Maximum Outflow, by Adam Rogers

Maximum Outflow is one of the true dystopian stories in the Wired Magazine Future of Work series.  The protagonist is a maintainer in a closed post-apocalyptic environment--a terrarium, as one of the characters puts it.  He isn't thought to be very bright, but he is plenty observant and notices a very minor imbalance that threatens to kill everyone.  It's a very exciting and well-told story, possibly an award contender.