Fosfor adalah salah satu senyawa bahan organik yang penting karena akan diabsorbsi oleh fitoplankton dan masuk ke rantai makanan. Proses dekomposisi serasah akan menghasilkan bahan organik yang berperan dalam menentukan kesuburan perairan salah satunya adalah Fosfor. Tujuan penelitian ini adalah untuk menganalisis kandungan Fosfor yang terdekomposisi pada serasah daun mangrove jenis Avicennia alba di Kawasan Konservasi Mangrove dan Kepiting Kota Tarakan. Penelitian ini di lakukan pada bulan April sampai dengan Agustus 2023 dengan metode deskriptif kuantitatif. Penelitian ini dilakukan dilakukan di tiga stasiun, stasiun 1 terletak dialiran Sungai Pamusian, stasiun 2 terletak di dekat Menara Pengawas, dan stasiun 3 terletak di dekat pemukiman masyarakat. Hasil penelitian menunjukan laju dekomposisi serasah daun mangrove Avicennia alba di Kawasan Konservasi Mangrove dan Kepiting Kota Tarakan berdasarkan hari berkisar 0,71 g/hari – 0,17 g/hari, sedangkan kandungan Fosfor pada serasah daun mangrove Avicennia alba yang tersekomposisi selama 56 hari hari berkisaran
0,26 % - 0,27 %.

Kata Kunci: Avicennia alba, Fosfor, Laju Dekomposisi, Mangrove, Serasah

Phosphorus is one of the essential organic compounds absorbed by phytoplankton and enters the food chain. The litter decomposition process will produce organic materials that play a role in determining water fertility, one of which is Phosphorus. This study aimed to analyze the decomposed phosphorus content in the mangrove leaf litter Of the Avicennia alba type in the Mangrove and Crab Conservation Area of Tarakan. This research was conduetcd from April to August 2023 using a quantitative descriptive method. This research was conducted at three stations: Station l, located in the Pamusian River; Station 2, located near the Watchtower, and Station 3. which was located near the community settlement. The results of the study showed that the rate of decomposition of Avicennia alba mangrove leaf litter in the Mangrove and Crab Conservation Arca of Tarakan based on days ranged from 0.71 g/day - 0.17 g/day, while the Phosphorus content in Avicennia alba mangrove leaf litter that was composed for 56 days ranged from 0.26% - 0.27%. Keywords: Avicennia alba, Phosphorus, Decomposition Rate, Mangrove, Litter