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The global environmental changes owing to natural and anthropogenic influences are challenging the structure and functioning of the ocean ecosystem. The complex processes interacting within the physical, chemical, and biological environment at different spatio-temporal scales and their impact on the ocean ecosystem processes are yet to be investigated. A long term trend on phytoplankton biomass in terms of Chlorophyll-a concentration (Chl-a), phytoplankton compositions and the processes that control the variability is required for understanding the ocean ecosystem. This study investigated decadal trends (2002-2015) of phytoplankton composition and biogeochemical parameters over the Global Ocean (GO), Southern Ocean (SO), and the Arctic Ocean (AO) using ocean color remote sensing and assimilated data from the National Aeronautics and Space Administration (NASA) Ocean biogeochemical model. The results revealed the dominance of larger cell phytoplankton mainly diatoms throughout the SO and AO; however, the coccolithophores dominate in the remaining part of the GO. Analysis of nutrients showed that nitrate is not a limiting factor for the variability of phytoplankton biomass in the SO and AO. The low nitrate concentration influenced in the rest of the GO. The photosynthetically available radiation (PAR) limiting the phytoplankton biomass and composition in the SO and AO. Although the SO is known as the high nutrient low chlorophyll (HNLC) region of the GO, the low iron concentration along with the PAR co-limits the growth of phytoplankton biomass. Trend analysis showed that an increase in Chl-a and diatoms in the SO and AO. In contrast, it declined significantly in the other regions of the GO, in response to the consistent increase in sea surface temperature. The results indicated that, shifting of phytoplankton community from regional to global scale have a greater implication for climate change and marine ecosystem.

Seychelles–Chagosthermocline ridge is one of the major upwelling area in the tropical Indian Ocean, where planktons are known to have large impacts on ocean ecosystem and biogeochemical cycles. Time series assessments of surface zooplankton abundance, biovolume, and community composition were carried out in the Indian Ocean during June 2014. In this paper, we explore the impact of phytoplankton size class biomass on trophic interaction in the thermocline ridge of the southwestern tropical Indian Ocean. Relatively, nitrate was a dominant contributor than the nitrite in the dissolved inorganic nitrogen pool. Size fractionated chlorophyll-a concentration clearly showed that the study area was a typical oligotrophic open ocean, in which picophytoplankton biomass was dominated, accounted for approximately 72% of total Chl a. We assumed that picophytoplankton biomass was most likely influenced by dissolved inorganic nutrients (chiefly nitrite), that showed a strong linear relationship with picophytoplankton biomass ( r 2 = 0 .41). These results support the key role of nitrite is not only supplying and promoting the growth of smaller phytoplankton biomass, but also controls the structure of zooplankton communities. As a result surface waters were dominated by picophytoplankton biomass, in this condition result in zooplankton being dominated by Poecilostomatoida, constituting approximately 68% of the total zooplankton count. These organisms were widely distributed and occurred in large numbers and displayed a strong linear relationship with picophytoplankton biomass ( r 2 = 0 .87). These empirical results suggest that abundance of poecilostomatoids presumably supports tertiary trophic levels, suggesting they might play a key role as carbon drivers in the vicinity of the thermocline ridge of the Indian Ocean.

Phytoplankton composition plays a major role in biogeochemical cycles of the ocean. The intensity of carbon fixation and export is strongly dependent on the phytoplankton community. Yet, the contribution of different types of phytoplankton to the total production on various communities is still poorly understood in the Indian Ocean sector of Southern Ocean (SO). Therefore the variability of chlorophyll-a (Chl-a) and diatoms in the frontal ecosystems of the Indian sector of SO have been investigated along with the sea surface temperature (SST), sea surface wind (SSW), photosynthetically active radiation (PAR), and nutrients datasets for the period of 1998–2012. Combined analysis of in-situ, model and satellite observations indicate that the variability of Chl-a and diatoms were primarily influenced by light and wind. The Chl-a was higher at the sub-Antarctic front (SAF) followed by the sub-tropical front (STF) and the polar front (PF). The diatom concentration was higher at the SAF followed by the PF and STF. Maximum concentration of Chl-a and diatoms commonly observed at the SAF region are probably due to the moderate PAR, SST and wind. Dominance of diatoms at the PF may be attributed to their adaptability for low light conditions. The results from this study in the frontal ecosystems would help to understand the biogeochemical cycle of the Indian sector of the SO.

Southern Ocean is a crucial oceanic regime for understanding the role of biogeochemical cycles in influencing the global climatic changes, as this is a region where signatures of global changes are more pronounced. For an Improved understanding of the link between Southern Ocean processes, biogeochemical cycles and the global climate, scientific research was commenced. Some remarkable research work has been accomplished by Indian Scientific community in the Indian sector of the Southern Ocean (ISSO). A total of six successful multi-disciplinary and multi-institutional expeditions were carried out

The northern boundary of the Indian sector of Southern Ocean is marked by the juncture of the Agulhas Return Front and the southern Subtropical Front and is a hydrographically dynamic region. During austral summer, a time series study (13th to 15th January 2012) was conducted in this region (40°S, 58°30′E) to discuss the episodic biochemical changes with respect to nutrient dynamics within the upper (120 m) water column. The observed nitrate was depleted in the upper 50 m. However, there was an influx of nitrate at 6 and 18 h of Day-1. Even though the nitrate was exceptionally low, the system was moderately productive, suggesting the rapid utilization of nitrate at this location. Also, the Degree of Nutrient Consumption, was high at 50 m (~0.4), compared to the rest of the water column indicating the presence of aged/old upwelled waters at 50 m. The observed influx of nutrients could be attributed to the wind induced mixing as well as the wind stress curl driven pumping at the periphery of the anticyclonic eddy at the location. Although the influx of nutrients was low, there was the rapid utilization of nutrients, which triggered a switch in the phytoplankton community structure and subsequently influencing the biogeochemical cycle.

The Southern Ocean is a very dynamic ecosystem comprising many fronts and interfrontal areas. Among the fronts, the Subtropical Front (STF) is a region of eddy activity exhibiting high spatial-temporal variability. The variability of food web structure in such a dynamic system and their influence on the plankton community structure is of great interest. The present study attempts to explain the variability of microzooplankton (MZP) mediation in the planktonic food web, using multi-disciplinary data collected along the zonal sections 40° S and 43° S in the STF during the austral summer 2012. Even though both zonal sections are in the STF, significant differences in the variability of their thermohaline structures were observed due to the presence of multiple mesoscale features (viz. cyclonic and anticyclonic eddies), leading to discrete environments within the STF. Our study highlights the concept of the classical food web in productive environments (cyclonic eddies), which is governed by the duration of such dynamic events. On the contrary, in the generally less productive anticyclonic eddies, the variability of trophic level interaction varies between the periphery and core regions of the eddies. The present study reveals the biological consequences of frequent mesoscale features in terms of the variability of food web structure. It also elucidates the trophic importance of MZP in the two contrasting environments within the STF.

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 (0.77 mg 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 (NO3−3-), phosphate (PO43−3-) 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.

Phytoplankton, the primary producers in all aquatic systems, plays an important role in key biogeochemical processes that are linked to higher trophic levels and climate variability. The present study deals with the phytoplankton community structure in the Indian Ocean, particularly in the higher latitudes with respect to environmental variables to understand the region specific dominant community and its governing environmental settings. The study areas were selected along the latitudinal transect between 3oN and 53oS (northern Indian Ocean to Indian Ocean sector of Southern Ocean). The surface water phytoplankton community based on microscopy coupled with diagnostic pigment indices showed marked variation in community structure from tropical to polar latitudes of the Indian Ocean. The Prokaryotic diagnostic pigment (ProkDP) dominated in the Equatorial and South Equatorial regions, the Flagellate diagnostic pigment (FlagDP) in the North Equatorial region (NER), Southern Tropical Indian Ocean and Sub-tropical Front (STF) region whereas, the Diatom diagnostic pigment (DiatDP) dominated only at the Polar Front (PF) region. The influence of a suite of environmental variables - temperature, nutrients, salinity and mixed layer depth (MLD), on the dominant phytoplankton groups at the STF and PF was observed. This understanding of community dominance from this poorly explored area with respect to influencing factors is very vital baseline information to design the perturbation experiments in future work to understand the phytoplankton process studies of each region.

During austral summer 2012, seawater samples were collected from the upper 120 m of the water column, across the frontal zones in the Indian Ocean sector of the Southern Ocean. Suspended particulate organic matter from seawater was characterised, from its organic carbon and nitrogen content, and δ13C(POM). There was a marked difference in δ13C(POM) across the three frontal zones. It was depleted from the Subtropical frontal zone (−24.48 ± 1.94‰) to the Subantarctic frontal zone (−25.53 ± 0.62‰) and the Polar frontal zone (−26.69 ± 1.22‰). Within the water column, Particulate Organic Carbon (POC) and Particulate Nitrogen (PN) decreased with depth, especially below the deep chlorophyll maxima. Principal Component Analysis of δ13C(POM) and the environmental variables showed two clusters, one cluster, within the Subtropical and Subantarctic frontal zones, whereas the other cluster was in the Polar frontal zone. Statistical analysis of δ13C with particulate organic carbon, nitrogen, apparent oxygen utilization, chlorophyll a and phytoplankton community, indicated that biological community structure strongly influenced the particulate organic matter characteristics in the upper water column. The study also revealed that both autotrophic and heterotrophic processes play a significant role in the waters of the Subtropical and Subantarctic frontal zones. Furthermore, physical factors like eddies and advection influence the community structure and food web dynamics of the region. However, in the Polar frontal zone, autotrophic processes were dominant, supported by the abundance of nutrients and prevalence of larger phytoplankton community (diatoms).

The Ninth Symposium on Polar Science/Ordinary sessions: [OB] Polar biology, Wed. 5 Dec. / 2F Auditorium, National Institute of Polar Research

Upper layer diapycnal mixing in the Subtropical Front (STF) was estimated using microstructure shear profiles collected from the Indian sector of the Southern Ocean (ISSO) during the austral summer of 2012. Observations were made in the northern and southern boundary of the highly mesoscale turbulent STF, which is characterized by the presence of the dynamic Agulhas Return Current. During the observational period, the STF was populated with alternating cyclonic and anticyclonic eddies. In this mesoscale turbulent region, the average eddy diffusivity at the base of the euphotic zone was 5.5 × 10−5 m2 s−1. The average diapycnal nitrogen flux at the base of the euphotic zone, calculated using direct turbulence measurements, and nitrate (NO3) and nitrite (NO2) concentrations, was 6.4 × 10−5 μ mol m−2 s−1. The satellite-derived primary production in the STF was ~1000 mg C m−2 day−1. The observed diapycnal nutrient flux could only sustain

The juncture of the Agulhas Return Front (ARF) and Subtropical Front (STF) in the Indian Ocean sector of Southern Ocean (SO) is characterized by high mesoscale turbulence, which results in sporadic, short lived phytoplankton proliferation. The biota, mainly the phytoplankton community from such a complex hydrodynamic region and its response to the mesoscale turbulence, are areas of interest for investigation. Hence, during the sixth Indian expedition to SO, a two-day time series was occupied at the ARF and STF merged region (40°S 58°30′E) from 13 to 15 January, 2012. The vertical profiles of phytoplankton (based on pigment indices) indicated variation in the percentage contribution of phytoplankton functional groups (Micro, Nano and Pico). Though the overall community structure was dominated by nanoplankton, as exhibited by pigment indices and CHEMTAX analysis, drastic shifts in the community were observed at 120 m depth at six hourly intervals. The oscillation between Flagellates (nanoplankton) to prokaryotes (picoplankton) and then to diatoms (microplankton) at this depth in three consecutive observations coincided with the significant variations in phosphate and nitrate concentrations, along with increase in abundance of the grazer community (ciliates and heterotrophic dinoflagellates). From the present study, it is evident that the flagellate group is the ideal one to survive in such a complex regime. However, the observed small interval oscillation in the phytoplankton community could be a coupled effect of bottom-up (vertical advection that alters the nutrient flux), and top-down (increased abundance of microzooplankton) factors.

Surface water samples were collected onboard ORV Sagar Nidhi from 11 stations in the Tropical Indian Ocean (TIO) and 9 stations in the Indian Ocean sector of Southern Ocean (IOSO) (between 3°N and 53°S) during the austral summer 2012 to understand δ13C and δ15N signatures of particulate organic matter (POM) in the two different oceanic regimes. Organic carbon and nitrogen concentrations (%C and %N) and δ13C and δ15N values (δ13C(POM) and δ15N(POM)) of POM were analysed along with a suite of supporting oceanographic parameters. Overall, large-scale variations in %C, %N, δ13C(POM) and δ15N(POM) values were observed in surface waters, which may be influenced by physical and biological processes. The δ13C(POM) values ranged from −26.82‰ to −21.40‰ and were higher in the subtropical waters (40–43°S), with an increase of approximately 3‰. δ15N(POM) values varied over a wide range, from −5.09‰ to 4.09‰, with higher δ15N(POM) in the nitrate-depleted subtropical waters. δ13C(POM) had a significant relation with temperature and an inverse relation with total CO2 (tCO2) beyond 40°S. The elemental C:N ratio ranged from 4.4 to 10.8 in the tropical waters, with higher ratios in equatorial waters (1°N–3°S). The biological community structure and diverse biological processes influence the spatial variations of δ13C(POM) and δ15N(POM) values in the study area. The selective enrichment of carbon of POM in tropical surface waters may be due to the dominance of picoplankton and associated biological processes. Beyond 40°S, a combination of physical factors including SST, eddies along with the changing biological community, especially the dominance of dinoflagellates in the subtropical front and diatoms in the polar front appears to control the latitudinal variability of POM characteristics.