Seasonal variations in response of periphytic algal community to nutrient enrichment in the river Ganga (Varanasi, India)

Effect of nutrient enrichment on periphyton was studied using chemical diffusing substrates that released, singly or in combination, PO43--P and nitrogen, both NO3--N and NH4+-N. The study was carried out in March and June 2014 as these two time periods showed abundance of different groups of algae. Whereas diatoms and green algae were abundant in March, cyanobacteria dominated in June. Nitrogen was primarily growth limiting in river water as its enrichment enhanced the growth of periphyton. Phosphate limitation was secondary in nature. Enrichment of NH4+-N caused greater enhancement of periphytic growth than that of NO3--N. Nutrient enrichment reduced species diversity, further enhancing total biovolume of the periphytic group that was predominant at that particular point of time. Nutrient enrichment markedly increased cyanobacterial populations in June due inter alia to high temperature. In spite of nutrient enrichment, the periphytic biomass did not reach the level (chl a >= 100-150 mg m(-2)) considered as the nuisance, except in March when combined enrichment of PO43--P and NO3--N greatly enhanced the periphytic biomass.