Plankton terbagi menjadi dua kelompok besar yaitu plankton tumbuhan (fitoplankton) dan plankton hewani (zooplankton). Plankton merupakan salah organisme di suatu perairan yang sangat penting dalam menentukan kesuburan pada suatu perairan. Penelitian ini bertujuan untuk mengatahui struktur komunitas plankton yang terdiri dari kelimpahan, indeks keanekaragaman, indeks keseragaman, indeks dominansi serta keterkaitannya dengan kondisi kualitas perairan di Sungai Ngingitan Kelurahan Mamburungan Kota Tarakan. Penelitian dilakukan pada tanggal 24 Februari hingga 7 April 2021. Pengambilan sampel plankton dilakukan 3 stasiun pengamatan yaitu stasiun 1 (hulu sungai), stasiun 2 (hilir sungai), dan tasiun 3 muara. Identifikasi dilakukan di Laboratorium Nutrisi dan Pangan Fakultas Perikanan dan Ilmu Kelautan Universitas Borneo Tarakan. Kelimpahan Fitoplankton didapatkan berkisar 4-37 sel/L. Kelimpahan zooplankton didapatkan berkisar 1-34 Ind/L. Nilai indeks keanekaragaman fitoplankton berkisar 0,608-1,226. Nilai indeks keanekaragaman zooplankton berkisar 0,637-1,318. Nilai Indeks keseragamaan fitoplankton berkisar 0,588-0,686. Nilai Indeks keseragamaan zooplankton berkisar 0,812-0,918. Nilai indeks dominansi fitoplankton berkisar 0,455-0,868 nilai indeks dominansi zooplankton berkisar. 0,321-0,556. Kelimpahan plankton di Sungai Ngingitan termaksuk dalam katagori rendah.
Kata Kunci: Kelimpahan, Keanekaragaman, Keseragaman, indeks Dominansi, Kelurahan Mamburungan

Plankton is divided into two major groups, namely plant plankton (phytoplankton) and animal plankton (zooSplankton). Plankton is one of the organisms in a waters that is very important in determining the fertility of a waters. This study aims to determine the structure of the plankton community consisting of abundance, diversity index, uniformity index, dominance index and their relationship to water quality conditions in the Ngingitan River, Mambirdan Village, Tarakan City. The study was conducted from February 24 to April 7, 2021. Sampling of plankton was carried out by 3 observation stations, namely Station 1 (upstream), Station 2 (downstream), and Station 3 estuary. Identification was carried out at the Food and Nutrition Laboratory, Faculty of Fisheries and Marine Sciences, University of Borneo Tarakan. Phytoplankton abundance was found in the range of 4-37 cells/L. The abundance of ooplankton was found in the range of 4-34 Ind/L. phytoplankton diversity index values ranged from 0,608 to 1,226 The value of the zooplankton diversity index ranged from 0,637 to1,318. Phytoplankton uniformity index values ranged from0,588-0,686. zooplankton uniformity index values ranged from0,812 to 0,918. phytoplankton dominance index values ranged from 0,321 to 0,556. Zooplankton dominance index values ranged. 0,321 to 0,556. The abundance of plankton in the Ngingitan River is in the low category. Keywords: Abundance, Diversity, Uniformity, Dominance index, Mambirdan Village