Tufted Duck (Aythya fuligula)
Although the Tufted Duck is widespread, almost always on fresh water lakes and ponds, throughout Dorset it is far more common in winter with extra reinforcements arriving from the frozen north.
It is a quite distinctive duck too, Its white sides being a marked contrast to the dark metallic blue/black of the back and head. Early in the year its breeding crest is clearly visible too. The only duck you could confuse this with is the Scaup which is a close relative but generally found on salt water, out at sea in bays and estuaries.
This photograph is, of course, of the male. The female is very similar but the white is replaced by a less conspicuous buff and the crest is much shorter.
The Tufted Duck is a diving duck, as opposed to a dabbling duck, so it will frequently disappear beneath the surface of the water before popping up again a feet away from where it originally dived.
A small compact duck but with a big character.