Re: 1. what effect does freshwater have on saltwater plants such as kelp and

I am not an aquatic botanist, but I suspect that if you we're transfer a
saltwater plant such as kelp or algae, these organisms will be very 
unhappy.
The reason for this is because these plant have lived and evolved in
saltwater for millions of years adapting their structure and internal
cellular processes to a life bathed in salt.  When a saltwater plant is
moved into freshwater, the plant is not used to these conditions and may 
not be equipped to trnsport the low amounts of salts in the water which are
essential for the plant cells to live.  The other huge probelem is that the
"osmotic pressure" in the two water conditions is very different and these
plants have developed a system of dealing with one of these conditions.
Since saltwater plants are in a salt solution all the time, my guess is 
that their internal cellular salt concentrations are high compared to 
freshwater.
The freshwater will want to equilibrate with the salt in the plant's cells
and rush into the cells, causing them to swell and perhaps burst.  This 
also changes the salt concentration in the plant's cells (bad).  This sudden
change in internal cellular salt concentration and cell volume in addtion 
to the plant not being equipped with the cellular machinery to live in
freshwater is most likely what contributes to the plant's death.  Hope this
helps you.