka County. Other metro lakes are monitored by watershed organizations, counties, soil and water conservation districts and the state.

Still, Carver County’s mix of As and Fs offer a lesson in how widely lakes, even in close proximity, can vary. Conditions in and immediately surrounding a lake, especially the human activity on its shores, determine its health.

“Every lake has its own story,” said Madeline Seveland, education coordinator for Carver’s Water Management Organization (WMO).

Condtions that influence lake water quality include landscape as well as its dimensions and history. Deeper lakes fare better because harmful materials settle to the bottom rather than mingle with the water.

Before regulations were tightened in the 1980s, stormwater and untreated sewage were dumped into some lakes, practices that continue to impair some lakes. Carp feeding on lake floors can set loose pollutants that swish through the water.

But the biggest factor is human activity onshore. Farms, industrial areas, parking lots and lawns can throw a lake’s natural ecosystem out of whack.

“A majority of the time it’s going to be some kind of human-caused problem,” said Paul Moline, manager of Carver County’s Planning and Water Department. “Typically it’s going to be fertilizer from agricultural uses or sediments that are running off something that’s not stabilized, or stormwater from pavement.”

Runoff can contain nutrients such as phosphorus and nitrogen, which encourage algae growth. “It’s like a vitamin — in the right amounts, it’s great for the body, but if it’s more it becomes unhealthy for the body,” said Tim Sundby, the Carver WMO’s water resources program analyst. The WMO requires industries and developers to remove or treat water on their property.

Nutrients trigger a process called eutrophication, which clouds water, reduces oxygen, kills fish and emits odors. It can spawn toxic blue-green algae, which can sicken humans and animals. And it makes lakes less desirable as recreation spots.

The lakes that got an F grade in Carver County “are all shallow lakes, so they’re less than 15 feet deep,” Sundby said. Hazeltine and Big Woods lakes, close and connected, also are surrounded by industrial and commercial property as well as Hazeltine National Golf Club. Rain that washes over roofs, paved areas and closely mowed course turf can run into lakes and carry with it nutrients, pollutants and debris.

A couple of other connected Carver lakes that got F grades, Meuwissen and Benton, are more rural and affected by farm runoff. Untreated sewage was deposited in Lake Benton until the mid-20th century.

“Now we’re going back and trying to reverse those impacts that [people have] had on those lakes over the years,” Sundby said.

Treatments that can improve lakes include restoring former wetlands and creating grassy buffers that capture water before it runs off. Another approach is to minimize pollutants, said Brian Johnson, who works in the Met Council’s water monitoring program.

Three of Carver County’s Grade A lakes are in Chaska: Brickyard Clayhole, Fireman’s Clayhole and Courthouse. They were mined for clay during Chaska’s heyday as a brickmaking center, from about 1850 to 1950. The former mines are about 60 feet deep and filled with groundwater from an aquifer. No pipes or storm sewers run into them.

Lake Waconia, the county’s other A-rated lake, has a more complex story. The water in Waconia is clear because the lake is infested with zebra mussels, among the state’s most unwelcome aquatic invasive species. They alter ecosystems, crowd out native aquatic animals for food, clog boat motors, ride from lake to lake on boat hulls and cut swimmers’ feet.

The Met Council’s grading system doesn’t look at invasive species, but it does measure how far down the water looks clear. Zebra mussels, when they first arrive, make water look clearer by feeding on substances like algae that cloud it.

“Sometimes a lake can be extremely clear, but that doesn’t mean it’s extremely good,” Sundby said.