Mana is added to a player's mana pool during their
Start Phase, or when a
site enters the
realm under their control. So, any sites already in
play won't get the extra mana until the start of your next turn. But, if
you play
Abundance and then play a site into Abundance's area of effect, the newly played site will generate extra mana immediately. Also, if you start your turn with a site in Abundance, and then during your
Main Phase the site is no longer in Abundance (e.g.
Cloud City moving away,
Scorched Earth, etc.), you don't
lose the mana that you gained during your Start Phase.