No announcement by poloniex but steem deposits seem to have been disabled at the moment.

Consequences

As Poloniex is the main place STEEM are traded this means that no new STEEM can get on the site to be sold but other assets can still be traded for STEEM, this will lead to the price rising. When deposits are allowed through this could also lead to a temporary crash.

Causes

As said poloniex have given no reason but I made a transfer to them before they shut down deposits last night at 20:00 UTC and it seemed to correspond to steemit loading very slowly, so maybe poloniex's reasons are due to this. Even this morning steemit seems to be loading poorly, if I open a new tab of steemit I am no longer logged in. It is possible if the automated program poloniex use to access steemit is having trouble that they would temporarily disable the deposits.

Where can I trade now

Bittrex is the second biggest exchange for STEEM trading and seem to be accepting deposits still.

Update 1

Interesting, I just noticed that poloniex are still accepting SBD (Steem dollars) so that would imply that the system is still fine. Or maybe as there is not much traffic on the SBD/btc pair that it didn't get flagged by their system.

Update 2

still disable and no communication from polo, also looks like steem dollars are disabled now

Update 3

From @ashaman's comment below