ABSTRAK Penelitian tentang kelimpahan fitoplankton di perairan Kuala Gigeng Aceh Besar telah dilakukan pada April sampai Juni 2013 bertujuan untuk mengetahui hubungan antara unsur hara nitrat dan fosfat terhadap kelimpahan fitoplankton di perairan tersebut. Analisis nitrat menggunakan metode "brucin" dan metode "stano clorida" untuk analsis fosfat. Metode sampling yang digunakan adalah purposive sampling untuk memilih lima stasiun pengamatan. Hasil penelitian menunjukkan bahwa terdapat 4 kelas fitoplankton yang terdiri dari kelas Chrysophyceae (9 genera), kelas Bacillariopyceae (7 genera), kelas Dinophyceae (4 genera) dan kelas Chlorophyceae (3 genera). Kandungan nilai nitrat berkisar antara 0,168-0,598 mg/L sedangkan nilai fosfat berkisar antara 0,014-0,026 mg/L. Kelimpahan fitoplankton berkisar antara 282,333 ind/l - 573,332 ind/l. Index determinasi yang dihasilkan dari korelasi produk momen pearson adalah sebesar 38,3% (psig0,05) yang menunjukkan bahwa terdapat hubungan yang sangat lemah antara unsur hara fosfat dengan kelimpahan fitoplankton. Kata kunci : nitrat, fosfat, kelimpahan fitoplankton. ABSTRACT The study investigate the abundance of phytoplankton in Kuala Gigeng bay Aceh Besar. The objective was of the research to explore the relation of the nitrate and phosphate nutrients to the abundance of phytoplankton in the area. The study conducted on April to June 2013. The "brucin” method was utilized to analyzed the nitrate and "the stano cloride” method to determine the phosphate. The phytoplankton sampling applied the purposive sampling method to choose five observation stations. The results showed that there were 23 genera from 4 classes of phytoplankton which consists of Chrysophyceae (9 genera), Bacillariopyceae (7 genera), Dinophyceae (4 genera) and Chlorophyceae (3 genera). The nitrate content value was about 0.168-0.598 mg/L, mean while the phosphate content value was about 0.014-0.026 mg/L. The abundance phytoplankton was about 282.333 ind/l - 573.332 ind/l. The determination index was generated from the pearson product moment correlation was about 38.3% (psig< 0.05) showed that there was the sufficient correlation between nitrate nutrient and the abundance of phytoplankton, and 2% (psig>0,05) showed that there was the very weak correlation between phosphate nutrient and the abundance of phytoplankton. Key words: nitrate, phosphate, the phytoplankton abundance