Fitoplankton merupakan produsen primer yang berperan penting bagi perairan. fitoplankton membentuk materi organik dari anorganik melalui proses fotosintesis dan dimanfaatkan oleh organisme lainnya. sedangkan tahap terakhirnya adalah dekomposer yaitu terjadi pelepasan nitrat dan fosfat. oleh karena itu penelitian ini bertujuan untuk menganalisis korelasi antara kelimpahan fitoplankton dengan kandungan nitrat dan fosfat di kawasan konservasi mangrove dan kepiting kelurahan pamusian. penelitian dilakukan pada juni – juli 2023. pengambilan sampel dilakukan pada 5 titik. metode kerja meliputi pengambilan sampel air untuk menguji kandungan nitrat, fosfat serta fitoplankton dan dilakukan juga pengambilan data pendukung sepert suhu, oksigen terlarut dan ph. sampel fitoplankton diidentifikasi kemudian dihitung kelimpahan dan keanekaragaman. hasil penelitian menunjukkan nitrat berkisar 0,07 – 4,12 mg/l dan fosfat berkisar 0,02 – 0,31 mg/l. fitoplankton yang ditemukan terdapat 9 genus dari 6 kelas yaitu bacillariophyceae 4 genus (synedra, rhizosolenia, cyclotella, dan navicula distars ), cyanophyta 1 genus (anabaena), coscinodiscophycidae 1 genus (isthmia), dinophyceae 1 genus (dinophysis acuminata), litostomatea 1 genus (loxophyllum), dan phytomastigophorca 1 genus (ceratium extensum). kelimpahan fitoplankton berkisar 9 – 13 sel/l, indeks keanekaragaman berkisar 0,362 – 1,333. berdasarkan pengujian statistik, terdapat korelasi antara nitrat dan fosfat terhadap kelimpahan fitoplankton dengan korelasi sebesar 0,856 sehingga termasuk dalam kategori korelasi tinggi. kata kunci: dekomposisi, keanekaragaman, mangrove, pamusian, plankton

Phytoplankton are vital autotrophic organisms that have a crucial function in aquatic ecosystems phytoplankton converts inorganic substances into organic matter, which other organisms consume by photosynthesis, and the last stage is releasing nitrates and phosphates hence, this study aimed to examine the relationship between the quantity of phytoplankton and the nitrate and phosphate levels in the mangrove and crab conservation area in pamustan village the study was carried out during june and july 2023 sampling was conducted at five distinct locations the study methodology involved the collection of water samples to analyze their nitrate, phosphate, and phytoplankton composition data such as temperature, dissolved oxygen levels, and ph are also gathered phytoplankton samples were recognized, and the quantity and diversity were subsequently quantified the findings signified that the nitrate levels varied between 0.07 and 4.12 mg/l, whereas the phosphate levels ranged from 0.02 to 0.31 mg/l. the phytoplankton identified consisted of 9 genera from 6 classes. these included 4 genera from the class bacillariophyceae (synedra, rhizosolenia, cyclotella, and navicula distars), 1 genus from the class cyanophyta (anabaena), 1 genus from the class coscinodiscophycidae (isthmia), 1 genus from the class dinophyceae (dinophysis acuminata), i genus from the class litostomatea (loxophyllum), and 1 genus from the class phytomastigophorca (ceratium extensum). the quantity of phytoplankton varies between 9 and 13 cells per liter, whereas the diversity index spans from 0.362 to 1.333. statistical testing reveals a significant association of 0.856 between nitrate and phosphate levels and the quantity of phytoplankton. this correlation falls into the strong correlation group. keywords: decomposition, diversity, mangrove, pamusian, plankton