The Loon is very territorial and you typically won't find more than one mating pair on a lake that's under 50 acres. Sometimes the juvenile loon will return to its natal lake the following year which might've been the case in the scenario below. The mating pair is very protective of their babies and will go after any Loon they perceive as a threat.
Here is the Loon eyeing his invader.
The chase ensues!
And don't come back!