[Myrddin]

If all the plants die, you lose the shade and water retaining capacity of the plants. The water evaporates. Eventually, the volatile fraction of the oil evaporates too. The trees referenced grow in fairly shallow water.

[driftdiver]

Mangroves do grow in shallow water but its tidal water. Its not going to dry up when the leaves die. Also, mangroves are swamps and the ground never gets bone dry. You have the tide coming in every 10-12 hours to keep things nice and wet.

the oil will kill the trees and the fish though.