Penelitian Ini Bertujuan Untuk Mengetahui Kondisi Struktur Komunitas Plankton Berdasarkan Kualitas Perairan Di Sungai Kayan. Pengambilan Sampel Dibagi Menjadi 3 Stasiun Dilakukan Selama 3 Bulan Pada Saat Air Pasang Dan Surut Dengan Cara Menyaring Sampel Air Ke Dalam Plankton Net Menggunakan Ember Bervolume 10 Liter Sebanyak 5 Kali Kemudian Di Konsentrasikan Menjadi 50 Ml Ke Dalam Botol Sampel Dan Diberi Larutan Formalin 4%. Dari Hasil Penelitian Di Dapatkan Kelimpahan Fitoplankton Yang Tertinggi Ditemukan Di Stasiun III Pada Bulan Agustus Sebanyak 4.125 Sel/L Dan Kelimpahan Terendah Ditemukan Di Stasiun III Pada Bulan Juli Dan September Sebanyak 2.250 Sel/L, Indeks Keanekaragaman Fitoplankton Tertinggi Terdapat Di Stasiun I Pada Bulan Juli Sebesar 1.057 Dan Terendah Terdapat Di Stasiun II Pada Bulan September Sebesar 0,53, Indeks Keseragaman Tertinggi Terdapat Di Stasiun II Pada Bulan Juli Sebesar 0,56dan Di Stasiun II Pada Bulan Juli September 0,38 Untuk Nilai Dominansi Tertinggi Terdapat Di Stasiun I Pada Bulan September Sebesar 0,38 Dan Terendah Di Stasiun II Pada Bulan Juli Sebesar 0,31.
Kata Kunci: Kelimpahan, Keanekaragaman, Keseragaman, Dominansi, Fitoplankton, Sungai Kayan

The Research Aims To Determine The Quality Of Water Conditions In The Kayan River By Using Biological Indicators Of Plankton. The Sampling Was Divided Into Three Stations Conducted For Three Months At The Time Of High Tide And Low Tide Samples By Filtering Plankton Net Into The Water Using A Bucket Of 10 Liters Volume 5 Times Later In The Concentrate Into A 50 Ml Sample Into A Bottle And A 4% Formalin Solution. The Result Showed That The Highest Abundance Of Phytoplankton Found In The Third Station In August As Many As 4.125 Cells / L And The Lowest Abundance Was Found In The Same Station On July And September, As Many As 2.250 Cells / L. Phytoplankton Diversity Index Was Highest In The First Station On July 1.057 And The Lowest Was At Station II On September At 0,53. Uniformity Index Was Highest In The Second Station On July By 0,56 And II Station On July September 0,38 While The Value Of Dominance Was Highest In The First Station On September At 0,38 A Lowest At The Second Station On July Of 0,31. Keywords: Abundance, Diversity, Uniformity, Dominance, Phytoplankton, Kayan