Nitrogen and Phosphorus Removal-capacity of Four Chosen Aquatic Macrophytes in Tropical Freshwater Ponds

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Abstract
The nutrient removal-capacity of four chosen aquatic macrophytes was tested in both natural and laboratory conditions. Laboratory experiments were performed under controlled conditions using ‘microcosm’ methods wherein the plants were grown in three different nutrient concentrations. For field experiments, three ponds were selected that had different levels of plant nutrient concentrations and accordingly were treated as polluted, moderately polluted, and relatively unpolluted, respectively, the object being to study the nutrient removal-capacity of chosen aquatic macrophytes living in ‘natural’ conditions. For the present investigation, four common and widespread aquatic plants growing in all three ponds were chosen: Water-hyacinth (Eichhornia crassipes [Mart.] Solms), Water-lettuce (Pistia stratiotes L.), Round-leafed Water-fern (Salvinia rotundifolia Willd.), and Lesser Duckweed (Lemna minor L.). These plants were selected also because of their frequent presence in aquatic bodies in the region and their high reproductive capacity.From the results it is revealed that, during the summer and rainy seasons, the highest content of nitrogen was removed by the Eichhornia, followed by the Pistia > Lemna > Salvinia, while during winter the highest content of nitrogen was removed by the Eichhornia followed by the Lemna > Pistia > Salvinia. Higher phosphorus removal was found in summer than in the rainy or the winter season. Phosphorus removal by the macrophytes was in the order of the Eichhornia > Pistia > Lemna > Salvinia, during the summer and rainy seasons, whereas the highest content of phosphorus was removed by Lemna in the winter months.The nutrient removal-capacity was rated to be highest by the Water-hyacinth, followed by the Pistia, then the Lemna, and lowest by the Salvinia. It was also evident that the nutrient removal increased with increasing nutrient concentration in the wastewater. The removal of nitrate by the selected macrophytes ranged from 42.0% to 96.2%, while phosphate removal ranged from 36.3% to 70.2%. A positive and significant correlation was obtained between the concentration of nitrate and phosphate in the waters and plant tissues that were studied, and it is thought that a useful strategy to employ might be to grow the Eichhornia and the Lemna together at least where winter temperatures were likely to be low enough to favour the Lemna at that season, though at other times it is apt to be a nuisance.