Located at the southern tip of Hokkaido, Japan, Hakodate Bay has experienced recurrent blooms of the toxic dinoflagellate Karenia mikimotoi since 2015. Therefore, understanding the primary drivers and improving monitoring approaches are essential for predicting the population dynamics and mitigating impacts on regional fisheries. In this study, we investigated the fifth recorded K. mikimotoi occurrence from October to December 2024, during which the maximum microscopic cell density reached 80.26 cells mL(-)& sup1; in mid-November. Abiotic factors, phosphate availability and solar radiation, emerged as the dominant drivers, with growth thresholds of 0.715-0.941 mu M and > 10.05 MJ m(-)& sup2;, respectively. Meanwhile, biotic interactions with diatoms and ciliates were not significant. To evaluate monitoring techniques, K. mikimotoi cell densities were enumerated using traditional microscopy and FlowCam. Although FlowCam reported consistently higher cell counts in 16 out of 24 samples, the difference was not statistically significant. We also applied the FlowCam-derived area-based diameter (ABD) to characterize K. mikimotoi cell size in natural waters. Cell size showed a significant temporal variation: ABD increased from October to late November, declined through December, and reached a maximum of 29.45 mu m by late December. A weak correlation was observed between ABD and cell density. Instead, larger cells tended to occur under low-phosphate conditions (size threshold: <0.599 mu M). This indicates that cell size changes can be a valuable indicator of nutrient stress. Our results suggest that FlowCam is an effective tool for monitoring the proliferation and nutrient status of K. mikimotoi populations.

Diatoms are considered an important food source for the commercially valuable sea cucumber Apostichopus japonicus. However, food sources for juvenile A. japonicus in the wild remain understudied, despite their importance for effective stock enhancement. In this study, seasonal diatom assemblages and viability in the feces of juvenile A. japonicus and in the feeding environments (biofilm, water column) were investigated using direct microscopy to evaluate diatom availability and utilization by the juveniles. Additionally, a laboratory feeding experiment was conducted to examine the digestibility of the predominant diatom (Tabularia) in the feces during the field survey. Field surveys have shown that diatoms are a major food item for juveniles, although their contribution varies seasonally with shifts in dominant food sources. Notably, Tabularia spp. occurred at a higher relative proportion in fecal assemblages than in the feeding environments and exhibited high mortality in the feces (96.3 ± 2.4%). Consistently, laboratory experiments showed marked degradation of intracellular contents in Tabularia sp. after excretion, supporting its digestibility. These findings have broadened our understanding of optimal food sources for A. japonicus juveniles as well as their feeding ecology in natural environments, informing the selection of more suitable diets and potentially enhancing stock enhancement practices.

While freshwater input into the western Arctic Ocean is increasing, studies evaluating the impacts of different freshwater sources—namely river water and sea-ice meltwater—on microplankton remain limited. In this study, we examined microplankton and hydrographic conditions during the late summers of 2021, 2023, and 2024 to evaluate the influences of different freshwater sources. Microplankton were identified and counted using an inverted microscope. In addition to water temperature, salinity, and nutrient concentrations, the fractions of river runoff (frro) and sea ice meltwater (fsimw) were estimated. High microplankton abundance (56,000 cells L−1) was observed in the group strongly influenced by river water (frro: 3%) but less affected by sea ice meltwater (fsimw: 7%), whereas the group strongly influenced by both freshwater sources (frro: 4%, fsimw: 15%) exhibited low abundance (530 cells L−1). In the former group, a lower overall freshwater proportion may have inhibited the full development of salinity stratification, allowing enhanced nutrient supply from deeper layers through upwelling. Therefore, nutrient-demanding taxa such as Chaetoceros dominated in the group. In contrast, strong salinity stratification in the latter group may have limited nutrient supply to surface waters. Consequently, heterotrophic taxa such as ciliophora tended to dominate. To our knowledge, this study provides one of the first evaluations of how different freshwater sources influence microplankton in the western Arctic Ocean. We compared the effects of freshwater accumulation on microplankton across different regions. Our results indicate that phytoplankton proliferation can be either promoted or suppressed depending on differences in freshwater sources and their complex interactions with environmental factors. This study provides a detailed assessment of how distinct freshwater sources, such as river water and sea ice meltwater, influence microplankton dynamics in the western Arctic Ocean. Furthermore, in 2021, sea ice melt was unusually delayed, and extensive sea ice remained even in the late summer. We also examined whether this unusually extensive sea ice influenced the microplankton community structure. Together, these results highlight the importance of considering both the magnitude and source of freshwater input, as well as sea-ice conditions, when evaluating microplankton dynamics in the western Arctic Ocean.

Autumn phytoplankton blooms are increasingly observed in the Arctic Ocean due to more frequent strong wind events over open waters. A recent study on the Chukchi shelf suggested that diatoms in sediments can resuspend to the surface during such events, potentially acting as a seed population for blooms. To investigate how resuspended diatoms could influence the dynamics of autumn bloom, we conducted an in situ incubation experiment under two conditions: (1) only nutrients are supplied to and (2) nutrients and microalgae-containing bottom-sediments are inoculated to phytoplankton community. Both experimental conditions resulted in increased microalgal biomass, particularly diatoms. Sediment input enhanced initial biomass, accelerating the transition to the bloom state, whereas the photophysiological parameters, including the maximum specific growth rate and the maximum quantum yield of photosystem II, were similar in both treatments. When only nutrients were supplied, Arcocellulus spp. and common autumn diatoms in the Chukchi Sea, including Rhizosolenia spp. and Leptocylindrus spp., prevailed. Meanwhile, diatoms typically present on the seafloor, such as the Chaetoceros socialis complex, Chaetoceros spp., and Thalassiosira spp., dominated the assemblage when sediments were inoculated. These results indicate that sediment resuspension to the surface potentially influences autumn bloom characteristics, affecting bloom development rate, size structure, and diatom community diversity and composition.

The Arctic marine ecosystem is undergoing a major, rapid transformation driven by climate change, resulting in complex and unpredictable shifts in zooplankton communities, which are key pelagic food web components. We synthesized extensive multi-year zooplankton datasets (2008–2021) collected by a Korean icebreaker research vessel (IBRV Araon; August 2016–2021) and a Japanese research vessel (RV Mirai; September 2008, 2010, 2012–2017, and 2021) in the western Arctic Ocean to examine the effects of environmental factors on zooplankton distribution. We determined the effect of key environmental variables, including integrated mean temperature, mean salinity, and fluorescence, on zooplankton community structure. We identified six distinct zooplankton communities shaped by regional characteristics and interannual oceanographic variability. The pronounced seasonal transition of zooplankton communities from summer to autumn, particularly in the Chukchi Sea and the Chukchi Borderland in 2017 and 2021, was the major finding. During summer, Pacific water inflow into the Chukchi Sea significantly increased Pacific species (e.g., Metridia pacifica) and meroplankton (e.g., barnacle larvae), with barnacle larvae extending into the Chukchi Borderland in 2017 and 2021. Although small species (e.g., Pseudocalanus spp.) remained dominant during the summer, but no clear increasing trend was observed in total abundance within the Chukchi Sea. By September, these Pacific Ocean–influenced communities had decreased rapidly, suggesting their high environmental dependency and incomplete establishment in the region. This study integrates multi-year, seasonally diverse datasets collected across a broad spatial range, providing a comprehensive understanding of how Arctic zooplankton respond to climate-induced environmental changes.

Several large-scale studies have examined the spatial and temporal (seasonal and interannual) variability in macrozooplankton communities in the eastern Indian sector of the Southern Ocean. In this study, variability in these communities was analyzed using samples collected by the RMT8 during the KY1804 survey, conducted between 80° and 150° E during the austral summer of 2018–2019. Furthermore, these findings were compared with those of the BROKE survey conducted in 1996. Using cluster analysis, the macrozooplankton community was divided into six groups. In both surveys, the zooplankton communities varied between the southern and northern stations of the sampling areas, though their distribution patterns differed between the two years. During the KY1804 survey, Thysanoessa macrura was more prevalent in the western region, while Themisto gaudichaudii and chaetognaths were more abundant in the eastern region; Salpa thompsoni dominated in the eastern region during the BROKE survey. Water temperature had the strongest influence on the macrozooplankton community during the KY1804 survey, whereas salinity was the primary influencing factor during the BROKE survey. This difference may largely reflect differences in sampling timing and latitudinal coverage, though a southward shift in the southern boundary of the Antarctic Circumpolar Current between the surveys may also have contributed. Hydrographic changes over the two decades between 1996 and 2019 likely affected the macrozooplankton community in this region, though differences in spatial and temporal survey coverage complicate interpretation of the results.

IntroductionSea ice extent increased in the Pacific Arctic Ocean during 2021 owing to the reversal of the Beaufort Gyre, unlike in previous years. The increased sea ice concentration may restore the marine ecosystem to its previous state; nevertheless, the precise conditions and mechanisms involved remain unclear.MethodsIn this study, the 2008–2017 period was defined as “the sea ice retreat year,” and its zooplankton community distribution representative was estimated using generalized dissimilarity modeling (GDM). Subsequently, we assessed the effect of delayed sea ice melt on the zooplankton community by comparing the zooplankton community of the sea ice retreat year with that in 2021.ResultsIn GDM, numerous satellite parameters significantly affected the zooplankton distribution, with the highest effect during the open-water period and annual primary production (APP) and the lowest in water temperature. The effect of APP and temperature on zooplankton similarity was high around the Bering Strait owing to the advection of Pacific copepods (Eucalanus bungii, Metridia pacifica, and Neocalanus spp.) and synchronized inflow of warm Pacific water. Under significant warming scenarios (Shared Socioeconomic Pathway [SSP]1-2.6 and SSP5-8.5), GDM-based multiple effects predicted that the zooplankton communities in high latitudes will be more affected than those on the southern shelf (northern Bering Sea to southern Chukchi Sea). In 2021, the total abundance across the northern Bering Sea to the Chukchi Sea shelf region was lower than that of the community during the sea ice retreat year. However, certain species (Limacina helicina and Pacific copepods) increased locally (northern Bering Sea and Barrow Canyon) because of the increasing volume of Pacific origin water.DiscussionContrary to the reported increase trend on zooplankton, low primary productivity and phenological mismatch for zooplankton may prevail in the Pacific Arctic Ocean, resulting in a low abundance during autumn 2021.

. We examined the distributional relationship between gray whales and their prey, benthic amphipods and euphausiids, using data collected from shipboard surveys in the Pacific Arctic. We conducted four survey cruises (in 2007, 2008, 2012, and 2013) in the northern Bering Sea, Chukchi Sea, and adjacent waters, and collected data on cetacean sightings, including gray whales, benthic amphipods by benthic grabs, and euphausiids using plankton nets. We observed a total of 160 gray whales from 55 groups during the four surveys. Statistical analysis across the study area using a generalized linear model indicated that gray whale distribution was related to benthic amphipods, whereas their relationship with euphausiids was unclear. The Distributed Biological Observatory 3 (DBO3) area off Point Hope, Alaska, had the highest density of gray whales (0.035 individuals/km2, hereafter referred to as ind./km2) in the fall of 2012 compared with the densities of the other surveys (0.00 - 0.006 ind./km2). Higher biomass of euphausiids and higher average wet weight of a single euphausiid were also observed in the fall of 2012, which may have contributed to the high density of gray whales. Aligning with previous direct confirmation by visual whale surveys and prey sampling in the fall of 2003, the present study confirmed that, in the fall of 2012, the DBO3 area was an area where gray whales could feed on both benthic amphipods and euphausiids.

Zooplankton size spectra are crucial for evaluating marine ecosystem structure, with copepods being the predominant taxa within the marine zooplankton communities. Among copepods, Calanoida and Ergasilida are the two numerically dominant taxa. Despite their importance, limited information exists regarding the effects of these two taxa on the overall zooplankton size spectra. This study aimed to comprehensively evaluate the regional and vertical changes in zooplankton size spectra, normalised biomass size spectrum (NBSS), and size diversity across subarctic, transitional, and subtropical regions of the western North Pacific Ocean. Additionally, the study aimed to investigate the effects of size variations in two dominant copepod taxa, Calanoida and Ergasilida, on the overall zooplankton size spectra. To achieve this, vertically stratified zooplankton samples were collected from 12 layers, ranging from the sea surface to a depth of 3000 m, at five stations across the subtropical to subarctic western North Pacific. The samples were analysed using ZooScan to assess the overall zooplankton size spectra and the effects of Calanoida and Ergasilida on it. Across all stations, the NBSS slopes became moderate, and the size diversity increased with increasing depth, particularly evident for the oxygen minimum layer (OML) at approximately 1000 m depth. These patterns reflect a high proportion of large-sized zooplankton in the deeper layers and a lower predation pressure from micronektonic fish around the OML. Calanoida and Ergasilida accounted for 43.3% and 24.6% of the mean zooplankton abundance, respectively. Among the two taxa, Calanoida exhibited significant changes in body size depending on the station and depth, being larger in the subarctic region and deeper layers. Ergasilida showed minimal changes in body size relative to location and depth. The body size of Calanoida significantly influenced the overall zooplankton size spectra. Therefore, the dominance of large-sized Calanoida induced a moderate NBSS slope and high size diversity in the overall zooplankton size spectra. The results of this study indicate that the size of Calanoida, the dominant taxon in the zooplankton community, primarily governs the size spectra of the overall zooplankton community.

Since 2015, occurrences of Karenia spp. blooms have been documented in Hakodate Bay, potentially expanding to the Pacific coastal area of Hokkaido via the Tsugaru Warm Current. However, available information on the expanding distribution remains limited. This study aims to elucidate the distribution of Karenia spp. in Hidaka Bay, which is an area downstream of the Tsugaru Warm current from Hakodate Bay, using image analysis technology, specifically the FlowCam, following a comprehensive quantitative assessment to facilitate cell detection. The FlowCam could detect Karenia spp., with assured quantitative precision when the cell density exceeds 1 cell mL(-1). Counting resulted in lower calculated densities that varied widely, suggesting potential subsampling biases. Using the FlowCam's software enabled image filtration of Karenia spp. from other plankton species with a maximum accuracy of 96%. Optimal filtering performance in the software was achieved through a combination of Equivalent Spherical Diameter, Aspect Ratio, and Average Green values. Despite a loss of similar to 10% of Karenia spp. images using the filter function, the application of this function significantly reduces the effort required for the cell sorting-out process. Field observations conducted in September 2022 revealed the presence of Karenia spp. across all stations in Hidaka Bay and Hakodate Bay. Furthermore, the observed gradient in cell density from the warmer waters of Hakodate Bay towards Hidaka Bay suggests the potential role of the Tsugaru Warm Current in facilitating transportation of Karenia spp. cells into Hidaka Bay.

Suctorian ciliates are widely distributed in both freshwater and marine environments. In the open ocean, they attach to crustacean zooplankton, specifically copepods, as epibiont hosts. However, criteria for host preference in oceanic areas remain unclear. In this study, we conducted a molecular analysis, examined copepod community relationships and performed laboratory-rearing experiments on suctorian ciliates in the subarctic Pacific. Suctorians belonging to five genera and species were observed on the exoskeletons of 20 copepod species across 10 genera. Additionally, species-specific host preferences were observed. Molecular analyses revealed intraspecific variations and haplotypes in suctorian ciliates, with no correlation between the haplotypes and host copepods or geographical location. Suctorians did not prefer numerically dominant or large-sized copepod species but favored deep-sea or diel vertical migration (DVM) species, specifically carnivorous copepods. A high suctorian prevalence was observed during the late copepodite stages, specifically among adult females and in regions with cold waters. These characteristics indicate that suctorians rely on prey or host molting events. Laboratory experiments confirmed novel suctorian attachments and growth from swarmer to trophont within 24 h. This study indicates that suctorian epibionts prefer late copepodite stages in deep-sea habitats, exhibiting DVM, prolonged stage duration and low predation risk.

Sea ice in the Pacific Arctic Ocean has been rapidly decreasing over recent decades. However, knowledge of its effects on microplankton is limited. To elucidate the effect of sea ice reduction on the microplankton community of the Pacific Arctic Ocean, we examined the differences in the microplankton community and hydrography between 2019 and 2020. Based on the cluster analysis, the microplankton community was divided into six groups. In the southern Chukchi Sea, high cell densities were observed with high variability in group occurrence owing to the inflow of nutrient-rich Pacific water. In the northern Chukchi Sea, a 1-month inter-annual difference in sea ice melting timing induced changes in the microplankton community through hydrographical changes. Early sea ice melting stimulates the growth of phytoplankton species (Proboscia alata), which can utilize organic nitrogen compounds. In the marginal ice zone, a 10-day inter-annual difference in sea ice melting was observed, resulting in variations in hydrographic conditions; however, these changes did not affect the microplankton community. Our findings indicate that microplankton production and diversity respond differently to sea ice melting in varies by region in the Pacific Arctic Ocean.

Coastal environments in the Arctic are increasingly affected by the rapidly changing climate. Significant and complex impacts of atmospheric warming have been intensifying, with changes observed both in terrestrial and marine environments. Here, we describe the overview and highlight the study results of multidisciplinary research activities performed under the ArCS II project (Arctic Challenge for Sustainability II) in the Qaanaaq coastal region of northwestern Greenland. The Japanese Arctic projects GRENE-Arctic and ArCS have conducted research at this study site since 2012. In continuity with these previous efforts, field and satellite measurements were carried out to quantify glacier and ice sheet changes. Fish, marine mammals and seabirds, which are key natural resources to human livelihoods, were studied in collaboration with local fishermen and hunters to examine habitat use and clarify the potential responses of marine ecosystems to the changing environments. Greenlandic villages are also directly affected by the flooding of glacial streams and landslides, which were monitored to better understand the driving mechanisms and risks to Arctic societies in the future. Research was also carried out in Qaanaaq village to investigate waste management and housing conditions. The study results were shared with residents through workshops that took place in Qaanaaq and nearby smaller villages. Our results show that coastal environments in northwestern Greenland are changing with increasingly evident impact on human livelihoods. Further collaboration with the villagers, notably in co-designing research questions and interests, is crucial to anticipate, reduce and mitigate the impacts of environmental changes on Arctic communities.

The density of the hermit crab, Paguristes ortmanni , in the artificial reef of juvenile Apostichopus japonicus increased almost threefold from June to December 2018. Calcareous ossicles of A. japonicus were identified from the stomach contents in 28.8% of the hermit crabs (18.6% in males and 10.2% in females) in December; season hatchery‐produced juveniles are frequently introduced into the wild in Hokkaido, Japan. The mortality rate of juvenile A. japonicus by P. ortmanni was estimated to be 2.5 ± 2.4 individuals day −1 based on laboratory predation experiments. Interestingly, 3% to 5% of individuals survived despite being attacked and injured in all trials, escaping on the shells of hermit crabs. Over 50% of females in the ossicle‐not‐detected group had shield lengths (SLs) smaller than the smallest individual in the ossicle‐detected group. The average SL of the ossicle‐detected group in females was significantly higher ( p < 0.01) than that of the not‐detected group, indicating an increased predation risk for A. japonicus juveniles when larger female P. ortmanni were present. The present study offers new insights into the predatory behavior of P. ortmanni toward A. japonicus juveniles, showing that these sympatric hermit crabs present a considerably high mortality risk to A. japonicus juveniles. It also emphasizes the importance of implementing appropriate measures to protect juveniles from predators during the release process, providing an essential viewpoint for enhancing and rebuilding the wild population of commercially important endangered A. japonicus .

The Pacific gateway to the Arctic has a vast continental shelf spanning the northern Bering and the Chukchi Seas. Within this shelf region, diatoms are crucial in sustaining high primary production and facilitating the sinking particulate organic carbon flux from spring to summer. Consequently, the bottom sediments have abundant viable diatoms, including resting stages. Despite the importance of diatoms, our understanding of the dynamics of this organism in sediments and their capacity to initiate primary production in the Pacific Arctic shelf remains limited. In this study, we delved into the photophysiological capabilities of diatoms in the surface sediments collected from the Chukchi Sea in autumn through a laboratory incubation experiment at 3°C under the light conditions of 300 or 30 µmol photons m-2 s-1 for seven days. This experiment revealed that diatoms, mainly Chaetoceros, quickly resumed photosynthesis after light exposure and reached the maximum photosynthetic carbon fixation rates within only several days. These results suggest that diatoms in sediments have a significant potential to function as “seeds” for bloom formation in the sunlit water column. We further examined diatom communities, including resting spores, in the water column of the Chukchi Sea during autumn using scanning electron microscopy (SEM) and DNA metabarcoding techniques, as well as environmental parameters. Consequently, intense winds and subsequent water turbulence in the shallow Chukchi caused the predominance of Chaetoceros resting spores, probably derived from the sediments, in the diatom assemblages. As speculated from the incubation experiment mentioned above, diatom resting spores from the sediments can germinate immediately in the water column. Thus, settled diatoms could work as seeds for subsequent autumn blooms by being supplied from the seafloor along with nutrient-rich water. The recent delayed sea ice formation in the autumn Arctic leads to increased storm occurrence over open water and enhanced vertical mixing, resulting in more frequent autumn blooms. Therefore, diatoms in sediments could be one of the critical contributors to autumn blooms in the shallow Pacific Arctic.

Acoustic Doppler Current Profilers (ADCPs) provide backscatter strength data, which can be used to detect suspended materials in the water column. This study developed a novel approach to detect and distinguish between zooplankton diel vertical migration (DVM) and sediment resuspension by applying Complex Empirical Orthogonal Function (CEOF) analysis to 24-hour bandpass-filtered ADCP backscatter data. The temporal CEOF mode scores serve as objective scalar indices of the intensity of each phenomenon, enabling quantitative comparisons with environmental factors. We applied this approach to year-round ADCP array observations in the western Sea of Okhotsk. At most sites, DVM activity is represented by the first CEOF mode. Temporal variations of the first mode revealed that, during the sea-ice season, DVM significantly weakens over shelf regions but persists in offshore regions, even though shelf regions exhibit more active biological productivity than offshore regions during the warm season. This contrasting seasonal behavior likely reflects differences in the dominant zooplankton species and their traits. The quantitative assessment of our method revealed clear relationships with tidal currents through spectral analysis. In the northwestern Okhotsk Sea, where tidal currents are strong, DVM activity, represented by the first mode, was consistently reduced during spring tides. At Kashevarov Bank, where tidal currents exceed 1 m/s, the first mode represents sediment resuspension and intensifies ∼ 1–1.5 days after peak tidal current velocities, and the second mode captures DVM. Given the vast quantity of unused ADCP backscatter data worldwide, the method proposed here can help unlock the potential of these dormant datasets.

The Arctic marine ecosystems, extending from microbial communities to the system response to environmental and human pressures, were investigated in the Arctic Challenge for Sustainability II (ArCS II) project, a nationally coordinated Arctic project in Japan. New findings and hypotheses emerged: a) bottom sediments on a continental shelf contained a significant amount of the bloom-causing viable diatom, and more primary production may be occurring over a water column than previously thought, b) particle flux containing biogenic opal increased over the 2010s, c) large copepod Calanus glacialis/marshallae exhibited flexibility on grazing in the Pacific Arctic Ocean, suggesting their high adaptation to environmental changes, d) a novel environmental DNA (eDNA) technique succeeded in the identification of polar cod distribution, e) there was an increase in species richness over the last 20 years due to the poleward shift of habitat ranges of marine predatory species, f) Arctic marine ecosystems may have a larger sensitivity to external forcings around the Pacific and Atlantic gateways. This article reviews and highlights these findings in the context of specific science questions and delivers Japan's contribution to the integrated assessment of Arctic marine ecosystems.

Knowledge on the distribution of zooplankton in the many unique habitats of the Southern Ocean is essential for understanding food web dynamics, assessing the impacts of environmental change and for managing the exploitation of marine living resources. Variation in the distribution of zooplankton may occur in the horizontal as well as the vertical plane, and the latter may show a diel cycle (diel vertical migration or DVM). Conventional sampling methods, including several types of nets and acoustics, often undersample or ignore the top 10 m of the water column. The surface waters may, however, host a specific zooplankton community and therefore be an important foraging ground for higher trophic level predators. In order to investigate the importance of the surface waters for understanding the distribution of species and potentially improving abundance estimates, the upper two meters of the water column were sampled in the eastern Indian sector of the Southern Ocean using a Surface and Under Ice Trawl (SUIT). Findings were compared to the zooplankton community structure in the epipelagic (15-200 m). Results showed that the surface zooplankton community could largely be divided into two regions. The surface community of the western side of the sampling area hosted large numbers of Antarctic krill, Euphausia superba, which were only present in low densities in the epipelagic depth layer. Densities of Limacina helicina were also relatively high in the west. The copepod Calanus propinquus and the amphipod Themisto gaudichaudii were present in relatively large numbers throughout the sampling area. T. gaudichaudii was the dominant species of the surface in the eastern side of the sampling area in the absence of Antarctic krill. Apart from cirripedia nauplii, no species were uniquely found in the surface water compared to the 15-200 m depth layer. Surface water sampling revealed patterns in vertical distribution and DVM, and showed that these patterns changed between the first and second half of the expedition. This could partially be explained by environmental variables but was likely also a result of sampling time and location, and associated variation in the size and ontogeny of species. Results revealed the impact of undersampling the surface layer regarding knowledge on distribution and vertical migration patterns of zooplankton species.

The Southern Ocean is facing rapid environmental changes. However, few studies have been conducted on the spatiotemporal variability of mesozooplankton communities under recent climatic conditions, particularly in the eastern Indian sector. This study describes the spatiotemporal variability of the mesozooplankton community and the demographics of large copepods and krill in this sector, sampled through a Rectangular Mid-Water Trawl with 1 m2 mouth area (RMT1) during the austral summer of 2018/2019 as part of the KY1804 survey. Cluster analysis indicated that the mesozooplankton community was divided into five groups that showed only small longitudinal differences, as they were affected by oceanic fronts. Part of the variability was explained by physical (local upwelling) and biological features (e.g., the occurrence of species showing a specific spatial distribution, such as Euphausia crystallorophias). Horizontal changes in the zooplankton community structure were not attributed to temporal changes during the 2-month sampling period. The demographics of the dominant species, Calanoides acutus, Calanus propinquus, Metridia gerlacheri, and Thysanoessa macrura, exhibited significant temporal differences in abundance or mean stage index (MSI) between the early and late seasons. These differences matched the growth rates estimated in previous studies, suggesting that their growth during the study period was constant without regional differences. There were no evident changes in the abundance or demographics of Rhinalanus gigas, suggesting that they were in their reproductive season. These species-specific demographics could be explained by the species life cycles: growth in C. acutus and C. propinquus and reproduction in R. gigas during the austral summer. Abundances and MSIs confirmed the growth of dominant copepods and krill during the sampling period; however, no evident seasonal changes were observed in the zooplankton community structure. The findings of this study contribute to the understanding of lower trophic levels in marine ecosystems and the present carbon cycle in the eastern Indian sector of the Southern Ocean.