Deep sea phytoplankton community of the Sangihe-Talaud Islands waters

As a photosynthetic organism, phytoplankton faces unfavorable conditions at great depths, especially limited light availability. Nevertheless, earlier studies have found the existence of chlorophyll containing microorganisms in the aphotic zone. The region off the Sangihe-Talaud Islands is a pivotal region for fisheries resources and the Indonesian Throughflow (ITF). Until today, the deep-sea phytoplankton in the Sangihe-Talaud waters is poorly studied. The present study investigates the composition of species and abundance of phytoplankton in the deep sea (600 m) of the Sangihe-Talaud Islands waters. Plankton samples were collected using the Rosette Sampler (10 L) during the EWIN field campaign with the Indonesian RV Baruna Jaya VIII research vessel in October 2018. Environmental parameters like temperature, salinity, and fluorescence were also measured. The results showed that diatoms predominated with 54.2% of phytoplankton composition, with the most dominant species being Thalassiosira punctigera, Chaetoceros peruvianus, and Chaetoceros borealis. The highest phytoplankton abundance was observed at the sampling stations near islands. Temperature and salinity measurements exhibited nearly uniform values of 6.86±0.25oC and 34.53±0.01‰ in the study area. Furthermore, fluorescence values ranged from 0.03 to 0.04 mg/m3 , indicating there is no photosynthetic activity at 600 m. According to Principal Component Analysis (PCA), we suggest salinity is the main factor that influenced the phytoplankton community in the depth of 600 m. Temperature profile shows a weak influence on phytoplankton community in the study area.