Model of Phytoplankton Diversity in Belawan River, North Sumatera, Indonesia.

<b>Background and Objective:</b> Belawan River is located in Deli Serdang and Medan Districts, North Sumatra, Indonesia. Belawan River passes through residential areas, industry, steam power plants (PLTU), PDAM and encroachment. Waste is directly discharged into water bodies due to many human activities, so the quality of water and aquatic biota is disturbed. Phytoplankton diversity is influenced by water quality conditions because its existence depends heavily on the water conditions of the Belawan River. Analyzing the model of phytoplankton diversity in the Belawan River was the purpose of this study. <b>Materials and Methods:</b> The analysis was conducted in the form of phytoplankton diversity by obtaining phytoplankton species and their distribution and analyze the environment such as the water quality of the Belawan River. Sampling was taken by purposive random method with 5 different locations with 3 times the test with, namely starting from the upstream, middle and downstream Belawan River area in 2010, 2015 and 2020. Stages of research methods, obtained types of plankton, abundance (A), relative abundance (RA), presence frequency (PF), equitability (E) and diversity (H) and analysis of water quality (temperature, light penetration, light intensity, depth, current speed, salinity, pH, DO, oxygen saturation percent, BOD and COD). <b>Results:</b> Twenty three genera of phytoplankton were found in 2020. Phytoplankton abundance was highest in the <i>Chaetoceros</i> sp., genera with an abundance of 186 ind m² at station 5. Diversity (H) was highest at station 5 at 2.30 and lowest at station 5 at 1.87. Phytoplankton in five stations was relatively low. The DO has a very strong effect on phytoplankton's diversity. The model of phytoplankton diversity relationship with water quality is y = -2.09235475-0.283821248x₁-0.000034042331x₂²-0.000000317192297x₃²+ 0.0000138747473x₄²-0.0642412267x₅²+0.0436398590x₆-0.0107999363x₇²+0.0000469016376x₈². <b>Conclusion:</b> Dissolved oxygen (DO) has an effect on the diversity of phytoplankton in the Belawan River where the highest phytoplankton is <i>Chaetoceros</i> sp., of 23 genera which was found with a diversity value of 2.30 at station V.