MBTA subways, buses and commuter rail trains will operate on reduced schedules while ferries will be taken offline completely in an effort to enforce social distancing and meet reduced demand amid the coronavirus outbreak.

Starting Tuesday, the four core subway lines and commuter rail will run service less frequently throughout the day, including during the old rush-hour periods, and buses except for a handful of key routes will transition to Saturday schedules.

The change in plans was suddenly put in place on a transit system that typically hosts more than 1 million trips on an average weekday. Officials decided scaling back is the appropriate step to help limit the spread of COVID-19 through commuting crowds, particularly with ridership declining.

"The MBTA plays an important role in slowing the spread of the coronavirus while continuing to provide critical services to medical professionals and other employees in key industries that rely on public transit," MBTA General Manager Steve Poftak said in a press release. "While some of these changes are inconvenient, they maintain a responsible balance between protecting the health and safety of the MBTA workforce and our customers, and our goal of continuing to run safe and reliable service without major disruptions."

Subway trains will run Saturday schedules: every nine to 13 minutes on the Blue Line, every nine to 11 minutes on the Orange Line, every 7 minutes on the Red Line between Alewife and JFK/UMass, every 14 minutes between JFK/UMass and both Ashmont and Braintree, every 7 to 13 minutes on the Green Line's four branches west of Kenmore, and more frequently on the Green Line through downtown Boston.

Express bus routes 325, 326, 351, 352, 354 and 501 will not see service reductions and will continue regular weekday schedules. The RIDE paratransit service will also operate at full levels.

All other buses will operate on Saturday schedules regardless of the day of the week for the foreseeable future.

The Hingham and Hull ferries the MBTA offers are canceled until further notice. Commuters who rely on those services can instead use the commuter rail's Greenbush Line by boarding at West Hingham or Nantasket Junction.

Commuter rail service will operate less frequently as well on what a spokesman described as a "Saturday schedule plus."

"The plus being additional trains on top of a normal Saturday schedule to boost peak service and other additional trains off peak to allow for travel at those times," said Tory Mazzola, a spokesman for commuter rail operator Keolis, in an email.

A full summary of the MBTA schedule changes is available online at mbta.com/covid19.

MBTA officials did not provide figures, but acknowledged in a Monday press release that "recent reductions in ridership" played a role in the decision to decrease service.

During the first week of March — earlier on in the outbreak before workplace impacts became as widespread — average weekday ridership was about 2.5 percent lower than it was on February weekdays, a T spokesman said last week. That decline was driven by a 7 percent decrease on the Green Line.

Anecdotal evidence indicated trains had been far emptier in recent days. Gov. Charlie Baker on Sunday announced an order to close all K-12 schools for three weeks, ban most gatherings of 25 or more people, and prohibit on-premises dining and drinking at restaurants and bars, which is also likely to drive down public transit use.

The T's press office did not immediately respond to questions about the operating cost and budget implications of the reduced service plan or about how staffing and planned maintenance work will be affected.

Poftak told MBTA workers about the upcoming changes in a Sunday night memo, hinting that some employees may be asked to work remotely.

"Employees should continue planning for their families well being, especially now with the closure of all public school districts," he wrote. "But it's in the spirit of public service that I ask essential personnel, those of you that operate and keep our system running everyday, to continue doing the job you do best, for all of our riders. For personnel whose responsibilities include supporting our frontline employees, your supervisor will be discussing with you recent interim policy changes relative to maintaining continuity, remote service as well as possible teleworking."

The plans are subject to alteration based on observed conditions, the agency noted.

"As this situation evolves, the MBTA will continue assessing ridership needs with a particular focus on workforce access for hospitals, as well as food distribution locations operated by the City of Boston," the T wrote in its press release. "As part of that ongoing assessment, the T will continue monitoring customer volumes and make service adjustments accordingly; this means if the T experiences an increase in ridership, capacity will be added as necessary."