Nutrient dynamics of two aquatic angiosperms in an alpine lake, Taiwan

Seasonal variations of nutrients in the lakewater, porewater in the lake se diment, and tissues of dominant aquatic angiosperms, Schoenoplectus mucrona tus (L.) Pall. subsp. robustus (Miq.) T. Koyama and Sparganium fallax Graeb n., in the Yuan-Yang Lake were studied. Nutrients in the lakewater were sca rce and did not vary significantly between seasons. The porewater, in gener al, had higher concentrations of nutrients than the lakewater. The nutrient concentrations of porewaters were varied among sampling sites. Decompositi on of organic matters in the lake and geographical characteristics of the w atershed are the sources of differences in nutrient contents among sampling sites of porewaters in the lake. Higher contents of dissolved organic carb on and ammonium and potassium ions in the porewater were found in sites of water entrance areas. High contents of Ca and Mg ions in the lakewater and porewater were only found in the northwestern site of the lake. Nitrogen an d potassium, which are concentrated in the aboveground tissues, are the maj or elements in tissues of dominant aquatic angiosperms. Nitrogen and potass ium were consumed luxuriously before the growing season, and the contents i n tissues gradually decreased during the growing period. However, no obviou s seasonal change was found for Na, Ca, and Mg. The yearly budget of uptake and loss of nutrients through Sparganium fallax is estimated to be 13.8 g m(-2) N, 1.1 g m(-2) P, and 19.8 g m(-2) K in 1997, and 9.9 g m(-2) N, 0.6 g m(-2) P, and 9.2 g m(-2) K in 1998.