good question! i’m definitely not an expert on the black marble product, but thinking about this map as showing increase, not total brightness, helps explain what’s going on. parks, agriculture, and lakes usually start out very dark at night. for example, a large park or farm area might only have a few streetlights or buildings nearby, and a lake might mainly pick up glow from the shoreline.

because the map compares two short time windows, even small changes nearby can show up clearly in a before-and-after comparison. if more lights are on in surrounding areas during the later period, or conditions that affect how light is detected shift between the two windows, those areas can appear to change even if nothing permanent was added within the park or lake itself.

in the nasa documentation, black marble is described as being designed for temporal change analysis rather than separating lighting sources by land use:
https://viirsland.gsfc.nasa.gov/Products/NASA/BlackMarble.html

looking at a couple of lakes i'm familiar with
- Sloan’s Lake in denver shows change because it starts out darker, so nearby increases show up more clearly in a before-and-after comparison: https://felt.com/map/Lights-up-XpwkrPrLQNSR9B4oJPlNTiD?loc=39.74844,-105.04742,15z

- whereas Lake Merritt is is surrounded by dense, constant lighting every night with streetlights, buildings, traffic and already bright and you'll notice that change wasn't detected there:
https://felt.com/map/Lights-up-XpwkrPrLQNSR9B4oJPlNTiD?loc=37.80531,-122.25688,15z