If they were both males, it is highly unlikely that they would have not exhibited aggressive behavior from the first time the new swan arrived. You may have 2 young swans that have no idea what they are doing. Mating season is over for this season and won’t begin again until spring
Do you have access to a boat? If the aggresive behavior continues, you may have to intervene as the swan that is in “jail”, may not be able to feed, enter or exit water and become stressed leading to injury or illness. The other swan could seriously injure the swan.
If you have access to a boat or several kayaks, you might want to harrass the new swan so that it leaves the area. If it feels threatened by being chased ( do not chase to the point of over stressing the bird, but just enough to let it know it is not welcomed, it should leave. Do not feed while the new swan is around to discourage its staying. Please let us know how this progresses. The Regal Swan