The Minnehaha Creek Watershed District’s 10th annual report card for water quality in more than 100 west metro lakes, ponds and marshes shows a long-term positive trend.

Of the 42 lakes that were graded in both 2004 and 2013, 18 had grades that rose, while 14 declined and 10 didn’t change. The lakes are graded for clarity and for phosphorous content and algae growth.

Of the 66 graded in 2013, 19 were in the A category, including Calhoun and Harriet of the Minneapolis Chain of Lakes and 13 bays of Lake Minnetonka. Those grades describe waters that are appealing nearly all summer for a wide range of recreation. Only one water body, the south bay of Lundsten Lake in Carver Park Reserve, received an F, which denotes severe algae growth and minimal clarity.

The three criteria measured are related to one another, noted Kelly Dooley, water quality specialist for the district. Phosphorous, an ingredient in fertilizer that is often washed into bodies of water, drives algae growth, which limits clarity.

Dooley cautioned that the grades offer a limited view of the lakes’ water quality. They don’t show the impact of invasive species or other pollutants, she noted.

Dooley also said that because the water bodies tested are parts of groups of interconnected lakes, creeks and marshes, their grades tend not to change dramatically from year to year.

Cedar Lake in Minneapolis had one of the steepest changes from 2004 to 2013, moving from an A- to a C+. But Rachael Crabb, water resources supervisor for the Minneapolis Park and Recreation Board, which manages the lake, said 2004 was a year of unusually good water quality at the lake, where alum had been deposited several years before that to control phosphorous. The lake has an unusually large and heavily urban watershed extending through parts of Minneapolis and St. Louis Park, and may now be showing a “new normal” level of water quality, although several water quality improvement strategies are continuing, Crabb said.

Some lakes do not get a grade in some years, usually because the device used to measure clarity gets lost or doesn’t go deep enough for a meaningful measurement, Dooley said.