Polar regions are particularly sensitive to even small increases in temperature. Eastern Antarctic coastal waters may therefore be especially vulnerable to warming, posing an increased risk to planktonic communities. Despite this vulnerability, zooplankton studies in the coastal waters of Eastern Antarctica (Indian Ocean sector) have received comparatively less scientific attention than those in West Antarctica, particularly with respect to understanding the impacts of ongoing climate change on zooplankton. To address this information gap, we investigated zooplankton biovolume, community composition, and vertical distribution in relation to hydrographic parameters in the coastal waters of Antarctica during the sea-ice melting season (January and February) of 2017. The sea surface temperature conditions observed in the neritic zone were highly unusual and, to our knowledge, had not been reported previously. In the seasonal ice zone, freshening of surface waters likely constrained the biomass of both phytoplankton and zooplankton. Conversely, the elevated zooplankton biovolume recorded in the neritic and ice free zones was likely associated with higher chlorophyll-a concentrations compared to those in the seasonal ice zone. Calanus simillimus and Calanus australis are key taxa in the oceanic domain, whereas Stephos longipes and Paralabidocera antarctica serve as indicator species of the continental shelf region, where their occurrence is closely associated with floating sea ice. We hypothesized that the combined influence regional factors including sea surface temperature, food availability, sea ice extent, ongoing sea ice decline, and surface freshening would exert the strongest control on zooplankton abundance and species diversity in the coastal waters of East Antarctica. As a result, such environmental changes are expected to substantially affect biogeochemical process in coastal Antarctic ecosystem.

Eastern Antarctica plays a crucial role in Southern Ocean (SO) circulation and deep-water formation. Using Argo profiles, the present study reported the anomalous upper-layer warming in eastern Antarctica during the austral summer of 2019-20, with sea surface temperature (SST) exceeding 1 degrees C in some regions. The reanalysis data indicated enhanced intrusion of relatively warm Circumpolar Deep Water (CDW) onto the continental shelf, resulting in active ocean-shelf exchange processes during this time. This intrusion was associated with pronounced westerly wind, negative wind stress curl and strong southward surface current favoring on-shelf transport of warmer subsurface waters and enhanced vertical mixing. The atmospheric forcing was further linked to elevated mean sea level pressure over the region, consistent with a negative phase of the Southern Annular Mode (SAM), which likely modulated regional wind patterns and ocean circulation. The coupled oceanatmosphere anomalies coincided with reduction in sea-ice extent and notable variations in chlorophyll concentration, highlighting the sensitivity of the coastal Antarctica region to short-term climatic variability.

Polynyas are within the sea ice cover, typically formed by wind-driven sea ice divergence or upwelling of warm subsurface waters. They play a crucial role in ocean-atmosphere interactions, climate regulation and marine ecosystems by substantially enhancing primary production. Open-ocean polynyas in the Southern Ocean are rare and are typically associated with deep convection, which disrupts conventional circulation pathways and impacts regional heat and carbon budgets. The Cosmonauts Sea (30 degrees E-60 degrees E) is an exception, with open-ocean polynyas forming annually. Using satellite-derived sea ice observations, we examined the spatiotemporal variability of polynyas in this region over the past two decades. The Cosmonauts Sea polynya exhibited large spatial and interannual variability, with the largest event occurring in 2016 (139,000 km(2)). An Argo float near the polynya recorded deep mixed layers (>400 m) and near-complete erosion of stratification, and the presence of dense water. This event coincided with anomalously intense cyclonic wind stress curl due to synoptic scale storms and a prolonged positive Southern Annular Mode (SAM) phase (2014-2016), both generally associated with reduced sea ice concentrations. While the southward shift of the Antarctic Circumpolar Current (ACC) during 2015 acted as a preconditioning mechanism, bringing warmer water towards the polynya region and inducing upwelling by vortex stretching. Additionally, anomalously high shortwave radiative fluxes (similar to+20 Wm(-2)) were observed in the summer preceding the 2016 event. The deep convective mixing observed during this event, together with the presence of dense water, indicates that the Cosmonauts Sea could be a potential dense water formation site.

The Southern Ocean (SO) is a globally important region connecting the Pacific, Atlantic and Indian Oceans, having extreme seasonal changes and is crucial with regard to the carbon dynamics. Organic carbon plays a vital role in various biogeochemical processes and the carbon biological pump in the ocean. This study discusses the variability of total organic carbon (TOC) and its partitioning into particulate and dissolved fractions across the different fronts of the Indian sector of the Southern Ocean during two austral summers. A considerable interannual variation in the TOC distribution was noted, TOC concentration ranged from 84.4 to 127.8 mu M in 2013 and from 76.3 to 121.2 mu M in 2015, with considerable interannual variation in the TOC distribution. This study implied that the dominant phytoplankton community substantially contributed to the changes in composition and partitioning of organic carbon in the epipelagic waters and microbial respiration in the mesopelagic waters. Besides, the zooplankton community and its grazing effect influenced the organic carbon dynamics within the water column. Also, a significant correlation of total organic carbon with temperature, apparent oxygen utilisation and dissolved inorganic carbon, suggests the interplay of physical mixing processes like water mass intrusion and eddies. With biochemical responses altering the TOC distribution. Importantly, even small climate changes like warming in the SO can stimulate alterations in hydrographic processes, the food web structure and carbon pathways, impacting the fate of organic carbon in the oceanic ecosystem.

This study examines the mesoscale eddy variability in the Southern Subtropical Front (SSTF) and Subantarctic Front (SAF) regions of the Indian Sector of the Southern Ocean (ISSO), using highresolution underway Conductivity-Temperature-Depth (uCTD) data collected during the 11th Indian Scientific Expedition to the Southern Ocean (February-March 2020). Two mesoscale eddies-one cyclonic (41-44 degrees S, similar to 300 km) and one anticyclonic (45-46 degrees S, similar to 100 km)-were identified and analyzed. These features appear to be recurring, with formation driven by baroclinic instability influenced by bathymetry. The anticyclonic eddy exhibited significant modification of local thermohaline structure through water mass mixing. Eddy-induced meridional heat transport was estimated to be similar to+ 0.075 PW north of 42 degrees S (northward) and similar to- 0.075 PW south of 42(degrees)30 ' S (southward). Notably, the Subtropical Surface Water (STSW) was advected from the SSTF to SAF via the cyclonic eddy's periphery, resulting in regional modification of water mass between 42 degrees S and 45 degrees S. Enhanced primary productivity was observed along the cyclonic eddy's boundary, with chlorophyll-a concentrations reaching 0.8 mgm(-3). These findings highlight the dynamic role of mesoscale eddies in modulating frontal systems, cross-frontal exchange, and biological productivity, offering critical insights into the physical-biogeochemical coupling in the ISSO under a changing climate.

The Circumpolar Deep water (CDW) is one of the largest water masses in the Southern Ocean. The cross-frontal flow of the CDW is a vital mode of redistribution and transport of heat, salts, nutrients, etc. in the ocean. The present study aims to understand the role of the CDW intrusion on nutrient dynamics and biogeochemical processes at the Polar frontal region. The study noted a patch of high nutrients layer at similar to 300-400 m in the Polar Front-II, underlying the dicothermal layer or the temperature minimum layer. This high nutrient patch was attributed to the intrusion of the CDW at the Polar front, influencing the physico-chemical processes in the water column. It was evident that, the density gradient, stratification and overall, the combined effect of physicochemical and biological factors as a consequence of intrusion of the CDW contributed to the formation of the high nitrate patch at the Polar Front-II. Even though, the CDW intrusion did not significantly impact the photic layer dynamics during this study, the possible barrier effect during austral summer contribute to the formation of the high nitrate layer. This feature may be responsible for trapping nutrients in the subsurface due to stratification, and made available to the photic region via upwelling processes, consequently triggering changes in the biological community structure and biogeochemical processes.

The optical characteristics of coastal Antarctic waters exhibit complexity due to the dynamic hydrography influenced by meltwater intrusion, which alters nutrient levels, thermohaline structure, and optically active substances (OAS) regimes. Studies on bio-optical variability and its implications on phytoplankton productivity (PP) are scanty in coastal polar regions. On this backdrop, time-series measurements (72 h at 6 h intervals) of bio-optical properties such as phytoplankton biomass (chlorophyll-a), absorption (aph), and total suspended matter (TSM) concurrently with PP were measured to understand their interplay and variability in relation to the ambient physicochemical settings in the under-sampled Prydz Bay, coastal Antarctica. Our findings revealed thermohaline stratification within the bay, likely attributed to the inflow of less saline meltwater from nearby glaciers and minimal wind activity. The consistent presence of sub-surface chlorophyll maximum (SCM) beneath the stratified layer underscored the light-acclimatization response of shade-adapted phytoplankton. Surface waters exhibited higher TSM compared to deeper layers, indicating glacial melt influence, while the depth of the sunlit layer remained relatively stable, suggesting limited water mass movement and/or variability in OAS at the study site. An inverse relation between chlorophyll-a and chlorophyll-a-specific phytoplankton light absorption (a*ph(λ)) manifested ‘pigment package effect’ within the prevailing phytoplankton community, implying reduced light-absorption efficiency and consequent lower PP. Compared to chlorophyll-a, the phytoplankton light absorption (aph(λ)) emerged as a better proxy for explaining PP variability. Nutrient availability was not limiting, which was conducive to micro (large) phytoplankton growth. Classification of phytoplankton size classes (micro, nano, and pico) based on the B/R ratio (aph at Blue (443 nm)/Red (676 nm) region) confirmed the dominance of larger (micro) phytoplankton that are more susceptible to package effect, thus have implications on reduced PP potential of this polar marine ecosystem.

Indian work in Antarctica has covered mainly atmosphere, biology and geoscience domains of sciences in central Dronning Maud land and Princess Elizabeth land (PEL) of eastern Antarctica. While observations of synoptic weather, geophysical and glaciological parameters have continued in both the sectors, thematic earth science studies focusing on crustal evolution and Gondwana fit have gained attention in the PEL where Neoproterozoic as well as Pan-African tectonic and metamorphic events that have established granulite grade metamorphism with peak conditions of ∼ 900 °C and 11 kbar followed by two stages of decompression. In the mafic granulites. The earth’s declining magnetic field and space weather studies have dominated the geophysical investigations. Ice sheet dynamics and deglaciation history have for the first time indicated that the Antarctic ice shelf too are losing ice and shrinking. The recent results of the Southern Ocean expeditions have revealed that the AABW have become fresher (∼ 0.002), warmer (0.04 °C), and sub ducted by ∼ 50–20 m toward the end of the past decade in the Indian Sector of Southern Ocean. Studies in the Arctic have mostly been conducted in the atmosphere and biological fields.

Despite its importance, zooplankton diversity, biomass and abundance are still poorly known in the remote Indian sector of the Southern Ocean (SO). Therefore, systematic observations of zooplankton and hydrography have been made in the Indian sector of the SO, with a focus on Copepoda. We collected the data along 57 degrees E transect during austral summer and examined various oceanographic locations in the central Indian Ocean sector of the SO lying in the east of the Kerguelen Plateau. The contribution of copepod abundance was substantial (85 %) to the total zooplankton biomass across the entire study region. We observed surface stratification in the Subtropical Front (STF) and Subantarctic Front (SAF); on the other hand, subsurface stratification was encountered in the Polar Front (PF) and South of PF (S-PF). As a result, nutrients were lower in warmer waters than in colder waters. On the other hand, zooplankton abundance was higher in colder waters than in warmer waters. The highest zooplankton grazing impact was recorded in the PF zone, where zooplankton removed similar to 81 % of the daily phytoplankton production, while north of the PF zooplankton grazing was generally low. Four copepod species were shown to be highly connected to oceanic fronts. The vertical distribution of zooplankton abundance and species composition was chiefly driven by physical and biological conditions in the Indian sector of the SO. As a result, our findings indicate that physical processes play an important role in organizing plankton diversity, which ultimately determines the fate of the foodweb structure and the biological carbon pump in the studied area.

Precise evaluation of in-situ bio-optical variables is crucial to better understand the biogeochemical facets of every ocean. Despite its global ecological importance, such observations are scarce in the Southern Ocean (SO) to date. Considering this caveat, the present study addresses two fundamental processes of the oceanic Carbon (C) cycle: primary productivity (PP) and phytoplankton light absorption (aph) through a 72 h time-series experiment in coastal Antarctica during the austral summer 2018. The 13C-based estimates PP measured onboard were higher than the 14C counterparts at all the sampling points. The persistent negative relationship between chlorophyll-a (Chl-a)-specific aph at 675 nm (a*ph (675)) and Chl-a at each timepoint indicated impact of pigment packaging on the absorption. Given the conventional link between the pigment packaging effect and the larger phytoplankton, we attempted a derivation of phytoplankton size fractions through two optical approaches. First of which (i.e., the spectral ratio of aph at blue to red (B/R)) band implied a clear predominance of microphytoplankton (with ratios 1.4-2.4, 77% of the total) followed by picophytoplankton (2.7-5.3), and nanophytoplankton (2.6-3.0). The second approach (i.e., a global absorption-based model) revealed the presence of the picophytoplankton community at 10, nanophytoplankton at 13, and the existence of microphytoplankton at 15 timepoints. We highlight the necessity of a thorough assessment of phytoplankton absorption and the magnitude of pigment packaging effect before applying any bio-optical algorithm to global datasets. This study, therefore, improves the current understanding of deviations in global bio-optical approaches contemplative of phytoplankton absorption.

Vertical distribution of zooplankton biovolume, abundance and community composition in the twilight zone are important to better understand their role in carbon and energy transfer that mediate the food web structure in the Prydz Bay. Qualitative and quantitative zooplankton studies were carried out in January 2017, using a multi plankton sampler (mouth area: 0.25 m(2); 200-microns mesh size) in the ice-free domain of the Prydz Bay. In this study, we investigated the diel variation of zooplankton biovolume and community structure between the surface water layer and 500 m depth at every six hours intervals for a period of 48 h at about 68 degrees S latitude, 74 degrees E longitude in the Prydz Bay. Besides, relationship between the zooplankton assemblages and diffrent water masses were also discussed. We found an abrupt increase in zooplankton biovolume within the mixed layer depth on day 1 (13:15 h). Apart from high, localized contributions from chaetognaths, appendicularians and polychaete larvae, mesozooplankton was numerically dominated by copepods, and densities were highest near the surface most of the times. Two water masses were identified based on the signatures of temperature and salinity: Antarctic Surface Water (ASW) and Shelf Water (SW). ASW had a higher proportion of Calanoides acutus, Calanus propinquus and Oithona similis, while Metridia gerlachei, and Heterorhabdus austrinus were mostly encountered in SW. Our results will help to predict the short term temporal variations of copepods assemblages and other zooplanktons according to the dynamics of hydrographic features in Prydz Bay. (c) 2022 Elsevier B.V. All rights reserved.

An important factor for predicting the effect of increased CO2 on future acidification of the ocean is a proper understanding of the interactions controlling production and dissolution of calcium carbonate minerals (CaCO3). The production and dissolution of CaCO3 in the ocean can be assessed over large spatial scales by measuring seawater calcium concentrations and total alkalinity (AT), yet past studies suggest that there could be large discrepancies between calcium and AT-based balances of the CaCO3 cycle in the North Pacific and Indian Oceans. Here, we analyse water column samples collected along transects in the North Pacific, Southern Ocean, tropical Indian Ocean and Red Sea for their concentrations of calcium, nutrients, and AT. We find that there is an excess calcium over AT anomaly in the top 1000 m of the tropical Indian Ocean water-column. The source of this anomaly is the dissolution of subsurface gypsum deposits in the Red Sea. We find no evidence for calcium-over-AT anomalies in the North Pacific, in contrast to previous studies. Our results show that, in most cases, calcium and AT data agree well and can be used to reconstruct the marine CaCO3 cycle. (C) 2021 Elsevier Ltd. All rights reserved.

Knowledge of Southern Ocean carbon cycling is limited by a paucity of phytoplankton primary productivity (PP) and spectral absorption data in this globally-important region. We measured C-13-based PP in the Indian sector of Southern Ocean (ISSO) during austral summer 2017, examining its link with spectral absorption coefficients and phytoplankton size structure derived from an absorption-based global model. Phytoplankton productivity was assessed at both coastal (60 degrees S-69 degrees S) and frontal stations (40 degrees S-60 degrees S), characterized by silicate- replete and -deplete water masses, respectively (indicated by measured nutrient ratios) to capture a range of phytoplankton growth conditions. Bio-optical relationships were used as indicators of phytoplankton community size structure and to assess the extent of cellular pigment packaging - a phenomenon reported previously for phytoplankton in this region. Blue-Red (B/R) ratios of phytoplankton absorption (a(ph)) spectra indicated that microphytoplankton (more prone to "package effects") were the dominant size class at most sites sampled. Overall, PP was better explained by a(ph) (R-2 = 0.85) than total chlorophyll-a (R-2 = 0.64) in surface waters. The a*(ph) (675)-chlorophyll-a relationship explained package effects more effectively in frontal regions (R-2 = 0.63) than stations further south (R-2 = 0.30). The global absorption-based model captured smaller (pico, nano) phytoplankton size classes but failed to identify larger microphytoplankton, underscoring the need for region-specific algorithm modifications. Our findings improve existing understanding of spatio-temporal trends in PP and bio-optical variability within the Indian Sector of the Southern Ocean (ISSO) - knowledge that is essential to improve capacity to retrieve PP from satellite-based models in this region.

Black Carbon (BC) is an absorbing aerosol which has significant impact on the Earth - Atmosphere radiation balance and hence on climate. The variation of BC mass concentration and contribution of fossil fuel and biomass burning have been investigated over the Indian ocean sector of the Southern Ocean during austral summer. BC mass was in the range of 300-500 ng m-3 between 23.3oS to 24.5oS followed by decrease in BC to 150 ng m-3 as moving to higher southern latitudes till 41oS latitude. An increase in BC mass from 250 to 450 ng m-3 was found between 41 and 50oS due to trap of air masses by cyclonic wind and transport of aerosols from the southern part of African and eastern Madagascar regions. Higher BC concentration (250-350 ng m-3) was observed in the latitude range of 57-60oS which can be attributed to convergence of north-westerly and south-easterly winds. The dominant contributor to BC was fossil fuel, which was > 80% during half of the total observations, while > 20% biomass burning contributed to one fifth of observations. The coastal Antarctic region showed higher BC mass concentration with mixed type of contributions of biomass and fossil fuel. Such accumulation of BC near the Antarctic coast can have a crucial impact on the sea-ice albedo which significantly affect the Antarctic climate system locally and global climate in general.

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 (r2=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 (r2=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.

Mesoscale eddies influence the nutrient distribution and modulate the phytoplankton growth. The present study addressed the influence of cyclonic and anticyclonic eddies on the variability of particulate organic matter composition at the Subtropical Front of the Indian Ocean sector of the Southern Ocean during austral summer 2012 and 2013. It was observed that the concentration of particulate organic carbon was lower at the aged cyclonic eddy (29.62 to 59.42 mu g/L) compared to that observed at the freshly formed cyclonic eddies (36.03 to 194.19 mu g/L). Likewise, at the matured anticyclonic eddies the particulate organic carbon was comparatively lesser (15.10 to 58.94 mu g/L) than that noted at the freshly formed eddy (29.54 to 104.44 mu g/L). The isotopic signatures of POM (delta 13C(POM) & delta 15N(POM)) were significantly different at the eddy regions. An enrichment of delta 13C(POM) was observed at the surface of cyclonic eddies with the highest delta 13C(POM) (-21.40%o) at a - 3 month old cyclonic eddy. However, an enrichment of delta 15N(POM) was observed at the depth of deep chlorophyll maxima of anticyclonic eddies with the highest delta 15N(POM) (4.39%o) at the -2 month old anticyclonic eddy. The variability in the POM characteristics and the dominant biochemical processes during this study were attributed to the difference in the eddy properties such as age, intensity and its origin. The study also indicated that eddy properties and the associated upwelling and downwelling processes altered the nutrient dynamics and supported a shift in the biological community structure that played a significant role in the variability of POM characteristics at the eddy influenced regions like the Subtropical Front.

Understanding of the coastal waters of Antarctica is crucial in determining the ocean-ice system response to climate change. In this study, using hydmgraphy data we have provided the observational evidence of anomalous warming (>1 degrees C) in the Prydz Bay during austral summer. Even though the warming was centred at 68 degrees S, 74 degrees E, the presence of warmer waters were also noticed close to the Amery Ice Shelf. The study further explored the causes and impact of this anomalous warming using satellite and re-analysis data. It was suggested that the advection of warmer waters towards the shelf region of the Prydz Bay between 72 degrees E and 75 degrees E was the main driver for the mixed layer warming. The surface heat flux played a weak role in the observed warming. The warm water advection and surface circulation during the observation period were anomalous compared to the normal pattern. The analysis further confirmed that warming caused a drastic reduction in sea-ice and chlorophyll in the Prydz Bay region. The study ascertains that regional oceanographic processes have significant implications on the sea-ice and biological productivity of the coastal waters of Antarctica.

Marine iodine speciation has emerged as a potential tracer of primary productivity, sedimentary inputs, and ocean oxygenation. The reaction of iodide with ozone at the sea surface has also been identified as the largest deposition sink for tropospheric ozone and the dominant source of iodine to the atmosphere. Accurate incorporation of these processes into atmospheric models requires improved understanding of iodide concentrations at the air-sea interface. Observations of sea surface iodide are relatively sparse and are particularly lacking in the Indian Ocean basin. Here we examine 127 new sea surface (≤10 m depth) iodide and iodate observations made during three cruises in the Indian Ocean and the Indian sector of the Southern Ocean. The observations span latitudes from ∼12°N to ∼70°S, and include three distinct hydrographic regimes: the South Indian subtropical gyre, the Southern Ocean and the northern Indian Ocean including the southern Bay of Bengal. Concentrations and spatial distribution of sea surface iodide follow the same general trends as in other ocean basins, with iodide concentrations tending to decrease with increasing latitude (and decreasing sea surface temperature). However, the gradient of this relationship was steeper in subtropical waters of the Indian Ocean than in the Atlantic or Pacific, suggesting that it might not be accurately represented by widely used parameterizations based on sea surface temperature. This difference in gradients between basins may arise from differences in phytoplankton community composition and/or iodide production rates. Iodide concentrations in the tropical northern Indian Ocean were higher and more variable than elsewhere. Two extremely high iodide concentrations (1241 and 949 nM) were encountered in the Bay of Bengal and are thought to be associated with sedimentary inputs under low oxygen conditions. Excluding these outliers, sea surface iodide concentrations ranged from 20 to 250 nM, with a median of 61 nM. Controls on sea surface iodide concentrations in the Indian Ocean were investigated using a state-of-the-art iodine cycling model. Multiple interacting factors were found to drive the iodide distribution. Dilution via vertical mixing and mixed layer depth shoaling are key controls, and both also modulate the impact of biogeochemical iodide formation and loss processes.

The community composition of zooplankton with an emphasis on copepods was assessed in the frontal zones of the Indian sector of the Southern Ocean (SO) during summer 2013. Copepods were the dominant group in both the bongo net and multiple plankton sampler across the entire region. High zooplankton abundance was recorded along each transect in the Polar Front (PF). Community structure in this front was dominated by common taxa, including Ctenocalanus citer , Clausocalanus spp., Calanoides acutus , Calanus propinquus , Calanus australis and Rhincalanus gigas , which together accounted for > 62% of the total abundance. Calocalanus spp., Neocalanus tonsus and C. propinquus were indicator species in the Sub-Tropical Front (STF), Sub-Antarctic Front and PF, respectively. A strong contrast in population structure and biovolume was observed between then PF and the STF. The community structure of smaller copepods was associated with the high-temperature region, whereas communities of larger copepods were associated with the low-temperature region. Thus, it seems probable that physical and biological characteristics of the SO frontal regions are controlling the abundance and distribution of zooplankton community structure by restricting some species to the warmer stratified zones and some species to the well-mixed zone.