Studies of the hydrobiology of a tropical lake in north-western Queensland. III. Growth, chemical composition and potential for harvesting of the aquatic vegetation

Abstract

The biomass of aquatic plant species and the standing crop of submerged species were estimated in Lake Moondarra from 1977 to 1979. In July 1978, the biomass of Salvinia molesta exceeded 53 000 t fresh weight, with a density of 167 t ha^-1 (0.81 kg dry wt m^-2). Hydrilla verticillata reached a maximum density of almost 3 kg dry wt m^-2 and Potamogeton crispus more than double that density. Photosynthetic activity of a range of species was high, and was still detectable at water depths allowing only very low light penetration. Organic nitrogen concentrations were high (0.91-4.37% dry wt), and phosphorus (0.07-0.47% dry wt) appeared to be a limiting factor for growth of the aquatic plants. Heavy metal concentrations varied with species, location and season but, although generally higher than world-wide 'mean' values (48 µg g^-1 copper, 11 µg g^-1 lead, 14.3 µg g^-1 zinc), were significantly less than those of aquatic macrophytes in heavily polluted environments. H. verticillata was more efficient than S. molesta in removal of nitrogen, phosphorus and zinc, whereas for copper and lead the reverse was true. The potential for using this property of the aquatic plants and subsequent harvesting in order to remove nutrients and metals from the lake is also discussed.