Blake Lively Posts Heartfelt Tribute to Michelle Trachtenberg

Blake Lively's remembering her former costar Michelle Trachtenberg ... saying she always gave 200% effort and she always stood up for what she believed in.

The actress took to Instagram just hours after it was announced that Michelle had passed to pay homage to the star ... sharing a photo of their first day meeting and talking about her former "Gossip Girl" costar.

Blake says she brought an electricity to every room she walked into ... always living life to the fullest -- laughing hard at jokes, taking on authority head-on when she saw injustice and added she was proud to be an actor even when the industry can be difficult.

Lively also mentions a specific detail ... saying Michelle always wore a caramel scented lip gloss -- a scene she now associates with Trachtenberg clearly.

Lively tells her fans not to take any chance to see old friends for granted ... because tragedy can strike when you least expect it -- and, you'll always treasure time with loved ones.

Blake ends her post by saying the world lost a "deeply sensitive and good person in Michelle."

Trachtenberg and Lively worked together on multiple episodes of "Gossip Girl" ... with Michelle playing Georgina Sparks -- a troublesome, hard-partying friend of Blake's Serena van der Woodsen.

While originally conceived as the series' main antagonist, Michelle's character had a shorter run on the series of just 28 episodes -- though her character ultimately found her way into the good graces of the protagonists' inner circle.

Michelle was pronounced dead early Wednesday morning after police and paramedics responded to a call for a woman in cardiac arrest, and found Michelle unresponsive, law enforcement sources told us. She was pronounced dead on the scene.

Another source told us Michelle received a liver transplant in the past year ... unclear if that played a role in her death, but we're told cops are looking at this as a natural death.

Ed Westwick -- another of Michelle's "Gossip Girl" costars -- as well as a handful of others, also posted for Michelle Wednesday.

Trachtenberg is also known for her roles in "Buffy the Vampire Slayer," "17 Again," "Inspector Gadget," "EuroTrip," "Mercy" and many more.

Michelle was 39.