Basically you are correct. Plants take up N,P,K and other trace elements in minute quantities. The removal of metals is also of high importance.

The increased surface area also provides a home for many species micro-life as a new trophic level is formed. E.g. phytoplankton and zooplankton will take up nutrients as they cycle and feed the rest of the system with proper and suitable food sources in both size and nutrient quality.