Since Siena bookmarks 30 website pages from the research she does on her browser every day, at the end of March, with 31 days, she would have bookmarked 30*31 = <<30*31=930>>930 pages.
If Siena already has 400 bookmarked pages in her bookmarks library now, she will have 930+400 = <<400+930=1330>>1330 pages in her bookmarks library at the end of March.
#### 1330