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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 ., (© 2024. The Author(s), under exclusive licence to Springer Nature Limited.)

Biological ocean data collected from ships find reuse in aggregations of historical data. These data are heavily relied upon to document long term change, validate satellite algorithms for ocean biology and are useful in assessing the performance of autonomous platforms and biogeochemical models. Existing aggregate products have largely been restricted to the surface ocean, omit physical data or have limited biological data. We present the first version of a BIOlogical ocean data reforMATting Effort (BIO-MATE) to begin to fill a gap in subsurface bio-physical data aggregates in a reproducible way. BIO-MATE uses open-source R software that reformats openly sourced published datasets from oceanographic voyages. These reformatted biological and physical data from underway sensors, profiling sensors, pigments analysis and particulate organic carbon analysis are stored in an interoperable BIO-MATE data product for easy access and use. Specific QA/QC protocols can now be easily applied to the BIO-MATE data product to support a variety of surface and subsurface applications., (© 2024. The Author(s).)

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., (© 2023. The Author(s).)

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., (© 2021. The Author(s), under exclusive licence to Springer Nature Limited.)

Ocean temperature extreme events such as marine heatwaves are expected to intensify in coming decades due to anthropogenic global warming. Reported ecological and economic impacts of marine heatwaves include coral bleaching, local extinction of mangrove and kelp forests and elevated mortalities of invertebrates, fishes, seabirds and marine mammals. In contrast, little is known about the impacts of marine heatwaves on microbes that regulate biogeochemical processes in the ocean. Here we analyse the daily output of a near-global ocean physical-biogeochemical model simulation to characterize the impacts of marine heatwaves on phytoplankton blooms in 23 tropical and temperate oceanographic regions from 1992 to 2014. The results reveal regionally coherent anomalies of shallower surface mixing layers and lower surface nitrate concentrations during marine heatwaves. These anomalies exert counteracting effects on phytoplankton growth through light and nutrient limitation. Consequently, the responses of phytoplankton blooms are mixed, but can be related to the background nutrient conditions of the study regions. The blooms are weaker during marine heatwaves in nutrient-poor waters, whereas in nutrient-rich waters, the heatwave blooms are stronger. The corresponding analyses of sea-surface temperature, chlorophyll a and nitrate based on satellite observations and in situ climatology support this relationship between phytoplankton bloom anomalies and background nitrate concentration. Given that nutrient-poor waters are projected to expand globally in the 21st century, this study suggests increased occurrence of weaker blooms during marine heatwaves in coming decades, with implications for higher trophic levels and biogeochemical cycling of key elements., (© 2020 John Wiley & Sons Ltd.)

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.

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.

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.

Some species may have a larger role than others in the transfer of complex effects of multiple human stressors, such as changes in biomass, through marine food webs. We devised a novel approach to identify such species. We constructed annual interaction-effect networks (IENs) of the simulated changes in biomass between species of the southeastern Australian marine system. Each annual IEN was composed of the species linked by either an additive (sum of the individual stressor response), synergistic (lower biomass compared with additive effects), or antagonistic (greater biomass compared with additive effects) response to the interaction effect of ocean warming, ocean acidification, and fisheries. Structurally, over the simulation period, the number of species and links in the synergistic IENs increased and the network structure became more stable. The stability of the antagonistic IENs decreased and became more vulnerable to the loss of species. In contrast, there was no change in the structural attributes of species linked by an additive response. Using indices common in food-web and network theory, we identified the species in each IEN for which a change in biomass from stressor effects would disproportionately affect the biomass of other species via direct and indirect local, intermediate, and global predator-prey feeding interactions. Knowing the species that transfer the most synergistic or antagonistic responses in a food-web may inform conservation under increasing multiple-stressor impacts., (© 2018 Society for Conservation Biology.)

We have little empirical evidence of how large-scale overlaps between large numbers of marine species may have altered in response to human impacts. Here, we synthesized all available distribution data (>1 million records) since 1992 for 61 species of the East Australian marine ecosystem, a global hot spot of ocean warming and continuing fisheries exploitation. Using a novel approach, we constructed networks of the annual changes in geographical overlaps between species. Using indices of changes in species overlap, we quantified changes in the ecosystem stability, species robustness, species sensitivity and structural keystone species. We then compared the species overlap indices with environmental and fisheries data to identify potential factors leading to the changes in distributional overlaps between species. We found that the structure of the ecosystem has changed with a decrease in asymmetrical geographical overlaps between species. This suggests that the ecosystem has become less stable and potentially more susceptible to environmental perturbations. Most species have shown a decrease in overlaps with other species. The greatest decrease in species overlap robustness and sensitivity to the loss of other species has occurred in the pelagic community. Some demersal species have become more robust and less sensitive. Pelagic structural keystone species, predominately the tunas and billfish, have been replaced by demersal fish species. The changes in species overlap were strongly correlated with regional oceanographic changes, in particular increasing ocean warming and the southward transport of warmer and saltier water with the East Australian Current, but less correlated with fisheries catch. Our study illustrates how large-scale multispecies distribution changes can help identify structural changes in marine ecosystems associated with climate change., (© 2017 John Wiley & Sons Ltd.)

Fluorometers are widely used in ecosystem observing to monitor fluorescence signals from organic compounds, as well as to infer geophysical parameters such as chlorophyll or CDOM concentration, but measurements are susceptible to variation caused by biofouling, instrument design, sensor drift, operating environment, and calibration rigor. To collect high quality data, such sensors need frequent checking and regular calibration. In this study, a wide variety of both liquid and solid fluorescent materials were trialed to assess their suitability as reference standards for performance assessment of in situ fluorometers. Criteria used to evaluate the standards included the spectral excitation/emission responses of the materials relative to fluorescence sensors and to targeted ocean properties, the linearity of the fluorometer's optical response with increasing concentration, stability and consistency, availability and ease of use, as well as cost. Findings are summarized as a series of recommended reference standards for sensors deployed on stationary and mobile platforms, to suit a variety of in situ coastal to ocean sensor configurations. Repeated determinations of chlorophyll scale factor using the recommended liquid standard, Fluorescein, achieved an accuracy of 2.5%. Repeated measurements with the recommended solid standard, Plexiglas Satinice® plum 4H01 DC (polymethylmethacrylate), over an 18 day period varied from the mean value by 1.0% for chlorophyll sensors and 3.3% for CDOM sensors.

A novel high-light (HL)-adapted Prochlorococcus clade was discovered in high nutrient and low chlorophyll (HNLC) waters in the South Pacific Ocean by phylogenetic analyses of 16S ribosomal RNA (rRNA) and 16S-23S internal transcribed spacer (ITS) sequences. This clade, named HNLC fell within the HL-adapted Prochlorococcus clade with sequences above 99% similarity to one another, and was divided into two subclades, HNLC1 and HNLC2. The distribution of the whole HNLC clade in a northwest to southeast transect in the South Pacific (HNLC-to-gyre) and two 8°N to 8°S transects in the Equatorial Pacific was determined by quantitative PCR using specific primers targeting ITS regions. HNLC was the dominant HL Prochlorococcus clade (2-9% of bacterial 16S rRNA genes) at the three westernmost stations in the South Pacific but decreased to less than 0.1% at the other stations being replaced by the eMIT9312 ecotype in the hyperoligotrophic gyre. The highest contributions of HNLC Prochlorococcus in both Equatorial Pacific transects along the latitudinal lines of 170°W and 155°W were observed at the southernmost stations, reaching 16 and 6% of bacterial 16S rRNA genes, respectively, whereas eMIT9312 dominated near the Equator. Spearman Rank Order correlation analysis indicated that although both the HNLC clade and eMIT9312 were correlated with temperature, they showed different correlations with regard to nutrients. HNLC only showed significant correlations to ammonium uptake and regeneration rates, whereas eMIT9312 was negatively correlated with inorganic nutrients.

The availability of iron is known to exert a controlling influence on biological productivity in surface waters over large areas of the ocean and may have been an important factor in the variation of the concentration of atmospheric carbon dioxide over glacial cycles. The effect of iron in the Southern Ocean is particularly important because of its large area and abundant nitrate, yet iron-enhanced growth of phytoplankton may be differentially expressed between waters with high silicic acid in the south and low silicic acid in the north, where diatom growth may be limited by both silicic acid and iron. Two mesoscale experiments, designed to investigate the effects of iron enrichment in regions with high and low concentrations of silicic acid, were performed in the Southern Ocean. These experiments demonstrate iron's pivotal role in controlling carbon uptake and regulating atmospheric partial pressure of carbon dioxide.

Sea ice and oceanic boundaries have a dominant effect in structuring Antarctic marine ecosystems. Satellite imagery and historical data have identified the southern boundary of the Antarctic Circumpolar Current as a site of enhanced biological productivity. Meso-scale surveys off the Antarctic peninsula have related the abundances of Antarctic krill (Euphausia superba) and salps (Salpa thompsoni) to inter-annual variations in sea-ice extent. Here we have examined the ecosystem structure and oceanography spanning 3,500 km of the east Antarctic coastline, linking the scales of local surveys and global observations. Between 80 degrees and 150 degrees E there is a threefold variation in the extent of annual sea-ice cover, enabling us to examine the regional effects of sea ice and ocean circulation on biological productivity. Phytoplankton, primary productivity, Antarctic krill, whales and seabirds were concentrated where winter sea-ice extent is maximal, whereas salps were located where the sea-ice extent is minimal. We found enhanced biological activity south of the southern boundary of the Antarctic Circumpolar Current rather than in association with it. We propose that along this coastline ocean circulation determines both the sea-ice conditions and the level of biological productivity at all trophic levels.

During the 1997-98 El Nino, the equatorial Pacific Ocean retained 0. 7 x 10(15) grams of carbon that normally would have been lost to the atmosphere as carbon dioxide. The surface ocean became impoverished in plant nutrients, and chlorophyll concentrations were the lowest on record. A dramatic recovery occurred in mid-1998, the system became highly productive, analogous to coastal environments, and carbon dioxide flux out of the ocean was again high. The spatial extent of the phytoplankton bloom that followed recovery from El Nino was the largest ever observed for the equatorial Pacific. These chemical and ecological perturbations were linked to changes in the upwelling of nutrient-enriched waters. The description and explanation of these dynamic changes would not have been possible without an observing system that combines biological, chemical, and physical sensors on moorings with remote sensing of chlorophyll.