It's a place of contrast, by definition (if the above and below would be compatible, there would be no bridge).
So if normal life passes above, then, what lives beneath it? We could imagine species living between the margins, probably mirroring the transience in whatever ways, in spectral colors. Dragonflies like a summer morning's mist. Sly snakes floating among rotting branches. Oil on the water, hinting of some bacteria or other. Maybe some weird guy who drinks beer.
By making it easier to pass above, a
Hidden away. Left alone. A peace earned by being cast aside. (I don't know if any of this is true, if it holds)
Some bridges are hard to get to, i like those the most. If you need to hike a bit to get there, and not many care for the trouble. So it'll be seeping with character. Some strange smelling plants maybe, littered with beer bottles of some forgotten kind, and graffiti signed a decade ago. (I'm once again imagining an early sky, dewdrops on the dark grass, pants soaked up to the knees from pushing through it)