Water quality held steady from previous years despite a 12 percent increase in rainfall in 2010.
About 15 percent of 64 lakes in the west metro area have severe algae problems and very limited recreational use, according to the latest report card from the Minnehaha Creek Watershed District.
The annual listing was released just before Minnesota's fishing opener on Saturday.
The report gave failing marks of D or F to 10 lakes, C grades to 25 and A or B grades to 29. The Minneapolis chain of lakes got mixed results. On Lake Minnetonka, bays in the eastern half generally rated better than those in the west.
The grades are based on water clarity, algae growth and total phosphorus that comes from fertilizers and organic wastes such as leaves.
Lakes with A grades have clear water and are generally healthy, well-suited for fishing, swimming and boating through the summer.
Lakes with failing grades have cloudy water and are choked with algae, offering few recreation opportunities. In terms of fishing, algae and other excessive plant growth robs water of oxygen, degrading habitat and making it difficult for popular sport fish to survive.
Scientists take multiple water samples from each lake between May and September and analyze the data.
The 2010 results do not vary much from previous years, said Kelly Dooley, Minnehaha watershed research technician. "We're very fortunate to have such consistent water quality in the district."
The district monitors the lakes to be on the lookout for unusual problems, she said, and to see if water-quality improvement programs are making a difference.
Those include efforts to reduce fertilizer runoff from yards by planting rain gardens and shoreline vegetation, installing rain barrels and picking up leaves and pet waste.
Dooley cautioned against reading too much into year-to-year grade changes, which can sometimes be affected by snowmelt, rain and other weather factors.
Lake quality needs to be observed over several years, she said, and in that respect most lakes in the district are improving.
In Minneapolis, Harriet and Calhoun lakes received A grades and Cedar got a B. Hiawatha, Nokomis and Lake of the Isles received C marks.
Almost half of the lakes in the survey are bays or ponds that are part of Lake Minnetonka and are tested separately. Carsons, Grays, Lafayette and Crystal bays got A grades, but Halsted and Jennings bays received D's.
Numerical scores for each lake or bay were averaged and put on a curved grading scale.
To find out how each lake and bay fared in 2010, go to www.startribune.com/a391.
Tom Meersman • 612-673-7388