Danau Manuk Bungkul berada disekitar permukiman warga di Desa Manuk Bungkul dan dimanfaatkan sebagai sumber mata pencaharian. Danau Manuk Bungkul ini memiliki beragam biota, sehingga biota yang ada saling berinteraksi dalam proses rantai makanan. Rantai makanan yang paling utama yaitu fitoplankton dan tahap akhir dalam rantai makanan adalah dekomposer yang akan menghasilkan unsur hara berupa nitrat dan fosfat. Tujuan penelitian ini adalah untuk mengetahui korelasi antara kandungan nitrat dan fosfat dengan kelimpahan fitoplankton di Danau Manuk Bungkul, Kecamatan Sembakung, Kabupaten Nunukan. Penelitian ini dilakukan pada bulan Oktober – November 2019, dengan menggunakan metode eksploratif, deskriptif dan kuantitatif. Pengambilan sampel dilakukan pada 3 titik stasiun. Metode kerja meliputi pengambilan sampel air untuk menguji kandungan nitrat dan fosfat serta pengambilan sampel fitoplankton, dilakukan juga pengambilan data pendukung seperti suhu, oksigen terlarut dan pH. Sampel fitoplankton diidentifikasi kemudian penghitungan kelimpahan dan keanekaragaman. Hasil penelitian menunjukkan kandungan nitrat berkisar 0,004 - 0,111 mg / l dan fosfat berkisar 0,001 - 0,319 mg / l. Fitoplankton yang ditemukan terdapat 6 spesies dari 4 kelas, yaitu Bacillariophyceae 3 spesies, Chlorophyceae 1 spesies, Euglenoidae 1 spesies dan Dinophyceae 1 spesies. Kelimpahan fitoplankton berkisar 3-60 sel / l, indeks keanekaragaman berkisar 0,120 - 1,436. Berdasarkan pengujian statistik, terdapat korelasi antara nitrat dan fosfat terhadap kelimpahan fitoplankton.
Kata Kunci: Danau Manuk Bungkul, Fitoplankton, Fosfat, Nitrat

Lake Manuk Bungkul is located around the residential area of ??Manuk Bungkul Village and is used as a source of livelihood. Lake Manuk Bungkul has a variety of biota, so that the biota that exist interact with each other in the process of the food chain. The most important food chain is phytoplankton and the final stage in the food chain is a decomposer which will produce nutrients in the form of nitrates and phosphates. The purpose of this study was to determine the correlation between nitrate and phosphate content with the abundance of phytoplankton in Manuk Bungkul Lake, Sembakung District, Nunukan Regency. This research was conducted in October - November 2019, using exploratory, descriptive and quantitative methods. Sampling was carried out at 3 station points. The work method includes taking water samples to test the nitrate and phosphate content as well as taking phytoplankton samples, also taking supporting data such as temperature, dissolved oxygen and pH. Phytoplankton samples were identified and then the abundance and diversity were calculated. The results showed that the nitrate content ranged from 0.004 to 0.111 mg / l and phosphate ranged from 0.001 to 0.319 mg / l. There were 6 phytoplankton species from 4 classes, namely 3 species of Bacillariophyceae, 1 species of Chlorophyceae, 1 species of Euglenoidae and 1 species of Dinophyceae. The abundance of phytoplankton ranged from 3-60 cells / l, the diversity index ranged from 0.120 to 1.436. Based on statistical tests, there is a correlation between nitrate and phosphate on the abundance of phytoplankton. Keywords: Manuk Bungkul Lake, Phytoplankton, Phosphate, Nitrate