Interesting development... Turkey confirms they've "officially" closed off the Dardanelles Straits citing the Montreaux Agreement:
Turkish President Recep Tayyip Erdogan and Turkey’s Foreign Minister Mevlut Cavusoglu both confirmed the closure of the Bosphorus and the Dardanelles to warship from any country, citing provisions in the 1936 Montreaux Convention, in separate remarks this evening. Cavusoglu had indicated that a formal decision on this matter was imminent late yesterday, which came a day after Ukrainian President Volodymyr Zelensky had Tweeted out his thanks for Turkish authorities taking this step.
“When Turkey is not a belligerent in the conflict, it has the authority to restrict the passage of the warring states’ warships across the straits,” Cavusoglu said. Yesterday, he stressed that the Turkish government’s position was that what has been happening in Ukraine is a “war” rather than a “conflict,” a difference in language that matters when it comes to the Montreaux Convention.
https://www.thedrive.com/the-war-zone/44497/turkey-blocks-warships-from-sailing-into-the-black-sea-over-war-in-ukraine
However... that agreement also allows passage by countries that have ports on the Black Sea, and of course it's arguable that Russia has one in Sevastopol in Crimea. Most analytical hacks I've seen believe that Turkey won't do anything to physically block the Russian ships going in/out, citing that reason. But then why would Erdogan do it? Furthermore, it's well-known that Erodgan has cozied up to Putin quite a bit in recent years, so this would seem strange. And in fact, it probably won't change much - Russia will probably pass back/forth. So the only thing left is that Turkey finds itself precariously perched in the middle - despite being a NATO country, they have clearly allied with Russia on many fronts (including trade and purchasing military weapons0, and probably don't want to be kicked out of NATO. Possibly as well NATO's unified response has scared Erdogan a bit, so he made this "PSA". I have doubts though as to whether it'll change much... Russian ships generally have passed through it to exercise muscle in the Mediterranean to "keep NATO down." But since now that's gone, they wouldn't have much reason to cruise around.
Still, you now have the prospect of a definitive military strategic move made by a NATO member, no matter how undependable, and Erdogan's use of "war" to invoke the Convention has to be a bit of a slap in Putin's face. My guess is that unless Russia tries to shell Istanbul, not much will happen here. But still...