This is my Google webmaster tools report from last 15 days. How is this possible? Almost no change in SERP but -30% change in all impressions? Any ideas?
1. Google Webmaster tools stats are not being updated every day, so you may see now some old data. 2. You may have been hit by the penguin algorithm. From what i can see in the table the impressions have decreased, clicks decreased and CTR increased, this is quite odd.
You may want to have an alternate page count (like your own page counter) to verify if the decrease is real.