Context. Highly migratory species (HMS), such as tuna, billfishes and sharks, are a major component of the top predator guild in oceanic ecosystems, but the trophic relationships of many populations remain poorly understood. Aims. This study aimed to characterise the trophic ecology and habitat use of 10 HMS collected from two subtropical oceanic regions around Aotearoa-New Zealand. Methods. We examined stable isotope ratios (delta C-13 and delta N-15) in muscle tissue of HMS considering differences in capture location and body size. Key results. Three trophic groups were distinguished: (1) swordfish, (2) tuna and Lamna nasus (porbeagle) in northern New Zealand and (3) marlins and other pelagic sharks. Despite stable isotope overlap among taxonomically similar species, subtle differences in foraging strategies likely contribute to variation in ecological roles. Trophic position estimates revealed that HMS occupy high trophic levels, though these estimates may be influenced by variations in delta 15N baseline. Stable isotope analysis indicated consistent trophic relationships across the two investigated oceanic regions, with evidence of ontogenetic shifts in resources use in some species. Conclusions. Stable isotope ratios varied widely among HMS, suggesting exploitation of diverse foraging habitats and extensive migration.

With the possibility of deep-sea mining of mineral resources occurring, it is necessary to understand potential impacts on benthic communities. Previous simulated mining experiments revealed direct benthic impacts; however, indirect impacts of sedimentation are not well understood. A disturbance experiment was conducted on Chatham Rise, New Zealand, to assess the resilience of benthic communities to sedimentation that could result from future deep-sea phosphorite mining. Macrofaunal and sediment samples were collected before, immediately after, and one year after a disturbance in areas directly physically disturbed and subjected to induced sedimentation (DI), and undisturbed or subjected to sedimentation only (UI). Macrofaunal abundance significantly decreased in DI areas but not in UI areas. However, abundance-based community structure changed in both areas; in DI areas this was mostly driven by decreases in dominant fauna, but in UI areas by changes in more sensitive fauna. One year after disturbance, abundance-based community structure had recovered in both areas and was correlated most strongly with variations in sediment C:N molar ratios. Biomass-based community structure in DI areas differed one-year post disturbance from communities sampled at the two earlier periods. These results are useful for informing the management of impacts from offshore industries where sedimentation is an issue.

Expansion of extractive industries to deep-sea environments will lead to increased stresses on seafloor ecosystems. We examined changes in environmental parameters following direct and indirect experimental benthic disturbance using a modified plough on the Chatham Rise (similar to 450 m water depth), Aotearoa/New Zealand. Measurements included sediment community oxygen consumption (SCOC), nutrient fluxes, macro-infauna community composition, and sediment organic carbon (C) and nitrogen (N) content. We observed an increase in SCOC immediately after the disturbance, alongside a decrease in macro-infauna abundance, biomass, species richness, and diversity, and an increase in sediment C:N. One-year after the disturbance, SCOC and sediment C:N ratios were similar to that of pre-disturbance levels, while macro-infauna abundance and biomass were greater. We observed a shift in the community structure at directly disturbed sites one-year post-disturbance with decreased biomass of Polychaeta and increased biomass of Ophiuroidea. No changes were observed in sediment nutrient fluxes. Our study indicates the Chatham Rise benthic ecosystem was initially negatively affected by the disturbance, but the recovery of many parameters after one-year shows the potential resilience of the ecosystem. Compared to many proposed deep-sea resource extraction sites the Chatham Rise experiences a relatively high-level of anthropogenic and natural disturbance which may explain the relatively rapid recovery. Seafloor resilience to human disturbances and recovery from impacts will depend on local environmental conditions that need to be evaluated by environmental impact assessments. This knowledge will be key to predicting and mitigating disturbance impacts. While the Chatham Rise is relatively well studied many other regions, including those planned for extractive activities, lack the information needed.

Submarine canyons transfer substantial amounts of sediment and organic carbon (OC) into the deep ocean, nourishing deep-sea ecosystems and contributing to the global carbon cycle through OC burial and sequestration. Tracking lateral OC transport through submarine canyon systems is challenged by the deep-ocean setting, difficulties with constraining episodic depositional events, and the need to assess the composition and age of marine and terrestrial organic matter. We apply innovative parallel ramped pyrolysis oxidation-accelerator mass spectrometry and pyrolysis-gas chromatography-mass spectrometry with isotope analyses to track OC age and sources in the 2016 Kaik & omacr;ura earthquake-triggered, canyon-flushing event that deposited along >1300 km of a submarine canyon-channel system, offshore Aotearoa New Zealand. Specifically, these techniques allow us to determine the ages, sources, and partitioning of OC within the Kaik & omacr;ura turbidite deposit and test hypotheses of how submarine canyon systems contribute to lateral OC flux and burial. Our results show that, despite considerable canyon floor erosion, substantial amounts of young OC were flushed into the deep sea, with relatively little (similar to 2 %) pre-Holocene OC contributions. Even without a direct connection between rivers and submarine canyons, most (similar to 55 %) of the OC in the Kaik & omacr;ura event bed is from terrestrial sources. However, the deposit also contains substantial amounts (similar to 22 %) of marine-derived OC and similar to 23 % of the material is of unassignable origin. Particle sorting imparts variability on the age and composition of OC within turbidite deposits and along the turbidity current flow path. Terrestrial-derived OC is preferentially older than marine-derived OC and concentrated in coarser particle sizes found more commonly at the deposit base and in proximal settings. Young, marine-derived OC is concentrated at the surface of the deposits and tends to be enriched in finer particle sizes. Such OC partitioning in turbidites supports the relevance of depositional models for predicting and quantifying distribution of OC in deep-sea deposits. Earthquake-triggered, canyon flushing events and resulting turbidites enhance OC burial efficiency and can sequester OC effectively, contributing an important carbon sink to the sedimentary carbon cycle.

Gravitational particle sinking is the main mechanism for carbon export in the biological carbon pump. However, the export dynamics of the particle-associated protist community are not fully understood. We used 18S rRNA gene metabarcoding to characterise the exported protist community within sinking particles and bathypelagic surficial sediments in oligotrophic subtropical and high-nutrient, low-chlorophyll subantarctic waters. Sinking particles were collected with formalin-fixed and preservative-free particle interceptor traps (fixed and live traps, respectively) to identify the community involved in particle export (fixed) and protist loss from remineralisation (live). We paired this with community analysis of the upper and lower water column (mixed layer and below mixed layer to mesopelagic, respectively) to compare the relative sources of exported protists. Amplicon sequences variants (ASVs) from upper water column samples accounted for 2 to 4-fold higher proportion of reads and ASV richness compared to lower water column samples in fixed trap and sediment samples, suggesting low influence of the suspended protist community from the lower water column on export. We further traced the export patterns of upper water column protist taxa by analysing the change in taxa relative abundance across the mixed layer to mesopelagic depths. Export patterns differed between taxa, which is similarly suggested by taxa-specific loss of ASV richness between fixed and live traps, but remained the same across biogeochemically-contrasting water masses. This could imply that the drivers for protist loss during export are related to characteristics consistent across environmental conditions, such as specific microbial interactions or inherent cell properties. ### Competing Interest Statement The authors have declared no competing interest.

Anthropogenic impacts are increasingly affecting deep-sea environments, including seafloor sediment disturbances by bottom trawling and seafloor mining. Fieldwork in the 'Resilience Of Benthic Ecosystems to Sedimentation' (ROBES) project were conducted in 2018-2020 on the 400 m-deep Chatham Rise crest, eastern Aotearoa New Zealand. Water column turbidity data, sediment traps on near-seabed moorings and benthic landers and surficial sediments from multi-corers provided baseline and post-impact information following an artificially induced seafloor disturbance event in June 2019 using a modified harrow plough in and around an elongated bathymetric depression, designated as the 'Butterknife'. During the disturbance, total mass and particulate organic carbon (POC) near-bed fluxes were elevated above long-term fluxes (2018-20). Long-term 2018-19 fluxes were generally less than in 2019-20, driven by interannual oceanographic variations. These results, coupled with 50-100 m-thick benthic boundary layers, suggest that lateral advection is a dominant process in the resuspension and redistribution of seafloor sediments on the rise. Further research is required to better understand the longer term dynamics of particle transport and deposition in response to human activities, such as bottom trawling or proposed phosphorite seafloor mining, especially in bathyal environments, such as the Chatham Rise.

The marine system plays a critical role in the global climate cycle, as a major control of atmospheric carbon dioxide (CO2). Marine primary production (photosynthesis) and remineralisation of organic carbon (respiration, degradation) determine the amount of CO2 sequestered in marine sediments and deep-water environments on century to millennial timescales. The stocks and fluxes of the marine carbon cycle are susceptible to global climate change impacts and other anthropogenic activities that modify key processes. Oceanographic studies of the marine carbon cycle in Aotearoa New Zealand's Exclusive Economic Zone (NZ EEZ) and Territorial Seas over past decades have provided broad knowledge across a complex and dynamic seascape, but there remain fundamental knowledge gaps that limit identification of and response to present and future anthropogenic threats. In particular, several areas of the EEZ have been under-sampled and there are currently insufficient data to establish baselines and variability for the marine carbon cycle. We recommend that new observational technologies and ocean modelling applications be fully developed and utilised to enable development of robust predictive capability of our ocean's response to human-induced perturbations. Future focus on oceanic nature-based solutions to accelerate CO2 uptake will require improved knowledge of the marine carbon cycle in NZ's EEZ.

The New Zealand Community Fault Model (NZ CFM) is a publicly available representation of New Zealand fault zones that have the potential to produce damaging earthquakes. Compiled through collaborative engagement between New Zealand earthquake-science experts, this first edition (version 1.0) of the NZ CFM builds upon previous compilations of earthquake-source active fault models with the addition of new and modified information. Developed primarily to support an update of the New Zealand National Seismic Hazard Model, the NZ CFM comprises two principal components. The first dataset is a two-dimensional map representation of the surface traces of 880 generalised fault zones. Each fault zone is assigned specific geometric and kinematic attributes, including uncertainties, supplemented with a subjective quality ranking focused primarily on the confidence in assigned slip rates. The second component is a three-dimensional representation of the fault zones as triangulated mesh surfaces that are projected down-dip from the two-dimensional mapped traces to a geophysically-defined maximum fault rupture depth. This article summarises the compilation and parameterisation of the NZ CFM, along with background on its relation to predecessor datasets, and forward applications to probabilistic seismic hazard assessment and physics-based earthquake models currently being developed for Aotearoa New Zealand.

Growing global demand for deep-sea resources may lead to increased pressure on benthic ecosystems. Here we examined changes in meiofaunal communities following an in situ physical disturbance experiment. A significant change in meiofaunal community structure in surface (0-1 cm) and subsurface (1-5 cm) sediments was observed immediately following the disturbance at both the directly and indirectly disturbed sites and adjacent potentially undisturbed/indirectly disturbed sites, reflecting a decrease in the abundance of several meiofaunal taxa. Abundance, taxon richness and diversity also decreased immediately following disturbance across all sites. Surface community parameters returned to pre-disturbance values one year post-disturbance, but only a partial recovery was observed for the subsurface community over the same period. The accumulation of recently resuspended and deposited sediment could have led to lower meiofaunal density and shifts in community structure. Passive dispersal and recolonisation from outside the study area may explain why the surface community recovered more fully than the subsurface community. We show that meiofauna were negatively affected by a relatively minor disturbance compared to proposed commercial seabed mining operations, but the finding that communities had largely recovered one year after disturbance has implications for resilience of fauna to much more pronounced impacts of potential large-scale mining.

Kaiko over bar ura Canyon, offshore Aotearoa/New Zealand, is a hotspot for deep-sea benthic biology with globally high faunal abundance. The Mw7.8 Kaiko over bar ura earthquake in 2016 triggered a severe disturbance that reshaped the canyon, evacuating an estimated 850 metric megatonnes of sedimentary material down canyon. The Kaiko over bar ura Canyon habitat is now recovering from this removal of both seafloor substrate and associated organisms. We measured post -event benthic macrofauna density together with biomass and sediment properties and related these to infaunal bioturbation activity, sediment community oxygen consumption (SCOC) and associated benthic macronutrient biogeochemical fluxes. Three legacy sites were used as reference along a depth transect of the canyon axis at 917, 1249, and 1480 m water depths. We found distinct differences in benthic functioning between sites. While many parameters measured followed the expected relationships of decreasing with depth some benthic biological activity metrics did not. Our data suggests that the sites differ based on the relative impact and type of disturbance experienced during the earthquake -triggered, canyon -flushing event. We find SCOC was linked to bioturbatory behaviour and sediment organic matter content, rather than taxa density or biomass. While the greatest bioturbation activity was measured at the shallowest site, the greatest SCOC was measured at the middle depth site due to elevated sediment organic matter content. These observations provide a wider benchmark for infaunal bioturbation and faunal -mediated biogeochemical processes in deep-sea canyon environments and provide rare insights of recovery trajectories for deep-sea benthic communities and ecosystem function after large seafloor disturbances.

Global climate mitigation efforts seeking to reduce greenhouse gas emissions require more renewable energy generation and utilisation. In Aotearoa New Zealand there are initiatives underway to develop offshore wind, or in the future, arrays of tidal turbines or wave energy converters, as a new energy resource. Here we synthesise available knowledge from international developments in offshore windfarm installations and discuss in a local Aotearoa New Zealand context. Aspects described include habitat modification, consequences of physical water column changes, and effects on benthic organisms, fish and fisheries, seabirds and marine mammals. Importantly, there is a need to adhere to Te Tiriti o Waitangi which defines Māori sovereign rights and expectations in terms of guardianship of resources (kaitiakitanga). Based on recent regulatory applications in marine spatial planning, where developments have been subject to the precautionary principle for environmental impacts, comprehensive environmental information will be critical for obtaining approval to proceed. The present synthesis identifies environmental pressure-points, footprints, and knowledge gaps, such as New Zealand-specific seabird and marine mammal behaviour and discusses potential opportunities to leverage the positive impacts of marine renewable energy developments.

Late Pleistocene and Holocene sedimentation records changes in glacio-eustatic cycles, oceanography, active tectonics, and sediment supply. Here, we use high-resolution seismic-reflection profiles in Te Tai-o-Aorere Tasman Bay to investigate sedimentary deposit distributions associated with sea-level cyclicity over & SIM;140 ka. We identify nine seismic stratigraphic units below the inner continental shelf, interpret their character based on stratal relationships and acoustic properties, and infer ages using global sea level. The sequence includes stacked deposits and unconformities associated with the shifting shoreline over glacial cycles. Prominent erosional features include a lowstand ravinement surface and two transgressive surfaces formed during sea-level rise following the last two glacial maxima of & SIM;20 ka and & SIM;140 ka. The youngest surface marks the base of post-glacial sediment accumulated since & SIM;12 ka in the study area. Post-glacial sediment includes five seismically mappable units, covering an area of & SIM;1400 km(2) and deposit volume of & SIM;9 km(3). Time-averaged sediment accumulation rates are estimated to range from a maximum of & SIM;1.6-2.8 m/kyr to & SIM;0.5-0.9 m/kyr. Sequence stratigraphy interpreted in Te Tai-o-Aorere Tasman Bay is representative of accumulation in a relatively wide-shelf, sheltered inner bay setting supplied by mountainous catchments, and provides stratigraphic context necessary for future stratigraphic and tectonics studies.

Increasing interest in seabed resource use in the ocean is introducing new pressures on deep-sea environments, the ecological impacts of which need to be evaluated carefully. The complexity of these ecosystems and the lack of comprehensive data pose significant challenges to predicting potential impacts. In this study, we demonstrate the use of Bayesian networks (BNs) as a modeling framework to address these challenges and enhance the development of robust quantitative predictions concerning the effects of human activities on deep-seafloor ecosystems. The approach consists of iterative model building with experts, and quantitative probability estimates of the relative decrease in abundance of different functional groups of benthos following seabed mining. The model is then used to evaluate two alternative seabed mining scenarios to identify the major sources of uncertainty associated with the mining impacts. By establishing causal connections between the pressures associated with potential mining activities and various components of the benthic ecosystem, our model offers an improved comprehension of potential impacts on the seafloor environment. We illustrate this approach using the example of potential phosphorite nodule mining on the Chatham Rise, offshore Aotearoa/New Zealand, SW Pacific Ocean, and examine ways to incorporate knowledge from both empirical data and expert assessments into quantitative risk assessments. We further discuss how ecological risk assessments can be constructed to better inform decision-making, using metrics relevant to both ecology and policy. The findings from this study highlight the valuable insights that BNs can provide in evaluating the potential impacts of human activities. However, further research and data collection are crucial for refining and ground truthing these models and improving our understanding of the long-term consequences of deep-sea mining and other anthropogenic activities on marine ecosystems. By leveraging such tools, policymakers, researchers, and stakeholders can work together toward human activities in the deep sea that minimize ecological harm and ensure the conservation of these environments.

The 2016 Mw7.8 Kaikōura Earthquake in Aotearoa New Zealand provides an opportunity to test widely applied turbidite sedimentation models because it triggered a co-seismic turbidity current. The resultant Kaikōura event bed (KEB), interpreted as a turbidite, is sampled for approx. 1300-km down-flow along the depositional system. Sediment core lithologies, computed tomography (CT), and particle-size data are used to test event-bed thickness, silt content, facies distribution and stacking patterns against the foundations of the turbidite conceptual model of Bouma (1962). KEB thickness is variable to approx. 100 km down-flow distance and attains a maximum thickness at approx. 700 km down-flow distance before thinning distally, similar to the predicted bell-shaped proximal to distal trend. Silt content is high throughout the KEB from canyon to fan. The KEB is dominated by laminated Td facies and Te facies that evolve down-system from laminated, then graded, to homogenous muds. CT and granulometry data ar e key to differentiating subtle density and textural variations within fine-grained deposits and reveal that KEB Td and Te facies in the KEB that are often not preserved or readily observed in older deposits. The KEB highlights a fine-grained sedimentary system that contrasts with more widely studied sandy turbidite basins. In particular, the KEB example reveals that Td and Te facies are ubiquitous in this fine-grained, silt-rich system. A varied conceptual model developed from the KEB may be applicable to many modern deep-sea turbidite systems and crucial for understanding present-day particulate transport to the deep sea and interpreting evidence from the stratigraphic record.

Turbidity flows can transport massive amounts of sediment across large distances with dramatic, long-lasting impacts on deep-sea benthic communities. The 2016 Mw 7.8 Kaikōura Earthquake triggered a canyon-flushing event in Kaikōura Canyon, New Zealand, which included significant submarine mass wasting, debris, and turbidity flows. This event provided an excellent opportunity to investigate the effects of large-scale natural disturbance on benthic ecosystems. Benthic meiofauna community structure before and after the event was analysed from a time series of sediment cores collected 10 years and 6 years before, and 10 weeks, 10 months, and 4 years after the disturbance. Immediately after the 2016 event abundances of all meiofauna dramatically decreased. Four years later the meiofauna community had recovered and was no longer distinguishable from the pre-event community. However, the nematode component of the community was similar, but not fully comparable to the pre-event community by 4 years after the disturbance. Community recovery was systematically correlated to changes in the physical characteristics of the habitat caused by the disturbance, using physical and biochemical variables derived from sediment cores, namely: sediment texture, organic matter, and pigment content. While these environmental variables explained relatively little of the overall variability in meiofauna community structure, particle size, food availability and quality were significant components. The minimum threshold time for the meiofauna community to fully recover was estimated to be between 3.9 and 4.7 years, although the predicted recovery time for the nematode community was longer, between 4.6 and 5 years. We consider the management implications of this study in comparison to the few studies of large-scale disturbances in the deep sea, in terms of their relevance to the efficacy of the marine reserve that encompasses Kaikōura Canyon, along with potential implications for our understanding of the impacts of anthropogenic seafloor disturbances, such as seabed mining.

Photosynthesis in the surface ocean and subsequent export of a fraction of this fixed carbon leads to carbon dioxide sequestration in the deep ocean. Ecological relationships among plankton functional groups and theoretical relationships between particle size and sinking rate suggest that carbon export from the euphotic zone is more efficient when communities are dominated by large organisms. However, this hypothesis has never been tested against measured size spectra spanning the >5 orders of magnitude found in plankton communities. Using data from five ocean regions (California Current Ecosystem, North Pacific subtropical gyre, Costa Rica Dome, Gulf of Mexico, and Southern Ocean subtropical front), we quantified carbon-based plankton size spectra from heterotrophic bacteria to metazoan zooplankton (size class cutoffs varied slightly between regions) and their relationship to net primary production and sinking particle flux. Slopes of the normalized biomass size spectra (NBSS) varied from -1.6 to -1.2 (median slope of -1.4 equates to large 1-10 mm organisms having a biomass equal to only 7.6% of the biomass in small 1-10 mu m organisms). Net primary production was positively correlated with the NBSS slope, with a particularly strong relationship in the microbial portion of the size spectra. While organic carbon export co-varied with NBSS slope, we found only weak evidence that export efficiency is related to plankton community size spectra. Multi-variate statistical analysis suggested that properties of the NBSS added no explanatory power over chlorophyll, primary production, and temperature. Rather, the results suggest that both plankton size spectra and carbon export increase with increasing system productivity.

ABSTRACT Submarine canyons are important deep-sea environments and conduits for transferring and accumulating sediment and organic matter and pollutants. Recent advances in observing, sampling, and analyzing modern canyon sediment transport systems illustrate near-seafloor dynamics and highlight the potential roles of submarine canyons in transporting and storing organic carbon, nutrients, and contaminants in the deep sea, with implications for deep-sea ecosystems and global carbon budgets. Kaikōura Canyon, offshore northeastern Te Waipounamu South Island, Aotearoa New Zealand, is a benthic biomass hotspot that experienced an earthquake-triggered, canyon-flushing event in 2016. On return to the canyon in October 2020, benthic landers, with sediment traps at 2 m above the seafloor, were deployed along the canyon axis in ∼ 900–1500 m water depths for a period of three weeks. These instrumented platforms provide a detailed view of near-seafloor sediment and organic-carbon transport between canyon-flushing events, showing that the canyon environment hosts dynamic physical processes and short-term sediment fluxes and transport. Variations in sediment and organic carbon flux down-canyon and over time include small-scale sediment transport events, some of which are interpreted as turbidity currents, occurring on much shorter timescales than earthquake recurrence. We compare Kaikōura Canyon results with other longshore-fed, shelf-incised global submarine canyons and deep-ocean sites, revealing differences and likely multiple controlling factors for near-seafloor sediment flux. This Kaikōura Canyon high-resolution, benthic lander timeseries dataset highlights the complexity of submarine canyons and their role in organic carbon flux to the deep ocean, even under high present-day sea-level conditions. Evolving insights underscore the need for more observational data and samples to further quantify submarine canyon sediment and organic-carbon transport and contribute to global evaluations of deep-sea canyon distributary systems.

The combination of iron limitation and microzooplankton grazing controls phytoplankton productivity and taxonomic composition in high-nutrient low-chlorophyll (HNLC) regions. While increased productivity and diatom contribution triggered by iron enrichment support this view, direct measurements of underpinning group-specific growth and grazing rates are scarce for the Southern Ocean. To assess these rates, we conducted dilution experiments coupled to high-performance liquid chromatography and flow-cytometry in sub-Antarctic waters on and off Campbell Plateau, southeast of Aotearoa-New Zealand. Off the plateau, growth and grazing were closely balanced for all groups despite a two-fold difference between slow- and fast-growing groups. On Campbell Plateau, where HNLC conditions were alleviated, the balance was disrupted, mainly by the preferential growth of diatoms and green algae, which was stimulated beyond grazing. Our results expand the recognized ability of diatoms to escape grazing control to picoplanktonic green algae that also avoid grazing and contribute significantly to phytoplankton productivity and biomass accumulation.

Sediment density flows are large scale disturbances that can have dramatic impacts on seafloor animal communities in the deep sea. Seafloor imagery collected in Kaikōura Canyon (New Zealand), before and after a sediment density flow event that included debris and turbidity flows triggered by a 2016 Mw 7.8 Kaikōura Earthquake, shows the recovery trajectory of the animal community in the canyon head in the weeks, months, and years following the disturbance. The canyon community appears resilient to this event, with models estimating full recovery within a minimum of 4.5–5.1 years and as long as 12 years. The implications of the resilience of this deep-sea community are discussed in the context of the local marine protected area, the surrounding fishery, and global seabed mining.

The Subantarctic Zone of the Southern Ocean plays a disproportionally large role on the Earth system. Model projections predict rapid environmental change in the coming decades, including ocean acidification, warming, and changes in nutrient supply which pose a serious risk for marine ecosystems. Yet despite the importance of the Subantarctic Zone, annual and inter-annual time series are extremely rare, leading to important uncertainties about the current state of its ecosystems and hindering predictions of future response to climate change. Moreover, as the longest observational time series available are only a few decades long, it remains unknown whether marine pelagic ecosystems have already responded to ongoing environmental change during the industrial era. Here, we take advantage of multiple sampling efforts – monitoring of surface layer water properties together with sediment trap, seafloor sediment and sediment core sampling – to reconstruct the modern and pre-industrial state of the keystone calcifying phytoplankton Calcidiscus leptoporus , central to the global marine carbonate cycle. Morphometric measurements reveal that modern C. leptoporus coccoliths are 15% lighter and 25% smaller than those preserved in the underlying Holocene-aged sediments. The cumulative effect of multiple environmental factors appears responsible for the coccolith size variations since the Last Deglaciation, with warming and ocean acidification most likely playing a predominant role during the industrial era. Notably, extrapolation of our results suggests a future reduction in cell and coccolith size which will have a negative impact on the efficiency of the biological pump in the Southern Ocean through a reduction of carbonate ballasting. Lastly, our results tentatively suggest that C. leptoporus coccolith size could be used as a palaeo-proxy for growth rate. Future culture experiments will be needed to test this hypothesis.