If I had a seasonal job with a bonus program and received a bonus last season, can my employer take away the bonus option half way through my second year?

Was the bonus program in writing--that is, was there a bonus letter or agreement, an employment contract which provided for a bonus, etc.? If there was, then there would likely be found to be a contract for the bonus, which contract is enforceable. In this, the employer could not discontinue the program.

However, if there was no actual contract for the bonus, and it's simply that the employer voluntarily had provided a bonus program in the past, the employer is not obligated to continue doing so; instead, the employer may discontinue the bonus program at will.