Plankton terbagi menjadi dua kelompok besar yaitu plankton tumbuhan (fitoplankton) dan plankton hewani (zooplankton). Plankton merupakan salah satu organisme di suatu perairan yang sangat penting dalam menentukan kesuburan pada suatu perairan. Penelitian ini bertujuan untuk mengetahui struktur komunitas plankton yang terdiri dari kelimpahan, indeks keanekaragaman, indeks keseragaman, indeks dominansi dengan kondisi kualitas perairan di Sungai Malinau Kecamatan Malinau Kota Kabupaten Malinau. Penelitian dilakukan pada tanggal 7 hingga 28 Desember 2019. Pengambilan sampel plankton dilakukan 3 stasiun pengamatan yaitu stasiun 1 Tikungan Sungai Malinau di Desa Malinau Hulu, stasiun 2 Percabangan Sungai Malinau di Desa Malinau Seberang, dan stasiun 3 Kawasan Penambangan Pasir dan Kerikil Malinau Kota. Identifikasi dilakukan di Laboratorium Nutrisi dan Pakan Ikan Fakultas Perikanan dan Ilmu Kelautan Universitas Borneo Tarakan. Kelimpahan Fitoplankton didapatkan berkisar 1-9 sel/L. Kelimpahan zooplankton didapatkan berkisar 2-5 Ind/L. Indeks keanekaragaman fitoplankton berkisar 0,996-1,446. Indeks keanekaragaman zooplankton berkisar 0,524-1,097. Indeks keseragamaan fitoplankton berkisar 0,849 – 0,957. Indeks keseragamaan zooplankton berkisar 0,757 – 0,973. Indeks dominansi fitoplankton berkisar 0, 278 – 0, 406. Indeks dominansi zooplankton berkisar 0,343 – 0,525. Kelimpahan plankton di Sungai Malinau termaksuk dalam kategori rendah.
Kata Kunci: Plankton, Struktur Komunitas, Sungai Malinau

Plankton is divided into two major groups, namely plant plankton (phytoplankton) and animal plankton (zooplankton). Plankton is one of the organisms which is very important in determining the fertility of a water. This study aimed at determining the structure of plankton community in terms of abundance, diversity index, uniformity index, dominance index with water quality condition in Malinau River, Malinau City of Malinau Regency. This study was conducted from 7th to 28th December 2019. The sampling of plankton was carried out in 3 observation stations, namely: Station 1 of Malinau River Bend in Malinau Hulu Village, Station 2 of Malinau River Branch in Malinau Seberang Village, and Station 3 of Sand and Gravel Mining Area of Malinau City. The identification was done at the Laboratory of Nutrition and Fish Feed, Faculty of Fisheries and Marine Science, University of Borneo Tarakan. The abundance of phytoplankton was found in the range of 1 to 9 cells/L. The abundance of zooplankton was found in the range of 2 to 5 Ind/L. The phytoplankton diversity index ranged from 0.996 to 1.446. The zooplankton diversity index ranged from 0.524 to 1.097. The phytoplankton uniformity index ranged from 0.849 to 0.957. The zooplankton uniformity index ranged from 0.757 to 0.973. The phytoplankton dominance index ranged from 0.278 to 0.406. The zooplankton dominance index ranged from 0.343 to 0.525. The abundance of plankton in Malinau River was in a low category. Keywords: Plankton, Community Structure, Malinau River