<![CDATA[Abstract. Sei Pulai reservoir is located on Bintan Island, Riau Islands Province. Water quality monitoring of Sei Pulai reservoir needs to be done considering that this reservoir is a source of raw water for the people of Tanjungpinang City and surrounding areas.The objective of this recent study were to determine the diversity and structure of phytoplankton community, also determine the water quality of litoral zone at Sei Pulai reservoir based on phytoplankton as bioindicator. Sampling points were chosen by purposive sampling method. Phtoplankton and water sample were collected in three parts namely inlet, middle, and outlet of the reservoir. Species richness of phytoplankton in littoral zone Sei Pulai reservoir consisted of 3 divisions and 18 species. Phytoplankton abundance in the inlet of reservoir showed moderate abundance. In theotehrs hand, the middle and outlet of the reservoir showed low abundance. Based on ecological index, phytoplankton community was categorized low diversity, moderate uniformity, and no dominance. The saprobic index value obtained from the analysis of the phytoplankton community more moderate organic matter or β/α-mesosaprobic. Keywords : bio-indicator, community structure, littoral zone, phytoplankton, saprobic index Abstrak. Waduk Sei Pulai terletak di Pulau Bintan, Provinsi Kepulauan Riau. Monitoring terhadap kualitas perairan waduk Sei Pulai perlu dilakukan mengingat waduk ini menjadi merupakan sumber air baku masyarakat Kota Tanjungpinang dan sekitarnya. Kajian yang dilakukan ini bertujuan untuk mengkaji jenis dan struktur komunitas fitoplankton serta kualitas perairan di zona litoral waduk Sei Pulai berdasarkan fitoplankton sebagai bioindikator. Lokasi pengambilan sampel ditentukan berdasarkan metode purposive sampling. Sampel fitoplankton dan air diambil pada tiga bagian waduk yaitu masukan air (inlet), tengah, serta keluaran air (outlet). Fitoplankton yang ditemukan di perairan zona litoral waduk Sei Pulai terdiri dari tiga divisi dan 18 jenis fitoplankton. Nilai kelimpahan fitoplankton di bagian inlet menunjukkan kelimpahan sedang. Bagian tengah dan outlet waduk menunjukkan kelimpahan rendah. Hasil penilaian indeks ekologi menunjukkan bahwa komunitas fitoplankton memiliki keanekaragaman rendah, keseragaman sedang, dan tidak ada yang mendominansi. Berdasarkan indeks saprobik diketahui bahwa di zona litoral Waduk Sei Pulai terjadi pencemaran bahan organik sedang (Kategori β/α-mesosaprobik). Kata kunci : bioindikator, fitoplankton, struktur komunitas, indeks saprobik, zona litoral]]>
