Water hyacinth growing in the lakes absorbs the nutrients (phosphorous, nitrogen) being released to the lake water from untreated sewage discharged in the lake, for their growth and reduces the nutrient concentration in the lake water. After a period of time these plants sink to the bottom of lakebed and decay affecting water quality. Biological/Chemical suppression of water hyacinth bring down the plants and the plants sink to the bottom of the lake. As the plants decay, it releases the nutrients back to the lake water there by impacting water quality.