At one point I'm getting something like 4500 visits a week, now it's just over a thousand a week. I'm guessing this is due to Google's 'Panda' update. I suspect part of the problem is that in terms of text content BioStor is actually pretty thin. For each article there is some metadata and a few links, so it probably looks a little like a link farm. The bulk of the content is in the page images, which of course, Google can't read.
I'd be interested to know of any other sites in the field that have been affected in the same way (or, indeed, sites which have seen no change in their traffic since October).