Phytoplankton community structure during monsoon transition period in the Lembeh strait of north Sulawesi, Indonesia

Phytoplankton community structure based on photosynthetic pigments in the Lembeh strait was studied in April 2017, during the period of monsoon transition. With data of the spatial distribution of phytoplankton community structures and environmental factors, the main factors leading to the changes in phytoplankton community structure and distribution of phytoplankton abundance were discussed. Our results show that the phosphate concentration in the north of the strait was higher than that in the south, while the distribution of nitrogen concentration was just the opposite. The silicate concentration had a significance high concentration in the bottom water of the northern strait, indicating that there was a sinking ocean current from the north. Different from previous surveys, the N/P ratio of water ranges from 2.6 to 4.56, proving that nitrogen is the nutrient limiting factor in this survey, which was due to increased phosphorus concentrations due to continuous heavy rains. The main pigments were Chl b, 19-hex-fucoxanthin and fucoxanthin during the period of monsoon transition. Chl a concentration was significantly higher in the north of the strait than in the south, and Chl a concentration at surface was higher than that at bottom. This is because the thrive of cyanobacteria in the northern strait, which were the main group in the Lembeh strait during monsoon transition. They can grow and reproduce in the nutrient-poor sea water, since they can use phosphonates as a source of phosphorus during metabolic processes. Compared with surveys conducted during SE monsoonal period, the abundance of phytoplankton in monsoon transition period was significantly lower. Because the upwelling brought sufficient nutrients to the water, which meet the require of phytoplankton growth, allowing phytoplankton to grow well in SE monsoon. On the whole, the results show that the changes in phytoplankton community structure and distribution of phytoplankton abundance during the monsoon transition period in the Lembeh strait is mainly affected by nutrients. Keywords: Photosynthetic pigments, Phytoplankton, Community structure, Monsoon transition, Lembeh strait