Agustinus Wohe, 2024. The Relationship between Nutrient Availability and Phytoplankton Abundance in Secure Pond in Pegat Batumbuk Village, Berau, East Kalimantan (supervised by Dewi Embong Bulan and Nurfadilah). Ponds are places used for brackish water cultivation activities in coastal areas. Shrimp-Carbon Aquaculture (SECURE) is an effort to restore mangroves through sustainable pond cultivation. The aim of this research is to determine the relationship between nutrient availability and phytoplankton abundance in SECURE Pond. The research was carried out from October 2023 to March 2024 in Pegat Batumbuk Village, Berau, East Kalimantan. The research method used was purposive sampling by taking phytoplankton data on the bottom and surface of the pond and measuring nutrient concentrations at 4 research stations. The research results showed that the number of phytoplankton identified was 22 species from 2 classes, namely Bacillariophyceae and Cyanophyceae. The average abundance of phytoplankton on the surface and bottom ranges from 98– 296 ind/L. The nutrient concentration consisting of nitrate is 0.042–0.209 mg/L, phosphate is 0.003–0.041 mg/L, nitrite is 0.00–0.050 mg/L, and ammonia is 0.002–0.122 mg/L. A simple linear regression test showed that ammonia, nitrite, nitrate and phosphate were not correlated with phytoplankton abundance. Keywords: SECURE Pond, Nutrients, Abundance, Phytoplankton