<![CDATA[Water fertility is the ability of a body of water to support and sustain the survival and growth of organisms such as phytoplankton and other marine plants. Phytoplankton are used as biological indicators to determine water quality, either by analyzing species diversity or by using certain indicator species.This research aims to assess the fertility level of the waters based on phytoplankton abundance in the waters of Tobati Village. The study was conducted from June to August 2025. Plankton samples were collected randomly at 4 stations with 3 replications. Water quality parameters were measured in situ, while nitrate and phosphate analyses were carried out at the Regional Health Laboratory of Papua. Identification and enumeration of phytoplankton were conducted at the Aquatic Resources Management Laboratory. Data analysis included total phytoplankton abundance, diversity index, evenness index, and water fertility criteria. The results showed 12 phytoplankton species including of the classes Bacillariophyceae and Dinophyceae. The highest total phytoplankton abundance in the waters of Tobati Village was found at station II with 8,855 cells/L, while the lowest was recorded at station I with 7,007 cells/L. The diversity index values at stations I and II were categorized as moderate, while at stations III and IV they were classified as low. The evenness index ranged from 0.14 to 0.42, indicating that phytoplankton species distribution across the four stations was uneven. Based on the findings, the average phytoplankton abundance across the four stations was 7,543 cells/L. The waters of Tobati Village are categorized as mesotrophic (moderately fertile). Thus, the fertility level of Tobati Village waters, South Jayapura District, Jayapura City, based on phytoplankton abundance, falls under the moderately fertile (mesotrophic) category, with no indication of pollution.]]>
