SUMMARY
Eutrophication / Fertility of waters is an event of increasing organic matter and nutrients (especially nitrogen and phosphorus elements) that accumulate in water bodies. Eutrophication can occur due to two factors, internal factors and external factors. External factors causing eutrophication are agricultural activities and aquaculture in the KJA system. Reservoir Ir. H. Juanda is the largest reservoir in Indonesia where most of the surface of the reservoir is used as a place for aquaculture with the KJA system. Currently, the number of marine cages in the Ir. H. Juanda has reached more than 33,888 units and continues to increase. The magnitude of the growth of fish farming with the KJA system allows eutrophication in the waters, especially in the KJA area. Based on this, this study aims to determine the content of nitrate and phosphate in the marine cage area and to determine the level of eutrophication. The selected locations are 3 stations for aquaculture with the KJA system, Panyingkiran, Pasir Jangkung and Pasir Canar. Nitrate levels were obtained based on the Brucine Spectrophotometer method according to SNI 06-2480-1991 while phosphate levels were obtained based on the Ascorbic Acid method in accordance with SNI 06-6989.31-2005. From the levels obtained, calculations and comparisons were made with the literature. The results showed that nitrate levels were 4.0167 mg/L, 3.7731 mg/L and 4.4998 mg/L, respectively, while phosphate levels were 0.0206 mg/, 0.0091 mg. /L, and 0.0956 mg/L. Based on these levels, it can be seen that the three stations are classified as mesotrophic waters based on nitrate levels, while based on phosphate levels, Panyingkiran station is classified as mesotrophic, Pasir Jangkung is classified as oligotrophic and Pasir Canar is classified as eutrophic. Keywords: aquatic eutrophication; KJA system aquaculture; nitrate; phosphate