Phytoplankton biomass and community composition in the frontal zones of Southern Ocean

Abstract

Phytoplankton biomass (chlorophyll-a) and community structure in the water column was analyzed across various frontal regions in the Subtropical Front (STF), Sub-Antarctic Front (SAF), Polar Front-1 (PF1) and Polar Front-2 (PF2) in the Indian Ocean sector of the Southern Ocean (SO) during the austral summer (January–February) of 2013 and 2015. The surface chlorophyll-a (Chl-a) was maximum in PF1 (0.77mg m−3) followed by the SAF (0.5 mg m−3), STF (0.26 mg m−3) and PF2 (0.21 mg m−3) in 2013. The Chl-a was maximum in the SAF (0.56 mg m−3) followed by PF1 (0.32 mg m−3), PF2 (0.28 mg m−3) and STF (0.20 mg m−3) during 2015, indicating that the average surface biomass was higher in 2013. The deep chlorophyll maximum (DCM) was located at 50 m in STF and SAF for 2013 and 2015 and was found at 75 m (100 m) in PF1 (PF2) during 2013. The diagnostic pigment (DP) index indicated that diatoms in the surface and water column increased from the STF to the PF. In contrast, smaller cell community flagellates and prokaryotes decreased from the STF to PF. Diatoms consistently increased from the surface to 120 m in SAF and PF, whereas flagellates were uniformly distributed throughout the water column. Prokaryotes were consistently distributed down to the DCM level and then progressively declined towards deeper regions. The nitrate (NO3−), phosphate (PO43−) and silicate (SiO4) increased considerably from the STF to PF through SAF. Results indicate the biomass and community variation in the frontal regions were due to the influence of physical, chemical, and biological processes under varied environmental conditions.