Iceland Time Lapses & Quiz

This montage contains some of the most beautiful outdoor time lapses I've ever seen. Photographer Joe Capra shot these over the summer in Iceland, using a Stage Zero dolly system for motion control (any of you is free to give me one of these as a gift).

http://vimeo.com/30581015

I do have one science question to put to you after viewing these. Because it's summer above the Arctic Circle, the sunlight is relatively constantover the course of each time lapse. However, in the shots that include the ocean, you can see hardly any tidal change. Why is this?