Are you receiving less traffic? The Panda and Penguin algorithm updates will affect your traffic levels but shouldn't affect your AdSense revenue directly.
In fact, I've usually found that AdSense CPC tends to increase a tiny bit after a big algorithm shift because all of the sites on the losing side now need to ramp up their AdWords spend in order to compensate for their traffic loss.
AdWords is a bid-based advertising platform. The more advertisers in competition for particular keywords, the higher the prices get driven up. The more AdWords advertisers are spending per click, the more AdSense pubishers get per click.
After Panda, for example, so many publishers lost a lot of their traffic, and attempted to make up for some of it by buying advertising campaigns through AdWords. Such an increase in AdWords budgets en masse drives up to the cost-per-click (CPC) of AdWords and, therefore, is passed to AdSense publishers (since AdSense displays AdWords ads).