Nitrate and phosphate from rivers as mitigation of eutrophication in Benoa bay, Bali-Indonesia

Abstract

Benoa Bay is estuary of six rivers in Bali. In addition to carrying water, river flows also carry nutrients (nitrate and phosphate) derived from activities along the watershed. High nutrient concentrations have potential for eutrophication that leads to the Harm Alga Booms. The aims of this study were to determine the concentration, load, and tropic level of nutrients (nitrate and phosphate) in river which ends at Benoa Bay. Measurement of nutrients in water used brucine method for nitrates and ascorbic acid method for phosphate. The results were then compared with ministerial decree of the environment No 51 in 2004 for marine biota. The result show concentration of nitrate and phosphate is fluctuated along years. Nitrate concentration fluctuated form undetected to 16.950 mg/l, meanwhile phosphate concentration from undetected to 3.275 mg/l. The highest concentration of nitrate and phosphate found in dry season and the lowest concentration found in wet season. Nitrate and phosphate load to benoa bay also fluctuated in each season and river. The highest nitrate load found in Mati River during second transition season (4.387 tons/day) and also in Buaji River during dry season (4.366 tons/day). The highest phosphate load to Benoa Bay also comes from Mati River in dry season (6.980 tons/day). The Eutrophic condition for nitrate generally found in Second Transition Season, meanwhile for phosphate generally found in all river and season. The concentration of nitrate and phosphate found in all rivers and season which ends at Benoa Bay generally exceeded the quality standard base on Ministerial decree of environment No 51 in 2004 for marine biota. These conditions are potential for trigger eutrophication process in Benoa Bay.