<![CDATA[Phytoplankton serve as primary producers and the foundational link in the food web. These organisms are frequently utilized as bioindicators for the fertility of aquatic environments. The presence of phytoplankton and the environmental conditions supporting their habitat play a critical role in assessing and managing the status and quality of marine waters.The objective of this study was to analyze the phytoplankton community structure in the waters of Youtefa Bay, Jayapura, encompassing species composition, abundance, diversity index, dominance index, and evenness index. This research was conducted in October 2023 across five sampling stations. Physico-chemical parameters, including temperature, salinity, and pH, were measured in situ. Phytoplankton samples were collected horizontally using a plankton net with a mesh size of 25 µm and subsequently preserved in 4% formalin.The results indicated that the phytoplankton in Youtefa Bay consisted of two classes Bacillariophyceae and Dinophyceae comprising a total of 21 species. Ceratium trichoceros and Thalassiothrix longissima were the species identified at all sampling stations. Total phytoplankton abundance ranged from 358.10 cells/m³ to 2,426.04 cells/m³. Ceratium trichoceros exhibited the highest species abundance at 4,074.59 cells/m³. Within the class Bacillariophyceae, Thalassiothrix longissima and Skeletonema costatum showed higher abundances compared to other diatom species, recorded at 414.15 cells/m³ and 267.80 cells/m³, respectively.The diversity index values in Youtefa Bay ranged from 1.17 to 1.75, the dominance index from 0.25 to 0.46, and the evenness index from 0.47 to 0.74. Water temperature ranged from 28.7°C to 30.2°C, salinity ranged from 17 to 34 ppt (average 30.6 ppt), and pH values ranged from 7.2 to 7.8. In conclusion, the water quality of Youtefa Bay is classified as good and suitable to support phytoplankton life. ABSTRAK Fitoplankton merupakan produsen primer dan awal mata rantai dalam jaring makanan. Organisme ini sering dijadikan sebagai indikator ukuran kesuburan suatu perairan. Keberadaan fitoplankton dan kondisi lingkungan pendukung habitatnya berperan dalam penilaian status dan kualitas perairan laut serta pengelolaannya. Tujuan penelitian ini ialah untuk menganalisis struktur komunitas fitoplankton di perairan Teluk Youtefa, Jayapura, yang meliputi komposisi jenis, kelimpahan, indeks keanekaragaman, indeks dominansi, dan indeks keseragaman fitoplankton. Penelitian ini dilakukan pada bulan Oktober 2023 di 5 stasiun penelitian. Parameter fisika-kimia air diukur secara insitu meliputi suhu, salinitas, dan pH. Sampel fitoplankton diambil secara horizonal menggunakan plankton net dengan mesh size 25 µm, dan selanjutnya diawetkan menggunakan formalin 4%. Hasil penelitian menunjukan, fitoplankton di perairan Teluk Youtefa terdiri dari 2 kelas, yaitu Bacillariophyceae dan Dinophyceae, yang terbagi atas 21 spesies. Spesies fitoplankton yang ditemukan pada semua stasiun penelitian yaitu Ceratium trichoceros dan Thalassiothrix longissima. Kelimpahan fitoplankton berkisar antara 358,10 sel/m³-2426,04 sel/m³. Ceratium trichoceros memiliki kelimpahan spesies tertinggi dibandingkan lainnya, yaitu 4074,59 sel/m³. Pada kelas Bacillariophyceae, spesies Thalassiothrix longissimadan Skeletonema costatum memiliki kelimpahan yang lebih tinggi dibandingkan spesies diatom lainnya, yakni 414,15 sel/m³ dan 267,80 sel/m³. Nilai indeks keanekaragaman fitoplankton di perairan Teluk Youtefa berkisar antara 1,17-1,75, indeks dominansi berkisar antara 0,25-0,46, dan indeks keseragaman berkisar antara 0,47-0,74. Nilai suhu perairan Teluk Youtefa berkisar antara 28,7°C-30,2°C, salinitas berkisar antara 17-34 ppt dengan rata-rata 30,6 ppt, dan nilai pH berkisar antara 7,2-7,8. Kualitas perairan Teluk Youtefa tergolong baik dan sesuai untuk mendukung kehidupan fitoplankton. Kata kunci: Fitoplankton, kelimpahan, keanekaragaman, dominansi, Teluk Youtefa]]>
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