Well, I conducted my thesis research on the effects of various environmental factors on Marsh vegetation, so I did a lot of reading on the subject.
1000 ppm is equal to 1 ppt. So that source is saying that the nonnative haplotype survived to 23 ppt. Not unbelievable. I've had bulrush survive at 23 ppt. BUT, surviving and thriving are two different things. Some varieties are more salt tolerant than others, but I don't think any Roseau will thrive in the salinities that oyster grass and wiregrass will. I think the fact that the lake salinities may be consistently around 18-20 ppt for most of the year is what keeps Roseau from being really established around Big Lake. That and the tidal regime.
We have some nonnative varieties of Roseau in Louisiana, but the Common Reed (Phragmites australis var. berlandieri) is the most common, and doesn't typically tolerate high salinities for extended periods. One study I know of also suggests that sulfides in the soils limit roseau, while oyster grass is not limited by sulfides. This also leads to the zonation you see in salt marshes.
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