ABSTRACTLongitudinal Gradient of nitrate (NO 3 - ) and phosphate (PO 4 3- ) in Air Hitam River was carried out in September 2021. This study aims to determine the Longitudinal Gradient of nitrate and phosphate in the Air Hitam River Pekanbaru City. There were 5 stations, namely Station 1 (upstream), Station 2, 3, 4 (in the middle of river), Station 5 (down stream). Sampling was done 3 times, once/3 days. Water quality parameter were measured such as nitrate and phosphate (main parameter) and temperature, transparency, pH, dissolved oxygen, BOD 5 , and free carbondioxide. Result shown nitrate concentration ranges from 0.1861-0.03181 mg/L, phosphate ranges from 0.2662-0.3208 mg/L, temperature ranges from 28-29ºC, pH ranges from 5.5-6, transparency : 18-28 cm, dissolved oxygen (DO) ranged 6,10-6,18 mg / L, free carbon dioxide (CO 2 ) ranged : 7,99 - 9,32 mg / L, BOD 5 ranged 5,5-6,0 mg /L. The longitudinal gradient of nitrate and phosphate does not show a characteristic pattern in the Air Hitam River. Based on the phosphate concentration, the trophic status of the Air Hitam River is hypereutrophicKeywords: nitrate and phosphate longitudinal gradient, water quality, Air Hitam River
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