Dynamic of water fertility in Koto Panjang Reservoir, Riau Province, Indonesia

Koto Panjang Reservoir, located in Riau Province, has multiple uses including hydroelectric power, irrigation, clean water sources, tourism, and fisheries. The high activity inside and outside the reservoir has increased the concentration of nitrate (N) and phosphate (P) so that it has an impact on water fertility. The high concentration of N and P causes a decrease in water quality which results in the low carrying capacity of the waters for floating net cage cultivation activities. This study aims to determine the dynamics of nitrate and phosphate as an indicator of water fertility in the Koto Panjang Reservoir. This research was conducted from 2016 to 2020 and sampling was conducted in March, June, September, and December. Sampling was carried out at 10 points and the determination of sampling points was based on purposive sampling. Water samples were taken from the surface of the water (1 m) using a water sampler and put into a 250 ml plastic bottle. Sampling, preservation, transportation, and sample analysis were carried out based on the standard methods of Standard Methods for The Examination of Water and Wastewater (APHA 2005). The data from the analysis results were tabulated and analyzed descriptively. The results of water quality data analysis show that the biological oxygen demand (BOD5) and chemical oxygen demand (COD) values exceed the permitted quality standards, while temperature, conductivity, pH, turbidity, and dissolved oxygen (DO) are still below the quality standard. The nitrate concentration value ranged from 0.37 - 0.56 mgL-1 while the phosphate concentration at each station ranged from 0.14 - 0.56 mgL-1. The value of nitrate and phosphate during the study fluctuated but the values for each sampling location were not much different. The concentration of N and P values in the Koto Panjang reservoir decreased with increasing numbers of cages. This shows that the high value of nitrate and phosphate in the Koto Panjang Reservoir does not only come from floating net cage activities but also from other activities such as plantations, agriculture, settlements, and tourism.