Watermen occupy the slips at St. Michaels, Md. (Katherine Frey/The Washington Post)

A slight increase in air temperature over the past half-century has caused waters to warm more than two degrees in tributaries to the Chesapeake Bay, a change that could reduce the expected benefits of the multibillion-dollar bay cleanup plan and eventually alter the behaviors of marine animals, a new study says.

The mean temperature of the bay’s tributaries is about 2 1/2 degrees higher now than in 1960 as a result of climate change, according to the study by two U.S. Geological Survey hydrologists. Although that doesn’t seem like much, warmer water allows phosphorous, a type of nutrient pollution, to rise from sediment in the bay at a faster rate.

“We expected that we’d find a link between rising air temperature and water temperature,” said John Jastram, who co-wrote the study with Karen Rice. But they were surprised to find that even in the northernmost parts of the 64,000-square-mile watershed, such as Pennsylvania near its border with New York, temperatures in brooks and streams increased significantly.

The effect can lead to “a chain reaction that can occur in different ways,” Jastram said. The oxygen-depletion events called “dead zones” are well-known. A lesser-known impact could be a change in the movement and behaviors of sport-fish species that drive local economies through lucrative yearly fishing events.

USGS staff measure water temperature during a flood on the Appomattox River near Matoaca, Va. (Douglas L. Moyer)

USGS hydrologist John Jastram collects a water-quality sample from a cableway over the Rappahannock River. (Douglas L. Moyer)

“As waters warm, species of fish that prefer warm water can move into the habitats of species that prefer cooler water,” Jastram said. For example, smallmouth and largemouth bass that swim in warmer streams could stray into the habitat of brook trout that enjoy cooler water tributaries, he said.

Bass flourish in water warmer than 80 degrees, but the trout cannot survive in waters with temperatures higher than 68 degrees. Between spring and the early days of summer, female bass seek warm waters of up to 90 degrees to lay thousands of eggs that males fertilize. Trout juveniles would not survive those conditions.

The study, published in the journal Climatic Change in mid-November, was announced Monday by the USGS. Jastram and Rice analyzed data collected by USGS stream gauges at about 125 sites in bay watershed states that include Maryland, Virginia, West Virginia, Delaware, Pennsylvania and New York, as well as the District of Columbia.

Rappahannock River near Fredericksburg, Va. (Amy E. Jensen)

Bay tributaries in the region — Difficult Run, Accotink Creek and the northwest branch of the Anacostia River — warmed at about a half-degree per decade from 1960 to 2010, slightly higher than the bay-wide average, the researchers said.

“We realized we had this trove of data” that policymakers could use to determine whether adjustments are needed in the approach to the bay’s cleanup, Jastram said. “I think it’s remarkable that we have data collected over such a long time to do these sorts of analyses.”

The Chesapeake Bay is the nation’s largest estuary, a storied body of water where fresh water pours from major rivers and mixes with the salty Atlantic Ocean. “More than 100,000 streams, creeks and rivers . . . thread through it,” supporting 3,700 species of plants and animals, according to the USGS.

The bay and the animals in it fuel a major recreation and fishing economy — blue crab and rockfish are major staples of the fishery, along with oysters, although to a lesser extent.

Blue crabs. (Steve Helber/AP)

Rockfish. (Matt McClain/The Washington Post)

But the bay is troubled by pollution that can pose health risks to swimmers and threaten the lives of animals in and around it. Crab populations rise and fall, often from pollution. The oyster stock in Virginia and Maryland has been devastated by a pair of waterborne diseases.

As part of a pact developed by the federal Environmental Protection Agency, the bay region’s six states and the District are collectively spending billions of dollars to upgrade wastewater treatment facilities that often dump stormwater mixed with raw human sewage into its waters. They also are regulating animal feed operations where manure flows into streams.

Those sources, along with sediment, combine to create nitrogen and phosphorous that make the bay unsafe, especially during summer. Warming waters are a significant concern because they can contribute to a pollution cocktail that could undermine the goals of the bay cleanup.

Oyster season in Tilghman Island, Md. (Katherine Frey/The Washington Post)

The bay cleanup plan is a factor in an ongoing dispute. Environmental organizations blame cities and farms for a majority of the pollution, while elected officials in agricultural counties say sediment pouring over the Conowingo Dam near Maryland’s border with Pennsylvania is the largest pollution source.

Millions of tons of sediment — loose dirt and sand from farms and cities, often from housing development work sites — have flowed down the Susquehanna River for decades, collecting behind the dam.

Maryland’s governor-elect, Larry Hogan (R), has called for dredging behind the dam, saying it would have a greater effect than other expensive efforts, including storm-water fees, restrictions on farm runoff and changes in septic disposal.

Jastram said the next step for researchers is to determine how much pollution is generated by warmer water and distributed to the bay through its tributaries.

“We have to determine how much of an impact this is having on nutrient pollution and how much does it confound our cleanup plan,” he said.

Correction: An earlier version of this story misstated the rate of warming of three of the tributaries — Difficult Run, Accotink Creek and the Northwest Branch of the Anacostia River. Their temperatures rose by 0.05 degrees per year from 1960 to 2010, not by a half degree per year. The article also incorrectly described the waterways as being in the District. Difficult Run and Accotink Creek are in Virginia, and the Northwest Branch is in Maryland. This version has been corrected.