INFLUENCE OF POTASSIUM SOURCE (SEDIMENT VS OPEN WATER) AND SEDIMENT COMPOSITION ON THE GROWTH AND NUTRITION OF A SUBMERSED FRESH-WATER MACROPHYTE (HYDRILLA-VERTICILLATA (LF) ROYLE)

K+ uptake by H. verticillata (L.f.) Royle from sediment vs. overlying water was evaluated in relation to the K+ demands incurred by this species during an 8-wk period of growth. The investigation was conducted on a heterogeneous assemblage of sediments and in 2 solutions differing fundamentally in the presence (2.3 mg l-1) and absence of K+. Biomass production and shoot morphology in Hydrilla varied significantly between solutions and among sediments. In contrast to N and P, which were readily mobilized from most sediments, K+ was mobilized from all sediments to only a minor extent by this species. Mobilization of K+ was proportional to interstitial water K+ concentration; yet on at least 4 of the 6 sediments examined, K+ supplied from sediments was insufficient to support the maximal growth of Hydrilla. The open water rather than the sediment appears to be the primary source of K+ supply to this species and perhaps to most other submersed freshwater macrophytes. Where K+ was supplied in solution, sediment-related differences in the growth of Hydrilla negatively correlated with sediment organic matter content over the range 0.2-56.2% dry sediment mass. Apparently, the organic composition of sediments may influence the species composition of aquatic macrophyte communities.