The mysterious blue of the VP reservoir

Neighbor MvB over at Seattlest asks a very good question about the Disneyland-esque shade of blue currently on display in the water of the Volunteer Park Reservoir. The question, summarized, is this -- is that cyan? We're thinking maybe the city is putting a little too much weed killer on the grass in the area. I've fished the Cedar River, source of that water -- stood right in it in fact (yuck, right?). Wasn't that color.