One might think so but it need not be, if what is happening is the plant roots lift nutrients from below, and eventually die and lie in the surface. My guess is that there would be growth though, since energy in the form of sunlight hits and is used by living things, plants in particular, for growth, effectively changing sunlight into mass ?
Hmm what is the definition of fresh water anyway ? If it's evaporated, condensed again, and fallen as rain then that seems fresh to me. I'm unsure how much is created anew from hydrogen and oxygen though. I'd suspect some.