The amount of pollution that washed into the Chesapeake Bay dropped dramatically in 2015, a trend advocates said is promising but also may have more to do with the weather than with bay cleanup efforts.

From 2014 to 2015, the amount of nitrogen that reached the bay fell by 25 percent, while phosphorus loads dropped by 44 percent, according to the Chesapeake Bay Program. The nutrients come largely from farm runoff and sewage treatment plants and are known to fertilize algae blooms, which cloud waters and create dead zones with little or no oxygen.


The amount of sediment fell most drastically, by 59 percent, over the same period. Sediment washes into the bay from construction sites and eroded shoreline and can also cloud waters and smother underwater plants.

Improvements in water quality that had been anecdotally reported by bay advocates were also confirmed: 37 percent of bay waters met quality standards from 2013 to 2015, a nearly 10 percent improvement over the previous assessment period.

"We are witnessing improvement in a number of our indicators — bay grasses, water clarity and water quality standards attainment, as well as a number of our fisheries such as blue crab population," Nick DiPasquale, director of the bay program, said in a statement. "But we must stay focused and ramp up our pollution reduction efforts if we are to be successful over the long term."

The findings back up recent improvements reported in the bay's blue crab population, underwater grass coverage and levels of dissolved oxygen.

Chesapeake Bay Foundation Vice President Kim Coble called the findings "very encouraging" and applauded efforts by state and local governments, farmers and residents to reduce the amount of pollution that washes into waterways.

But, she emphasized, "weather played a key role and that is something we can't count on every year."

In years with major tropical storms or other heavy rains across the bay watershed, the amount of pollution that ends up the bay tends to increase.

River flow was below normal in 2015, bay program officials noted. But it was not as low as scientists would have expected, given the pollution reduction they observed.

"The long-term decline in pollution loads can also be attributed to on-the-ground pollution-reducing practices," bay program officials wrote.