Nitrogen contamination in southern Minnesota is so severe that 27 percent of the region’s lakes and rivers could not be used for drinking water, according to an unexpectedly blunt assessment of state water pollution released Wednesday.

The Minnesota Pollution Control Agency (MPCA) said that, overall, 41 percent of the streams and lakes in southern Minnesota have excessive nitrogen, which can be toxic to fish and other forms of aquatic life and is the state’s most widespread form of water pollution.

Nitrogen is one of the nutrients that sweeps down the Mississippi River from the Upper Midwest and into the Gulf of Mexico, where it creates a polluted area known as the “dead zone” which now covers an area as big as Massachusetts.

Reducing nitrogen by just a third would result in much healthier lakes and rivers — but would cost tens of millions of dollars annually in each watershed, the MPCA said in a report released today.

Three fourths of the nutrient comes from fertilizers used in agriculture, particularly tile drainage that sends contaminated water from farm fields directly into ditches and streams, the report found. About 9 percent comes from wastewater treatment plants, and 1 percent from urban runoff.

And the trend is worsening in some parts of the state, the report found, particularly for the Mississippi River.

Nitrogen concentrations are modest in the upper reaches of the river, where there is little agriculture. Nonetheless, the river showed increasing concentrations between 1976 and 2010 — ranging between 87% and 268% — everywhere between Camp Ripley and LaCrosse. During recent years nitrate concentrations were increasing everywhere downstream of Clearwater at a rate of 1 to 4 percent per year.