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Global climate models project the intensification of marine heatwaves in coming decades due to global warming. However, the spatial resolution of these models is inadequate to resolve mesoscale processes that dominate variability in boundary current regions where societal and economic impacts of marine heatwaves are substantial. Here we compare the historical and projected changes in marine heatwaves in a 0.1° ocean model with 23 coarser-resolution climate models. Western boundary currents are the regions where the models disagree the most with observations and among themselves in simulating marine heatwaves of the past and the future. The lack of eddy-driven variability in the coarse-resolution models results in less intense marine heatwaves over the historical period and greater intensification in the coming decades. Although the projected changes agree well at the global scale, the greater spatial details around western boundary currents provided by the high-resolution model may be valuable for effective adaptation planning. Marine heatwaves are likely to intensify in a warmer world, but prediction of these events is hampered by course-scale modeling. Here the authors develop a fine scale, global model which shows that marine heatwaves will amplify with greater spatial variability, particularly at western boundary regions.

At high latitudes, the biological carbon pump, which exports organic matter from the surface ocean to the interior, has been attributed to the gravitational sinking of particulate organic carbon. Conspicuous deficits in ocean carbon budgets challenge this as a sole particle export pathway. Recent model estimates revealed that particle injection pumps have a comparable downward flux of particulate organic carbon to the biological gravitational pump, but with different seasonality. To date, logistical constraints have prevented concomitant and extensive observations of these mechanisms. Here, using year-round robotic observations and recent advances in bio-optical signal analysis, we concurrently investigated the functioning of two particle injection pumps, the mixed layer and eddy subduction pumps, and the gravitational pump in Southern Ocean waters. By comparing three annual cycles in contrasting physical and biogeochemical environments, we show how physical forcing, phytoplankton phenology and particle characteristics influence the magnitude and seasonality of these export pathways, with implications for carbon sequestration efficiency over the annual cycle. Distinct seasonality of export pathways from the different pumps in the Pacific Southern Ocean are revealed using year-round robotic profiler observations, contributing to understanding of particle export into the oceans’ interior.

In the Southern Ocean, large-scale phytoplankton blooms occur in open water and the sea-ice zone (SIZ). These blooms have a range of fates including physical advection, downward carbon export, or grazing. Here, we determine the magnitude, timing and spatial trends of the biogeochemical (export) and ecological (foodwebs) fates of phytoplankton, based on seven BGC-Argo floats spanning three years across the SIZ. We calculate loss terms using the production of chlorophyll—based on nitrate depletion—compared with measured chlorophyll. Export losses are estimated using conspicuous chlorophyll pulses at depth. By subtracting export losses, we calculate grazing-mediated losses. Herbivory accounts for ~90% of the annually-averaged losses (169 mg C m −2 d −1 ), and phytodetritus POC export comprises ~10%. Furthermore, export and grazing losses each exhibit distinctive seasonality captured by all floats spanning 60°S to 69°S. These similar trends reveal widespread patterns in phytoplankton fate throughout the Southern Ocean SIZ. Satellites can observe marine phytoplankton, but observations are sparse in seasonally dark, cloudy environments like the Southern Ocean. These authors use Argo floats to track the fate of phytoplankton blooms off Antarctica and determine 10% of biomass is exported, while 90% is prey to grazing.

Natural iron fertilization of the Southern Ocean by windblown dust has been suggested to enhance biological productivity and modulate the climate 1 – 3 . Yet, this process has never been quantified across the Southern Ocean and at annual timescales 4 , 5 . Here we combined 11 years of nitrate observations from autonomous biogeochemical ocean profiling floats with a Southern Hemisphere dust simulation to empirically derive the relationship between dust-iron deposition and annual net community production (ANCP) in the iron-limited Southern Ocean. Using this relationship, we determined the biological response to dust-iron in the pelagic perennially ice-free Southern Ocean at present and during the last glacial maximum (LGM). We estimate that dust-iron now supports 33% ± 15% of Southern Ocean ANCP. During the LGM, when dust deposition was 5–40-fold higher than today, the contribution of dust to Southern Ocean ANCP was much greater, estimated at 64% ± 13%. We provide quantitative evidence of basin-wide dust-iron fertilization of the Southern Ocean and the potential magnitude of its impact on glacial–interglacial timescales, supporting the idea of the important role of dust in the global carbon cycle and climate 6 – 8 . Nitrate observations over 11 years from autonomous biogeochemical ocean profiling combined with a Southern Hemisphere dust simulation find that iron supplied by dust supports about 30% of Southern Ocean productivity.

Droughts and climate-change-driven warming are leading to more frequent and intense wildfires 1 – 3 , arguably contributing to the severe 2019–2020 Australian wildfires 4 . The environmental and ecological impacts of the fires include loss of habitats and the emission of substantial amounts of atmospheric aerosols 5 – 7 . Aerosol emissions from wildfires can lead to the atmospheric transport of macronutrients and bio-essential trace metals such as nitrogen and iron, respectively 8 – 10 . It has been suggested that the oceanic deposition of wildfire aerosols can relieve nutrient limitations and, consequently, enhance marine productivity 11 , 12 , but direct observations are lacking. Here we use satellite and autonomous biogeochemical Argo float data to evaluate the effect of 2019–2020 Australian wildfire aerosol deposition on phytoplankton productivity. We find anomalously widespread phytoplankton blooms from December 2019 to March 2020 in the Southern Ocean downwind of Australia. Aerosol samples originating from the Australian wildfires contained a high iron content and atmospheric trajectories show that these aerosols were likely to be transported to the bloom regions, suggesting that the blooms resulted from the fertilization of the iron-limited waters of the Southern Ocean. Climate models project more frequent and severe wildfires in many regions 1 – 3 . A greater appreciation of the links between wildfires, pyrogenic aerosols 13 , nutrient cycling and marine photosynthesis could improve our understanding of the contemporary and glacial–interglacial cycling of atmospheric CO 2 and the global climate system. Oceanic deposition of wildfire aerosols can enhance marine productivity, as supported here by satellite and in situ profiling floats data showing that emissions from the 2019–2020 Australian wildfires fuelled phytoplankton blooms in the Southern Ocean.

The Amundsen Sea Embayment (ASE) experiences both the highest ice shelf melt rates and the highest biological productivity in West Antarctica. Using 19 years of satellite data and modelling output, we investigate the long-term influence of environmental factors on the phytoplankton bloom in the Amundsen Sea (ASP) and Pine Island (PIP) polynyas. We test the prevailing hypothesis that changes in ice shelf melt rate could drive interannual variability in the polynyas' surface chlorophyll-a (chl a) and Net Primary Productivity (NPP). We find that the interannual variability and long-term change in glacial meltwater may play an important role in chl a variance in the ASP, but not for NPP. Glacial meltwater does not explain the variability in neither chl a or NPP in the PIP, where light and temperature are the main drivers. We attribute this to potentially greater amount of iron-enriched meltwater brought to the surface by the meltwater pump downstream of the PIP, and the coastal ocean circulation accumulating and transporting iron towards the ASP.

Mesoscale eddies are ubiquitous in the iron-limited Southern Ocean, controlling ocean-atmosphere exchange processes, however their influence on phytoplankton productivity remains unknown. Here we probed the biogeochemical cycling of iron (Fe) in a cold-core eddy. In-eddy surface dissolved Fe (dFe) concentrations and phytoplankton productivity were exceedingly low relative to external waters. In-eddy phytoplankton Fe-to-carbon uptake ratios were elevated 2–6 fold, indicating upregulated intracellular Fe acquisition resulting in a dFe residence time of ~1 day. Heavy dFe isotope values were measured for in-eddy surface waters highlighting extensive trafficking of dFe by cells. Below the euphotic zone, dFe isotope values were lighter and coincident with peaks in recycled nutrients and cell abundance, indicating enhanced microbially-mediated Fe recycling. Our measurements show that the isolated nature of Southern Ocean eddies can produce distinctly different Fe biogeochemistry compared to surrounding waters with cells upregulating iron uptake and using recycling processes to sustain themselves. Eddies are common ocean features that isolate large swaths of seawater, but it is unclear how they influence productivity of phytoplankton trapped inside. Here Ellwood and colleagues use stable and radiogenic isotopes to characterize a Southern Ocean eddy, finding vanishingly low iron concentrations that drive low productivity across the region.

Marine heatwaves (MHWs), prolonged extreme thermal events, are reshaping ocean ecosystems, yet their influence on global productivity patterns remains poorly understood. Here, we use a global regression framework to disentangle linear thermal effect from nonlinear feedback and demonstrate that MHWs restructure the dominant drivers of ocean net primary production (NPP). MHWs induce a regime shift from sea surface temperature (SST)-independent to SST-dependent controls on NPP anomaly, reflecting an enhanced thermal effect in response to extreme warming. MHW suppressed the NPP anomaly across nutrient-poor low latitudes but increased it in nutrient-rich higher latitudes. The contrasting responses arise from differences in nutrient baselines, with low-nutrient regions exhibiting greater sensitivity to extreme warming. Together, these results reveal an emerging poleward redistribution of ocean productivity and highlight the need to incorporate MHWs into projections of marine ecosystem resilience and climate–biosphere feedbacks. Marine heatwaves (MHWs) systematically alter ocean net primary production (NPP) patterns meridionally, reflecting regional differences in baseline nutrient availability and pointing to an emerging poleward redistribution of ocean productivity.

Ocean fronts affect phytoplankton and higher trophic levels, including commercially important fisheries. As the oceans warm, uncertainty remains around the trends in fronts. Here we examine changes in sea surface temperature fronts (frequency, density, and intensity) and the concentration of chlorophyll, over recent satellite records (2003 – 2020) in ocean warming hotspots - areas that are warming faster than other parts of the ocean. Commonalities exist across hotspots with comparable dynamics. Most equatorial and subtropical gyre hotspots experienced a decline in frontal activity (frequency, density, strength) and chlorophyll concentration, while in high-latitude hotspots, frontal activity and chlorophyll concentration mostly increased. Continued warming may accentuate the impacts, changing both total biomass and the distribution of marine species. Areas with changing fronts and phytoplankton also correspond to areas of important global fish catch, highlighting the potential societal significance of these changes in the context of climate change.

Our review of the literature has revealed Southern Ocean subsurface chlorophyll-a maxima (SCMs) to be an annually recurrent feature throughout the basin. Most of these SCMs are different to the “typical” SCMs observed in the tropics, which are maintained by the nutrient-light co-limitation of phytoplankton growth. Rather, we have found that SCMs are formed by other processes including diatom aggregation, sea-ice retreat, eddies, subduction events and photo-acclimation. At a local scale, these SCMs can facilitate increased downward carbon export, primary production and food availability for higher trophic levels. A large proportion of Southern Ocean SCMs appear to be sustained by aggregates of large diatoms that form under severe iron limitation in the seasonal mixed layer. The ability of large diatoms to regulate their buoyancy must play a role in the development of these SCMs as they appear to increase buoyancy at the SCM and thus avoid further sinking with the decline of the spring bloom or naturally iron fertilised blooms. These SCMs remain largely unobserved by satellites and it seems that ship-based sampling may not be able to fully capture their biomass. In the context of the Marine Ecosystem Assessment of the Southern Ocean it is important to consider that this phenomenon is missing in our current understanding of Southern Ocean ecology and future climate scenarios. The broader implications of SCMs for Southern Ocean ecology will only be revealed through basin-wide observations. This can only be achieved through an integrated observation system that is able to harness the detailed information encapsulated in ship-based sampling, with the increased observational capacity of fluorometers on autonomous platforms such as those in the biogeochemical Argo (BGC-Argo) and the Marine Mammals Exploring the Ocean Pole to pole (MEOP) programs. The main challenge towards achieving this is the uncertainties associated with translating fluorescence to chlorophyll-a concentrations. Until this translation is resolved, the reporting of subsurface fluorescence maxima (SFMs) in place of SCMs could still yield valuable insights with careful interpretation.