What are the effects/significance of flora in the freshwater biome?

Plants provide a variety of "services".  They might be food for herbivores, they can provide hiding places (for both predators and prey), they produce oxygen that other aquatic organisms can use, they provide a surface for laying eggs/spawning and attract microorganisms for small fish, or aquatic salamanders or larger insects to eat.

Additionally, plants provide shade, which may keep water temperatures cooler (and cooler water holds more dissolved oxygen), and may help to remove excess nutrients and some pollutants from the water.  The stems of the plants (unless these are floating plants) will slow the water current, and can allow particles in the water to settle to the bottom (acting like a filter).  The roots may anchor substrate so the bottom doesn't erode as easily in flowing water.

And that's just what I can think of off the top of my head.

Some sources for the other info you were asking about:  http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/F/Freshwater.html , http://www.freshwaterecosystems.com/ , http://www.iowadnr.com/education/wrlpnd.html