AACL Bioflux, 2020, Volume 13, Issue 1. http://www.bioflux.com.ro/aacl 392 Community structure of seaweeds in dry season in Minahasa Peninsula, North Sulawesi, Indonesia Rene C. Kepel, Lawrence J. L. Lumingas, John L. Tombokan, Desy M. H. Mantiri Faculty of Fisheries and Marine Science, Sam Ratulangi University, Manado, North Sulawesi, Indonesia. Corresponding author: R. C. Kepel, renecharleskepel65@gmail.com Abstract. This present study was conducted to determine the biodiversity and community structure of seaweeds in dry season found along the intertidal zone of Minahasa Peninsula, North Sulawesi Indonesia. The line transect method was used to identify and quantify the seaweeds from the three established stations. Three transects were established for each station, and each transect was divided in ten quadrates. A total of 23 different species of seaweeds were identified in the study area belonging to Rhodophyta (Rhodomelaceae, Lithophyllaceae, Galaxauraceae, Gracilariaceae, Solieriaceae, Cystocloniaceae), Phaeophyta (Dictyotaceae, Scytosiphonaceae, Sargassaceae) and Ulvophyceae (Caulerpaceae, Halimedaceae, Dichotomosiphonaceae, Anadyomenaceae, Siphonocladaceae, Valoniaceae, Dasycladaceae). The most abundant seaweed species across the three stations are: Hypnea boergesenii, Boergesenia forbesii, and Gracilaria edulis. The seaweed species identified also have different densities ranging from 2.87 to 11.77 ind. per m 2 . G. edulis had the highest density, and Avrainvillea erecta, Cladostephus spongiosum, Actinotrichia fragilis and Caulerpa racemosa had the lowest density. Species richness index, diversity index, evenness index and dominance index were calculated to determine the diversity of seaweeds in the study area. Station 2 presented the highest species richness and station 3 presented the lowest species richness. Station 2 recorded the highest diversity and station 3 recorded the lowest diversity. The evenness index had the highest value in station 2, while the lowest value was in station 3. The dominance index had the highest value in station 3, while the lowest value was recorded in station 2. The three sampling stations are divided into 2 groups based on an abundance of 23 seaweed species. The two groups are Group I (Kampung Ambong, Poopoh) and Group II (Tumbak). Apparently, the two station groups are related to the type of sediment and coverage of seagrass. Key Words: diversity, dominance, evenness, richness, species density. Introduction. Algae are thallophytes (plants lacking roots, stems, and leaves) that have chlorophyll a as their primary photosynthetic pigment and lack a sterile covering of cells around the reproductive cells (Lee 2008). The algae are unicellular or multicellular organisms, which, with the exception of cyanophytes, have cellular organelles surrounded by membranes (Pereira & Neto 2015). Marine algae can be divided broadly into macroalgae and microalgae, according to their physical sizes. Marine macroalgae are commonly known as seaweeds, which commonly include brown, red and green seaweeds, based mainly on their characteristic pigmentation (Qin 2018). Marine macroalgal groups present a large diversity (Stengel & Connan 2015). Seaweeds, or macroalgae, are aquatic photosynthetic organisms belonging to the domain Eukaryota and the kingdoms Plantae (green and red algae) and Chromista (brown algae) (Pereira 2018). Seaweeds are generally found in the intertidal region and experience diverse chronic stresses, including desiccation, intense irradiance, ultraviolet radiation, salinity and submergence or exposure arising from periodic regular tidal rhythms (Kumar & Ralph 2017). Macroalgae, or seaweeds, grow on seabeds, rocky shores, and other substrates and represent multilayered, perennial vegetation growing photosynthetically (Singh et al 2015). They are characterized by quick growth and a relatively high ability to fix carbon dioxide (Chojnacka et al 2018). The approximately 10000 described marine macroalgal species are segregated by photosynthetic pigment