What Scoter?

Saw a Male Harlequin duck aggressively chasing this Scoter yesterday. Time and Time again the Harlequin would wait for the Scoter to surface, then run across the water towards it as soon as the Scoter dove the Harlequin duck did too. . . not quite sure why.

So, anyone who could tell me what species this is and maybe offer an explanation as to why it was being chased would be greatly appreciated.

(There were no female Harlequins around - only 2 other males who also chased each other)

- Luke

Here is the Harlequin in pursuit of the much larger scoter

Looks like a female Surf Scoter to me. Overall brownish head with faint white cheek patch. I don't know why the Harlie is badgering it; I'll speculate that the Harlie's gonads are firing up and the territorial instinct isn't being very selective.

It looks to me like an adult female white-winged scoter (Melanitta fusca). I don't know about the behavior.

I'm no expert on sea ducks, but after some time with Sibley I agree on White-winged, although the white patches are fainter than I would expect. The eye color seems pretty strong evidence.