Is the sediment natural or man made toxic?

Reminds me of the current Florida Iguana invasion. They don't tolerate cold weather, often fatal but also paralyzing them to the point they simply fall out of the trees. So after a cold snap the iguana population is much reduced. However there is no official effort when the numbers are reduced, to state wide eradicate them, sadly. The ones that survive are more tolerant of future cold snaps, thru natural selection.

Same possibility here in Venice, if they want to reduce the sediment/sludge, this would be a good time before all the gondolas go fully active I would think.