Inter-annual variability of phytoplankton and subsurface chlorophyll in the Indian sector of the Southern Ocean: evidence from austral summers 2018 and 2020

ABSTRACT The Southern Ocean (SO) plays a central role in regulating global heat and carbon exchange, yet the Indian sector remains one of its least observed regions despite spanning major oceanic fronts. Phytoplankton dynamics here are strongly influenced by light availability, nutrient supply and vertical mixing, all of which are being reshaped by rapid climate-driven changes. This study examines interannual variability in Subsurface Chlorophyll Maximum (SCM) characteristics and phytoplankton community structure during the austral summers of 2018 and 2020 across the Subtropical Front (STF), Subantarctic Front (SAF), Polar Front (PF) and the region south of the PF (SPF). Phytoplankton pigments were quantified using high-performance liquid chromatography (HPLC), and community composition was assessed via flow cytometry and microscopy. Modest interannual changes in upper-ocean structure resulted in pronounced differences in SCM depth, intensity and community composition. In 2018, shallow mixed layers (25–35 m) and deep euphotic zones (60–80 m) supported shallow, intense SCMs at the STF and SAF (Chlmax ≈ 1.4 mg m−3; Dmax 30–40 m), dominated by small flagellates and dinoflagellates. In 2020, deeper mixing (MLD 55–75 m) and reduced light penetration (Zeu 40–55 m) produced weaker, deeper SCMs at the STF (Chlmax 0.25–0.38 mg m−3), while the SAF developed a stronger, shallower SCM (Chlmax ≈ 1.6 mg m−3). Enhanced stratification at the PF and SPF in 2020 yielded stronger, shallower SCMs (Chlmax up to 0.74 mg m−3), accompanied by surface cooling and freshening indicative of melt water influence. Phytoplankton communities shifted accordingly: diatoms such as Fragilariopsis and Chaetoceros increased by 40–60% south of the PF in 2020, while picophytoplankton (Synechococcus, picoeukaryotes) peaked at the STF under warm, high-light conditions. Principal Component Analysis revealed a shift in dominant environmental controls—from density and salinity in 2018 to light and oxygen in 2020—indicating a transition from physically to radiatively regulated SCMs. These findings highlight the sensitivity of the Indian Sector of the Southern Ocean to small perturbations in stratification and light climate, with implications for regional carbon export, ecosystem functioning and future productivity under continued climate forcing.