It's actually all frozen. When the Dutch lakes freeze over in winter there's rarely any snowfall so the ice is like beautiful glass. Iceskating then makes a lot of scratches and marks which reflects the light and makes it look like "normal" ice.
What do you mean "water on both sides"? The water is only below the ice.
The band that's white is white because that's where people are skating, so it gets scratched. The virgin ice on either side remains clear.
For anyone who wants to visit: that's the one bit the description did get right: Paterswoldsemeer, Dutch city of Groningen. Currently not frozen, obviously. But if you visit during a very cold winter, you might be able to skate here too.
Born and raised in Groningen, I can tell you that the whole lake is frozen and the white part is just the damaged ice from ice-skating. Dutch people love to ice-skate on natural ice, whenever possible. That said, Groningen and this lake are a natural beauty in the Netherlands. As we say in Groningen: "There is nothing above Groningen." -> the "above"-part refers to that this is the most northern province of the Netherlands but in in Dutch this sentence also translates as "better than" :)
As a Dutch let me explain that everything you see is frozen, the white banner you see is caused by the little ice scratches on the ice (from the ice skates) that makes little pieces of ice/snow. The frozen ice left and right from the banner is still pure and transparant, like ice cubes.
I say it again.. It's a frozen lake without snow on the ice.. What so special about that?? It's just like any hockey rink or whatever?? It's a great picture but really nothing special about the ice or anything strange with the fact that you can walk on ice.. If it's just thicker than 5cm you can walk on it without any issues..