RESPONSES OF VALLISNERIA NATANS TO REDUCED LIGHT AVAILABILITY AND NUTRIENT ENRICHMENT

The effects of light and nutrient availability, 3 (light) @ 3 (nutrient), on growth and N, P and C concentrations ofVallisneria natans were investigated in a microcosm experiment.Growth of V.natans decreased with increased light limitation.Inhibition of increased nutrient loading on the growth of V.natans was observed only in high light treatments, while in mediumlight treatments there were no significant effects of nutrient enrichment on growth, and the growth of V.natans increased withnutrient enrichment in low light treatments.Low light intensity increased the concentrations of tissue total nitrogen, total phos-phorus, ammonia-N and free amino acid-N but lowered tissue soluble sugar and starch concentration.The concentrations of totalphosphorus, ammonia-N, free amino acid-N and starch increased with nutrient enrichment.The results indicated that both lowlight availability and high nutrient availability influenced the growth of V.natans significantly, and that there were interactionsbetween them, showing effects of nutrient enrichment depend on the light availability.Growth inhibition in nutrient enrichmentswas mainly caused by decreased biomass, but not ammonia toxicity or carbohydrates deficiency.Since the growth of V.natanswas severely inhibited by light insufficient in medium and low light treatments, there was no more suppression of nutrient enrich-ment on growth.