Description: Nitrogen fixation by diazotrophic cyanobacteria is a critical source of new nitrogen to the oligotrophic surface ocean. Research to date indicates that some diazotroph groups may increase nitrogen fixation under elevated pCO2. To test this in natural plankton communities, four manipulation experiments were carried out during two voyages in the South Pacific (30–35oS). High CO2 treatments, produced using 750 ppmv CO2 to adjust pH to 0.2 below ambient, and ‘Greenhouse’ treatments (0.2 below ambient pH and ambient temperature +3 °C), were compared with Controls in trace metal clean deckboard incubations in triplicate. No significant change was observed in nitrogen fixation in either the High CO2 or Greenhouse treatments over 5 day incubations. qPCR measurements and optical microscopy determined that the diazotroph community was dominated by Group A unicellular cyanobacteria (UCYN-A), which may account for the difference in response of nitrogen fixation under elevated CO2 to that reported previously for Trichodesmium. This may reflect physiological differences, in that the greater cell surface area:volume of UCYN-A and its lack of metabolic pathways involved in carbon fixation may confer no benefit under elevated CO2. However, multiple environmental controls may also be a factor, with the low dissolved iron concentrations in oligotrophic surface waters limiting the response to elevated CO2. If nitrogen fixation by UCYN-A is not stimulated by elevated pCO2, then future increases in CO2 and warming may alter the regional distribution and dominance of different diazotroph groups, with implications for dissolved iron availability and new nitrogen supply in oligotrophic regions.

Description: The goal of the Sea2Cloud project is to study the interplay between surface ocean biogeochemical and physical properties, fluxes to the atmosphere, and ultimately their impact on cloud formation under minimal direct anthropogenic influence. Here we present an interdisciplinary approach, combining atmospheric physics and chemistry with marine biogeochemistry, during a voyage between 41 degrees and 47 degrees S in March 2020. In parallel to ambient measurements of atmospheric composition and seawater biogeochemical properties, we describe semicontrolled experiments to characterize nascent sea spray properties and nucleation from gas-phase biogenic emissions. The experimental framework for studying the impact of the predicted evolution of ozone concentration in the Southern Hemisphere is also detailed. After describing the experimental strategy, we present the oceanic and meteorological context including provisional results on atmospheric thermodynamics, composition, and flux measurements. In situ measurements and flux studies were carried out on different biological communities by sampling surface seawater from subantarctic, subtropical, and frontal water masses. Air-Sea-Interface Tanks (ASIT) were used to quantify biogenic emissions of trace gases under realistic environmental conditions, with nucleation observed in association with biogenic seawater emissions. Sea spray continuously generated produced sea spray fluxes of 34% of organic matter by mass, of which 4% particles had fluorescent properties, and which size distribution resembled the one found in clean sectors of the Southern Ocean. The goal of Sea2Cloud is to generate realistic parameterizations of emission flux dependences of trace gases and nucleation precursors, sea spray, cloud condensation nuclei, and ice nuclei using seawater biogeochemistry, for implementation in regional atmospheric models.

Description: Elevated dimethyl sulfide (DMS) concentrations in the sea surface microlayer (SML) have been previously related to DMS air–sea flux anomalies in the southwestern Pacific. To further address this, DMS, its precursor dimethylsulfoniopropionate (DMSP), and ancillary variables were sampled in the SML and also subsurface water at 0.5 m depth (SSW) in different water masses east of New Zealand. Despite high phytoplankton biomass at some stations, the SML chlorophyll a enrichment factor (EF) was low (〈 1.06), and DMSP was enriched at one station with DMSP EF ranging from 0.81 to 1.25. DMS in the SML was determined using a novel gas-permeable tube technique which measured consistently higher concentrations than with the traditional glass plate technique; however, significant DMS enrichment was present at only one station, with the EF ranging from 0.40 to 1.22. SML DMSP and DMS were influenced by phytoplankton community composition, with correlations with dinoflagellate and Gymnodinium biomass, respectively. DMSP and DMS concentrations were also correlated between the SML and SSW, with the difference in ratio attributable to greater DMS loss to the atmosphere from the SML. In the absence of significant enrichment, DMS in the SML did not influence DMS emissions, with the calculated air–sea DMS flux of 2.28 to 11.0 µmol m−2 d−1 consistent with climatological estimates for the region. These results confirm previous regional observations that DMS is associated with dinoflagellate abundance but indicate that additional factors are required to support significant enrichment in the SML.

Description: Polar marine ecosystems are particularly vulnerable to the effects of climate change. Warming temperatures, freshening seawater, and disruption to sea-ice formation potentially all have cascading effects on food webs. New approaches are needed to better understand spatiotemporal interactions among biogeochemical processes at the base of Southern Ocean food webs. In marine systems, isoscapes (models of the spatial variation in the stable isotopic composition) of carbon and nitrogen have proven useful in identifying spatial variation in a range of biogeochemical processes, such as nutrient utilization by phytoplankton. Isoscapes provide a baseline for interpreting stable isotope compositions of higher trophic level animals in movement, migration, and diet research. Here, we produce carbon and nitrogen isoscapes across the entire Southern Ocean (〉40°S) using surface particulate organic matter isotope data, collected over the past 50 years. We use Integrated Nested Laplace Approximation-based approaches to predict mean annual isoscapes and four seasonal isoscapes using a suite of environmental data as predictor variables. Clear spatial gradients in δ13C and δ15N values were predicted across the Southern Ocean, consistent with previous statistical and mechanistic views of isotopic variability in this region. We identify strong seasonal variability in both carbon and nitrogen isoscapes, with key implications for the use of static or annual average isoscape baselines in animal studies attempting to document seasonal migratory or foraging behaviors.

Description: Significance Oceans represent 70% of our planet’s surface, housing a large spectrum of microorganisms that interact with the above atmosphere. Ocean microorganisms were proposed in the late 80’s to be at the center of a climate feedback loop involving dimethyl sulfide (DMS) that would form aerosols and modify cloud properties (CLAW hypothesis). In the present paper, we report observational evidence from semicontrolled experiments in the South Pacific that nitrate ions, yet hitherto not considered, is a key species involved in aerosol nucleation in the pristine marine atmosphere and which precursors are coemitted with DMS. Our results further indicate that nitrate ion formation would be related to short-term microbial processes, sensitive to environmental stressors, therefore potentially “closing the loop”. Abstract Our understanding of ocean–cloud interactions and their effect on climate lacks insight into a key pathway: do biogenic marine emissions form new particles in the open ocean atmosphere? Using measurements collected in ship-borne air–sea interface tanks deployed in the Southwestern Pacific Ocean, we identified new particle formation (NPF) during nighttime that was related to plankton community composition. We show that nitrate ions are the only species for which abundance could support NPF rates in our semicontrolled experiments. Nitrate ions also prevailed in the natural pristine marine atmosphere and were elevated under higher sub-10 nm particle concentrations. We hypothesize that these nucleation events were fueled by complex, short-term biogeochemical cycling involving the microbial loop. These findings suggest a new perspective with a previously unidentified role of nitrate of marine biogeochemical origin in aerosol nucleation.

Description: Future change in sea surface temperature may influence climate via various air-sea feedbacks and pathways. In this study, we investigate the influence of surface seawater biogeochemical composition on the temperature dependence of sea spray number emission fluxes. Dependence of sea spray fluxes was investigated in different water masses (i.e. subantarctic, subtropical and frontal bloom) with contrasting biogeochemical properties across a temperature range from ambient (13–18 °C) to 2 °C, using seawater circulating in a plunging jet sea spray generator. We observed sea spray total concentration to increase significantly at temperatures below 8 °C, with an average 4-fold increase at 2 °C relative to initial concentration at ambient temperatures. This temperature dependence was more pronounced for smaller size sea spray particles (i.e. nucleation and Aitken modes). Moreover, temperature dependence varied with water mass type and so biogeochemical properties. While the sea spray flux at moderate temperatures (8–11 °C) was highest in frontal bloom waters, the effect of low temperature on the sea spray flux was highest with subtropical seawaters. The temperature dependence of sea spray flux was also inversely proportional to the seawater cell abundance of the cyanobacterium Synechococcus, which facilitated parameterization of temperature dependence of sea spray emission fluxes as a function of Synechococcus for future implementation in modelling exercises.

Description: Author Posting. © The Author(s), 2010. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part II: Topical Studies in Oceanography 58 (2011): 753-763, doi:10.1016/j.dsr2.2010.10.015.

Description: The SOLAS air-sea gas exchange experiment (SAGE) was a multiple-objective study investigating gas-transfer processes and the influence of iron fertilisation on biologically driven gas exchange in high-nitrate low-silicic acid low-chlorophyll (HNLSiLC) Sub-Antarctic waters characteristic of the expansive Subpolar Zone of the southern oceans. This paper provides a general introduction and summary of the main experimental findings. The release site was selected from a pre-voyage desktop study of environmental parameters to be in the south-west Bounty Trough (46.5°S 172.5°E) to the south-east of New Zealand and the experiment conducted between mid-March and mid-April 2004. In common with other mesoscale iron addition experiments (FeAX’s), SAGE was designed as a Lagrangian study quantifying key biological and physical drivers influencing the air-sea gas exchange processes of CO2, DMS and other biogenic gases associated with an iron-induced phytoplankton bloom. A dual tracer SF6/3He release enabled quantification of both the lateral evolution of a labelled volume (patch) of ocean and the air-sea tracer exchange at the 10’s of km’s scale, in conjunction with the iron fertilisation. Estimates from the dual-tracer experiment found a quadratic dependency of the gas exchange coefficient on windspeed that is widely applicable and describes air-sea gas exchange in strong wind regimes. Within the patch, local and micrometeorological gas exchange process studies (100 m scale) and physical variables such as near-surface turbulence, temperature microstructure at the interface, wave properties, and wind speed were quantified to further assist the development of gas exchange models for high-wind environments. There was a significant increase in the photosynthetic competence (Fv/Fm) of resident phytoplankton within the first day following iron addition, but in contrast to other FeAX’s, rates of net primary production and column-integrated chlorophyll a concentrations had only doubled relative to the unfertilised surrounding waters by the end of the experiment. After 15 days and four iron additions totalling 1.1 tonne Fe2+, this was a very modest response compared to the other mesoscale iron enrichment experiments. An investigation of the factors limiting bloom development considered co- limitation by light and other nutrients, the phytoplankton seed-stock and grazing regulation. Whilst incident light levels and the initial Si:N ratio were the lowest recorded in all FeAX’s to date, there was only a small seed-stock of diatoms (less than 1% of biomass) and the main response to iron addition was by the picophytoplankton. A high rate of dilution of the fertilised patch relative to phytoplankton growth rate, the greater than expected depth of the surface mixed layer and microzooplankton grazing were all considered as factors that prevented significant biomass accumulation. In line with the limited response, the enhanced biological draw-down of pCO2 was small and masked by a general increase in pCO2 due to mixing with higher pCO2 waters. The DMS precursor DMSP was kept in check through grazing activity and in contrast to most FeAX’s dissolved dimethylsulfide (DMS) concentration declined through the experiment. SAGE is an important low-end member in the range of responses to iron addition in FeAX’s. In the context of iron fertilisation as a geoengineering tool for atmospheric CO2 removal, SAGE has clearly demonstrated that a significant proportion of the low iron ocean may not produce a phytoplankton bloom in response to iron addition.

Description: SAGE was jointly funded through the New Zealand Foundation for Research, Science and Technology (FRST) programs (C01X0204) "Drivers and Mitigation of Global Change" and (C01X0223) "Ocean Ecosystems: Their Contribution to NZ Marine Productivity." Funding was also provided for specific collaborations by the US National Science Foundation from grants OCE-0326814 (Ward), OCE-0327779 (Ho), and OCE 0327188 OCE-0326814 (Minnett) and the UK Natural Environment Research Council NER/B/S/2003/00282 (Archer). The New Zealand International Science and Technology (ISAT) linkages fund provided additional funding (Archer and Ziolkowski), and the many collaborator institutions also provided valuable support.

Description: Biomass accumulation was assessed by subtracting phytoplankton mortality (due to microzooplankton) from phytoplankton growth rates. Rates of phytoplankton growth and microzooplankton grazing were assessed daily with the dilution technique (Landry and Hassett 1982; doi:10.1007/BF00397668), following the two treatment approach (Landry, Haas et al. 1984 doi:10.3354/meps016127), at six depths within the euphotic zone. We implemented this mini-dilution approach to generate vertically resolved growth and grazing rates, but also conducted a full dilution experiment on the last day of each of the cycles (n = 5) to test linearity assumptions of the method. Seawater collected with the Niskin bottles attached to the CTD rosette at 02:00 h was used to fill a pair of 2.2-L polycarbonate bottles (100%, B and C) while a third bottle (A) was filled with 25% whole seawater diluted with 0.2-µm filtered seawater obtained immediately before by gravity filtration using an Acropak filter cartridge (Pall) directly from the same Niskin bottle. Nutrients (final concentrations in 2.2L bottles; nitrate 0.18 μM, ammonium 4.16 μM, phosphate 15.08, silicate 44.2 μM, and vitamins) were added to bottles A and B in order to ensure the assumption that the same phytoplankton intrinsic growth rate was occurring in WSW and FSW bottles despite dilution (Gutiérrez‐Rodríguez, Safi et al. 2020 doi:10.1029/2019JC015550). Bottles were then incubated in situ at the same six depths of collection using a drifting array. Rates were calculated from changes in Chl a concentration and picophytoplankton abundance between the beginning and end of the experiment assuming exponential growth of phytoplankton. Microzooplankton grazing rate was estimated from: µ = (kA – kB)/(1-x) where kA and kB are the observed net rates of change of chl a in bottles A and B, respectively, and x is the fraction of whole seawater in the diluted bottle A (0.25). Phytoplankton growth rate was estimated from µ =m+kB. Photoacclimation effects were corrected from changes in cell chl a fluorescence estimated by flow cytometry during incubations as a proxy of cell chl a content (Gutierrez-Rodriguez, Latasa et al. 2010 doi:10.1016/j.dsr.2009.12.013). These include estimating the photoacclimation index (Phi) from changes in FL3: FSC and calculating an average value from Phi index obtained for pico- and nanoeukaryotic populations weighted by their biomass contribution. Accumulation was calculated by subtracting the C-based estimates of microzooplankton grazing (from the dilution experiments) from the 14C-based NPP.

Description: Salps were collected using double oblique Bongo tows, with 0.7m diameter frames equipped with 202 µm nets, General Oceanics flow meters, and an RBR temperature depth recorder. Salp specimens (typically 10) from each tow had their guts excised, and chl a and phaeopigments gut contents were measured. A power function was used to fit the size-specific Gpig (chl a + phaeo) contents for each tow, allowing the estimation of Gpig for each size bin per tow, and this was multiplied by the abundance in each size bin. Gut passage time (GPT) was calculated using a modified equation, based on (von Harbou, Dubischar et al. 2011 doi:10.1007/s00227-011-1709-4) where GPT(h) = 2.607*ln(OAL, mm) - 2.6. Grazing was estimated as: G (h-1) = Gpig /GPT, and scaled using a Q10=2. Daily salp grazing rates were obtained by assuming 14 h of day and 10 h of night, coincident with the times and latitudes at which we sampled these communities during the Salp Particle expOrt and Ocean Production (SalpPOOP) campaign. Cycle estimates were calculated by first averaging all day and all night tows separately, and then adding the two estimates. Fecal pellet production was calculated by assuming an egestion efficiency of 0.36 (Huntley, Sykes et al. 1989 doi:10.1007/BF00238291, Pakhomov 2004 doi:10.1016/j.dsr2.2001.03.001, Pakhomov and Froneman 2004 10.1016/j.dsr2.2000.11.002) and converting to carbon values using C:Chl ratios from the phytoplankton growth and grazing experiments combined with NPP, and reported in mg C m-2 d-1. Data is reported by size after binning in 5mm size bins (ranging 1-135mm), and for oozooids and blastozooids separately.

Description: Water collection for nutrient analysis was done using a CTD rosette equipped with 24 10L Niskin bottles, at different depths throughout the water column depending on the cast, spanning the euphotic zone to a maximum depth of 200m. Multiple casts were done during five Lagrangian experimental cycles conducted during Salp Particle expOrt and Ocean Production (SalpPOOP), from October to November 2018 in the vicinity of the Chatham Rise (New Zealand). Water was filtered through 25mm Whatman GF/F filters onto clean polyethylene bottles (250ml) and frozen at -20 °C. Analysis was done at the NIWA Hamilton Water Quality Laboratory (New Zealand), using an Astoria Pacific API 300 microsegmented flow analyzer (Astoria-Pacific, Clackamas, OR, United States) following colorimetric the methods outlined in Law et al. (2011; doi:10.1016/j.dsr2.2010.10.018).