During the rainy season, the tail end of which is when the mangroves send out the propagules, there can be shifts in salinity of a few ppts over a period of a single day. I have measured fluctuations of 8ppt and larger after storms on five or six occassions. During one of the hurricane induced seawater surges into the coastal area, we had a rise from an average 22ppt to full strength salinity of 34ppt. Current year saplings in those areas looked fine and are still around. Sure, long term, the shifts are gradual. But they do happen, and they can happen quickly and persist for several days.
Of course, I'm not out there everyday.
>Sarah