Uptake Kinetics of \({\text{NH}_{4}^{+} }\), \({\text{NO}_{3}^{-} }\) and \({\text{H}_{2}\text{PO}_{4}^{-} }\) by Typha orientalis, Acorus calamus L., Lythrum salicaria L., Sagittaria trifolia L. and Alisma plantago-aquatica Linn