A study says the water quality has been diminishing for decades in several lakes in western Nova Scotia, where residents have complained about blue-green algae.

The Environment Department, which began sampling water quality in the area in 2008, released its report yesterday on 10 lakes within the Carleton River watershed in Yarmouth and Digby counties.

While blue-green algae is natural, the department says the problem is worsened by excess nutrients in the water, which can be caused by poor waste-management practices, fertilizers and faulty septic systems.

The report, which includes analysis of samples taken last fall, found nutrients are increasing overall and water quality has been deteriorating since the mid-1980s. One of the 10 lakes studied, however, was found to have “near pristine water quality.”

Because mink farms were recognized as nutrient sources, the provincial government inspected eight properties this year and found deficiencies on five, which it says have been addressed.

“In the interest of making life better for people who live in these areas, government is committed to doing what it can to improve the health of these lakes,” Environment Minister Sterling Belliveau said in a statement.