Deep chlorophyll maximum and primary productivity in Indian Ocean sector of the Southern Ocean: Case study in the Subtropical and Polar Front during austral summer 2011

To understand the implications of deep chlorophyll maximum (DCM) on primary productivity (PP), 3-hourly observations (over 72 h) were carried out at the Polar Front (PF) and the Subtropical Front (STF) of the Indian Ocean sector of Southern Ocean during the austral summer (February) 2011. Pronounced vertical variations in hydrographic parameters (temperature, salinity, density, dissolved oxygen) were observed between the fronts, whereas temporal (diurnal) variations of these parameters within the fronts were minimal during the study period. At the PF, a well-defined temperature minimum layer (TML), which is the winter residue of Antarctic Surface Water (AASW), was observed between 50 and 320 m. The DCM observed at the PF (~75 m) was more prominent than that at the STF and coincided with the upper boundary of the TML. The elevated chlorophyll a (Chl a) concentration in DCM is believed to result from the proliferation of the low-light adapted phytoplankton (shade flora). Due to the presence of a strong DCM the average column-integrated Chl a was nearly two times higher at the PF than at the STF yielding ~1.4 times higher column integrated PP (IPP) at the PF (211 mg C m−2 d−1) compared to the STF (152 mg C m−2 d−1). Higher Chl a and productivity at the PF was attributed to the dominance of diatoms as indicated from diagnostic pigment analysis by high performance liquid chromatography (HPLC). Owing to their higher sinking rate, the diatoms with sequestered CO2, would generate substantial export production/flux, thereby making the PF region as a sink for atmospheric CO2.