Why is the "the" taken out of here?
It’s the -n of sjön. Sjö means ’sea’. Note that this is pronounced with a short ö and a long n, whereas sjö has a long ö.
Oh, I didn't catch that when translating that, and put östersjöen......whoops, my mistake!
As I hear Östersjön (well, actually I could not catch it), I mistook it as "Östersund" where I once worked part-time in summer in that famous city hotel/ restaurant´s kitchen a few decades ago. I never heard of Östersjön.