Proses dekomposisi serasah akan menghasilkan bahan organik yang berperan dalam menentukan kesuburan perairan salah satunya adalah fosfor. Tujuan penelitian ini adalah untuk menguraikan kandungan bahan organik fosfor yang terdekomposisi pada serasah daun mangrove jenis R. mucronata di Kawasan Konservasi Mangrove dan Kepiting Kota Tarakan. Penelitian ini dilakukan pada bulan Juni sampai dengan Agustus 2022 dengan metode deskriptif kuantitatif. Penelitian ini dilakukan di tiga stasiun, stasiun 1 terletak di aliran 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 R. mucronata di Kawasan Konservasi Mangrove dan Kepting Kota Tarakan Berkisar 0,38-0,43 g/hari, sedangkan kandungan fosfor pada serasah daun R. mucronata yang terdekomposisi selama 56 hari berkisar 0,7-0,8 %.
Kata kunci: Fosfor, Mangrove, Laju Dekomposisi, R. mucronata, Serasah

The decomposition process of litter would produce organic material that plays a role in determining the fertility of waters. One of them is phosphorus. The purpose of this research was to decompose the content of decomposed organic material of mangrove leaf litter of Rhizophora mucronata species in the mangrove and crab conservation area in Tarakan City. This research was conducted from June to August 2022 using a descriptive quantitative method. It was conducted at three stations. Station I was located in Pamusian River, Station II was in the near of Supervisory Tower and Station III was in the near of community settlement. The result of the research showed that the decomposition rate of mangrove leaf litter R mucronata in the mangrove and crab conservation area in Tarakan City ranged 0.38 0.43 g/day, meanwhile the phosphorus content of mangrove leaf litter R mucronata that decomposed for 56 days ranged 0.7-0.8%. Keywords: Phosphorus, Mangrove, Decomposition Rate, R. mucronata, Litter