BONOROWO WETLANDS P-ISSN: 2088-110X Volume 9, Number 2, December 2019 E-ISSN: 2088-2475 Pages: 51-58 DOI: 10.13057/bonorowo/w090201 Community structure of phytoplankton in the surface and thermocline layers of Sangihe and Talaud waters, Indonesia LADY A. SRIWIJAYANTI 1 , DJUMANTO 1,♥ , RIZA Y. SETIAWAN 1 , MOCHAMAD R. FIRDAUS 2 , NURUL FITRIYA 2 , HAGY Y. SUGEHA 2 1 Departement of Fisheries, Faculty of Agriculture, Universitas Gadjah Mada. Jl. Flora, Bulaksumur, Sleman 55281, Yogyakarta, Indonesia. Tel.: +62-274-563062, ♥ email: lely412@yahoo.com 2 Research Center for Oceanography, Indonesia Institute of Sciences (P2O-LIPI). East Ancol, North Jakarta 14430, Jakarta, Indonesia Manuscript received: 9 July 2019. Revision accepted: 15 August 2019. Abstract. Sriwijayanti LA, Djumanto, Setiawan RY, Firdaus MR, Fitriya N, Sugeha HY. 2019. Community structure of phytoplankton in the surface and thermocline layers of Sangihe and Talaud waters, Indonesia. Bonorowo Wetlands 9: 51-58. The aim of this study was to determine the species dominance and distribution, and community structure of phytoplankton in the surface and thermocline layers of Sangihe and Talaud waters Indonesia. Phytoplankton samples were collected at the Sangihe-Talaud waters in October 2018 at 14 research stations. Water samples were collected at 5 m (surface) and thermoclines layers using rosette sampler equipped with Conductivity, Temperature, and Depth (CTD) recorder. Samples were concentrated to 40 ml using hand plankton net (mesh size 20 µm), then preserved with 4% formaldehyde. Phytoplankton species were identified using a guidebook based on morphological character traits. The cell count of each species of plankton was calculated using a Sedgwick rafter counting cell chamber. The result showed that there were 4 classes of phytoplankton (Bacillariophyceae, Dinophyceae, Cyanophyceae, and Raphydophyceae) which consisted of 59 species in the surface and 56 species in the thermocline, respectively. The abundance of phytoplankton at surface ranged from 77,333- 4,024,000 cell m -3 , meanwhile in the thermocline layer 8,000-542,222 cell m -3 . The average of phytoplankton diversity of the surface was 0.82 and the thermocline was 1.71. The surface layer was dominated by Leptocylindrus danicus (8.92 x 106 cell m -3 ), Trichodesmium erythareum (5.83 x 106 cell m -3 ), and Detonula converfacea (0.62 x 106 cell m -3 ). The thermocline layer was dominated by Chaetoceros affinis (2.74 x 105 cell m -3 ), Thalassionema nitzchioides (2.21 x 105 cell m -3 ), and Chaertoceros dichaeta (1.38 x 105 cell m -3 ). The low phytoplankton abundance was found at the stations 12 and 13 caused by higher salinity concentration. The highest phytoplankton abundance was found in the stations with warmer temperatures, both in the surface and in the thermocline. The shallow depth thermocline layer (75-100 m) has a higher abundance than the deeper thermocline layer (110-150 m). Temperature was the environmental parameter that has the greatest influence on the abundance and species of phytoplankton, the phytoplankton in the surface layer reached 10 times more abundant than the thermocline layer. Keywords: Phytoplankton, surface, thermocline, tropical INTRODUCTION Plankton is groups of microscopic organisms found in almost all types of waters, moving passively following the flow, their biomass in marine waters reach 98% of all micro-sized organisms (Sardet 2015). Phytoplankton is a group of plankton that can photosynthesize and contribute to almost half of the total global net primary productivity (Falkowski et al. 1998). As a primary produces, phytoplankton is a food source for all populations in the sea (Lagus et al. 2004; Sardet 2015; Rowe et al. 2017). The first consumer of phytoplankton is zooplankton, which is as food source of many marine biotas such as fish, shrimp, lobsters, crabs and various types of small fish. Many studies show that phytoplankton has a positive correlation between high commercial fish catches such as mackerel (Tangke 2012), sardinella (Putra et al. 2012), and tuna (Tangke et al. 2015; Tangke et al. 2016). In addition, the four types of high commercial fish mostly live in the thermocline layer. However, the existents of phytoplankton tend to follow the movement towards water currents. It also very affected by physical and chemical changes in the waters. Depth, temperature, and salinity are some crucial parameters that will determine the phytoplankton community structure both horizontally and vertically (Sardet 2015). The water column vertically has a different density gradient depending on the temperature and depth. Temperature will decrease to seawater depth, otherwise water pressure increase. At a certain depth, the temperature will drop dramatically and it called the thermocline layer. In addition to temperature, salinity also has a similar pattern, which will increase dramatically at a certain depth, and it is referred to as a halocline layer. The thermocline and halocline layers create unique conditions that make phytoplankton adaptable to survive. Phytoplankton communities make different adaptations so that there are variations in community structure between water columns based on their abilities and characteristics of life. Sangihe Talaud waters which are directly adjacent to the Mindanao Islands (southern Philippines), have water masses that are affected by North Pacific waters (Gordon 2005). This water mass will flow through the thermocline layer (Koch-Larrouy et al. 2007) so that it will provide