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Phytoplankton community structure is crucial to pelagic food webs and biogeochemical processes. Understanding size-based biomass distribution and carbon dynamics is essential for assessing their contributions to oceanic carbon cycling. This study quantifies plankton carbon (C) based size spectra, community composition, living to total particulate organic carbon (POC) and C:Chlorophyll a (C:Chl a ) ratios across biogeographical provinces in the Pacific sector of the Southern Ocean near the Subtropical Front (Chatham Rise, Aotearoa-New Zealand). We analyzed phytoplankton community composition using epifluorescence microscopy and flow cytometry, while quantifying size-fractionated Chl- a and POC to estimate normalized biomass, abundance size spectra, and C:Chl a ratios. On average, subtropical-influenced waters had lower macronutrients, higher total Chl a (1.1 ± 0.2 μg Chl a L -1 ) and were dominated by nanoplankton, which accounted for 45% of the total plankton community (35.2 ± 4.6 μg C L -1 ). In contrast, picoplankton dominated plankton communities within the subantarctic-influenced and accounted for 35% of the total plankton community (18.5 ± 0.9 μg C L -1 ) in these water with higher macronutrient concentrations and lower total Chl a concentrations (0.32 ± 0.06 μg Chl a L -1 ). Subantarctic-influenced regions had steeper (more negative) slopes for the normalized biomass size spectrum (average = -1.00) compared to subtropical-influenced waters (average = -0.78) indicating greater relative dominance of small taxa. The subantarctic-influenced region had ~2-fold higher surface average C:Chl a ratios compared to the subtropical-influenced region with picoplankton consistently having lower C:Chl a ratios, due to low Chl a values, than larger nano- or microplankton. Live plankton carbon contributed a median of 67% of total particulate organic carbon in the euphotic zone (non-living detritus comprises the remaining ~1/3), which is indicative of substantial primary production and rapid recycling by a strong microbial loop. Our study provides important insights into phytoplankton community structure, biomass distribution and their contribution to carbon sequestration in this region, highlighting the important roles of nanoplankton in subtropical productive waters and picoplankton in offshore subantarctic waters as well as a strong variation of C:Chl a across different phytoplankton size classes.

The oceanic frontal region above the Chatham Rise east of New Zealand was investigated during the late austral summer season in February and March 2012. Despite its potential importance as a source of marine-originating and climate-relevant compounds, such as dimethyl sulfide (DMS) and its algal precursor dimethylsulfoniopropionate (DMSP), little is known of the processes fuelling the reservoirs of these sulfur (S) compounds in the water masses bordering the subtropical front (STF). This study focused on two opposing short-term fates of DMSP-S following its uptake by microbial organisms (either its conversion into DMS or its assimilation into bacterial biomass) and has not considered dissolved non-volatile degradation products. Sampling took place in three phytoplankton blooms (B1, B2, and B3) with B1 and B3 occurring in relatively nitrate-rich, dinoflagellate-dominated subantarctic waters, and B2 occurring in nitrate-poor subtropical waters dominated by coccolithophores. Concentrations of total DMSP (DMSPt) and DMS were high across the region, up to 160 and 14.5 nmol L−1, respectively. Pools of DMSPt showed a strong association with overall phytoplankton biomass proxied by chlorophyll a (rs  =  0.83) likely because of the persistent dominance of dinoflagellates and coccolithophores, both DMSP-rich taxa. Heterotrophic microbes displayed low S assimilation from DMSP (less than 5 %) likely because their S requirements were fulfilled by high DMSP availability. Rates of bacterial protein synthesis were significantly correlated with concentrations of dissolved DMSP (DMSPd, rs  =  0.86) as well as with the microbial conversion efficiency of DMSPd into DMS (DMS yield, rs  =  0.84). Estimates of the potential contribution of microbially mediated rates of DMS production (0.1–27 nmol L−1 day−1) to the near-surface concentrations of DMS suggest that bacteria alone could not have sustained DMS pools at most stations, indicating an important role for phytoplankton-mediated DMS production. The findings from this study provide crucial information on the distribution and cycling of DMS and DMSP in a critically under-sampled area of the global ocean, and they highlight the importance of oceanic fronts as hotspots of the production of marine biogenic S compounds.

Elevated atmospheric CO 2 concentrations are contributing to ocean acidification (reduced seawater pH and carbonate concentrations), with potentially major ramifications for marine ecosystems and their functioning. Using a novel in situ experiment we examined impacts of reduced seawater pH on Antarctic sea ice-associated microalgal communities, key primary producers and contributors to food webs. pH levels projected for the following decades-to-end of century (7.86, 7.75, 7.61), and ambient levels (7.99), were maintained for 15 d in under-ice incubation chambers. Light, temperature and dissolved oxygen within the chambers were logged to track diurnal variation, with pH, O 2 , salinity and nutrients assessed daily. Uptake of CO 2 occurred in all treatments, with pH levels significantly elevated in the two extreme treatments. At the lowest pH, despite the utilisation of CO 2 by the productive microalgae, pH did not return to ambient levels and carbonate saturation states remained low; a potential concern for organisms utilising this under-ice habitat. However, microalgal community biomass and composition were not significantly affected and only modest productivity increases were noted, suggesting subtle or slightly positive effects on under-ice algae. This in situ information enables assessment of the influence of future ocean acidification on under-ice community characteristics in a key coastal Antarctic habitat.

As the sea surface microlayer (SML) is the uppermost oceanic layer and differs in biogeochemical composition to the underlying subsurface water (SSW), it is important to determine whether processes in the SML modulate gas exchange, particularly for climate active gases. Enrichment of dimethyl sulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP) has been reported in the SML, but it remains unclear how this is maintained whilst DMS is lost to the atmosphere. To examine this, a comprehensive study of DMS source and sink processes, including production, consumption, and net response to irradiance, was carried out in deck-board incubations of SML water at five locations in different water masses in the southwestern Pacific east of New Zealand. Net consumption of DMSP and production of DMS in the light and dark occurred at all sites. The net response of DMS and DMSP to irradiance varied between stations but was always lower than conversion of DMSP to DMS in the dark. In addition, DMS photolytic turnover was slower than reported elsewhere, which was unexpected given the high light exposure in the SML incubations. Although no relationships were apparent between DMS process rates and biogeochemical variables, including chlorophyll a, bacteria, and phytoplankton groups, net bacterial DMSP consumption was correlated with DMSP and DMS concentrations and also dinoflagellate and Gymnodinium spp. biomass, supporting the findings of a companion study that dinoflagellates play an important role in DMS cycling in the SML. However, net DMS production rates and accumulation were low relative to regional air–sea DMS loss, indicating that DMS cycling within the SML is unlikely to influence regional DMS emissions.

Establishing the relationship between marine boundary layer (MBL) aerosols and surface water biogeochemistry is required to understand aerosol and cloud production processes over the remote ocean and represent them more accurately in earth system models and global climate projections. This was addressed by the SOAP (Surface Ocean Aerosol Production) campaign, which examined air–sea interaction over biologically productive frontal waters east of New Zealand. This overview details the objectives, regional context, sampling strategy and provisional findings of a pilot study, PreSOAP, in austral summer 2011 and the following SOAP voyage in late austral summer 2012. Both voyages characterized surface water and MBL composition in three phytoplankton blooms of differing species composition and biogeochemistry, with significant regional correlation observed between chlorophyll a and DMSsw. Surface seawater dimethylsulfide (DMSsw) and associated air–sea DMS flux showed spatial variation during the SOAP voyage, with maxima of 25 nmol L−1 and 100 µmol m−2 d−1, respectively, recorded in a dinoflagellate bloom. Inclusion of SOAP data in a regional DMSsw compilation indicates that the current climatological mean is an underestimate for this region of the southwest Pacific. Estimation of the DMS gas transfer velocity (kDMS) by independent techniques of eddy covariance and gradient flux showed good agreement, although both exhibited periodic deviations from model estimates. Flux anomalies were related to surface warming and sea surface microlayer enrichment and also reflected the heterogeneous distribution of DMSsw and the associated flux footprint. Other aerosol precursors measured included the halides and various volatile organic carbon compounds, with first measurements of the short-lived gases glyoxal and methylglyoxal in pristine Southern Ocean marine air indicating an unidentified local source. The application of a real-time clean sector, contaminant markers and a common aerosol inlet facilitated multi-sensor measurement of uncontaminated air. Aerosol characterization identified variable Aitken mode and consistent submicron-sized accumulation and coarse modes. Submicron aerosol mass was dominated by secondary particles containing ammonium sulfate/bisulfate under light winds, with an increase in sea salt under higher wind speeds. MBL measurements and chamber experiments identified a significant organic component in primary and secondary aerosols. Comparison of SOAP aerosol number and size distributions reveals an underprediction in GLOMAP (GLObal Model of Aerosol Processes)-mode aerosol number in clean marine air masses, suggesting a missing marine aerosol source in the model. The SOAP data will be further examined for evidence of nucleation events and also to identify relationships between MBL composition and surface ocean biogeochemistry that may provide potential proxies for aerosol precursors and production.

During a survey cruise crossing the Subtropical Front over the Chatham Rise east of New Zealand, we made the first observation in High Nutrient Low Chlorophyll waters of massive sedimentation of picoplankton embedded in large ($>$0.5 mm diam.) organic aggregates (OAs) sensu Riley (1963). We estimate 9.3 mg C m-2yr-1of prokaryotic picoplankton biomass alone may be transported below the euphotic zone via this mechanism. Using confocal microscopy, we made direct observations of picoplankton within undisrupted individual OAs, collected in sediment traps fitted with acrylimide gels, which largely conserved particle structure. Prokaryotic picoplankton autofluorescence was well-preserved and concentrations were extremely high within large rapidly sedimenting aggregates, ranging from 1.06 × 108ml-1in the 120 m sediment traps in subantarctic waters to 7.5 × 106ml-1at 550 m in subtropical waters, yielding Enrichment Factors of 103-105relative to picoplankton concentrations in the water column. Aggregate picoplankton concentrations showed a well-constrained exponential decline with depth, which we speculate may represent an estimate of protozoan grazing rate within the aggregates. Picoplankton were found within heterotrophic flagellates, within copepod fecal pellets, and within organic matrices, all of which were incorporated in OAs. The key event of picoplankton incorporation into initial particles occurs in the upper water column, very likely through grazing with low assimilation efficiency. Once OAs are formed, their changing porosity and reduction in picoplankton cell numbers via heterotrophy is likely to be a key factor mediating picoplankton carbon fluxes in moderately productive ecosystems, and in determining the overall particle structure of sedimenting OAs.

This study was made possible by funding from the Ministry for Business, Innovation and Employment (MBIE) of New Zealand, NIWA Coast and Oceans Food Webs (COES) and Ocean Flows (COOF), and the Royal Society of New Zealand Marsden Fast-track award to MD, and by U.S. National Science Foundation awards OCE- 1756610 and 1756465 to MS and KES.

Small phytoplankton, consisting of pico and nano size fractions, are diverse in size and taxonomy. Yet, the differences in their productivity and taxonomic diversity are poorly described. Here, we measured the cell-specific carbon fixation rates of small phytoplankton (picocyanobacteria Synechococcus, picoeukaryotes and nanoeukaryotes) populations and unveiled their associated community composition in oligotrophic subtropical (ST) and high-nutrient low-chlorophyll (HNLC) subantarctic (SA) waters. We coupled 24 h in situ radiolabelled 14C incubations to flow cytometry sorting (FCM-sorting) and DNA metabarcoding from the same incubated samples, offering a direct account of the community associated with the carbon fixation rates measured. In both water masses, nanoeukaryotes had the highest cell-specific carbon fixation rate, followed by picoeukaryotes and Synechococcus (2.24 +/- 29.99, 2.18 +/- 2.08 and 0.78 +/- 0.55 fgC cell-1 h-1, respectively). The cell-specific carbon fixation rates and growth rates of Synechococcus were 3-fold higher in ST compared to SA waters, while the rates of picoeukaryotes and nanoeukaryotes had no significant difference between the biogeochemically contrasting water masses. Despite distinct community composition between picoeukaryote and nanoeukaryote populations, the FCM-sorted picoeukaryote community in SA waters and the nanoeukaryotic community in both ST and SA waters were dominated by taxa with reported phago-mixotrophic strategies (Chrysophyceae, Dinophyceae and Prymnesiophyceae), suggesting phago-mixotrophy might alleviate nutrient stress in iron limited conditions for discrete small photosynthetic eukaryote populations.Competing Interest StatementThe authors have declared no competing interest.Footnotes* https://github.com/deniseong/TAN1810_C14* http://flowrepository.org/experiments/1773

I investigated spatial and temporal variability in benthic and pelagic (phytoplankton) microalgal assemblages in 2 harbours and a coastal embayment off the east coast of the North Island of New Zealand in late spring, summer and autumn. For the first time pelagic microalgal assemblages were sampled just above the sediment in the layer from which suspension feeders are likely to feed (~10 cm). Benthic microalgal assemblages were also assessed within the sediments. Strong links were observed between pelagic and benthic microalgal populations, with the amount of benthic microalgae found in suspension correlated with current speed and turbidity. Conversely, a number of species found within the benthic counts were tychopelagic species whose life habit is to rely on currents which re-suspend them from the benthic to pelagic environment. In late spring, pelagic water samples from 1 harbour and the embayment it opened into were dominated by dinoflagellate taxa, while diatoms dominated the second harbour location. In autumn, all the locations were dominated by diatom taxa but the 2 harbours were most similar in community composition. In summer, local conditions favoured the development of different microalgal populations in each location. Both pelagic and benthic microalgal assemblages generally decreased in similarity as spatial scale increased, with populations being most similar within a sampling site and least similar between locations. Benthic microalgae assemblages were more variable than pelagic assemblages within and between sampling sites at the locations, but had a greater similarity between locations (40 to 64%). Our results provide a new insight into the scales of variability of microalgal populations within and between different nearshore environments, and identify some of the forcing factors that drive changes in microalgal composition. By sampling microalgal populations just above the sediment we have begun to characterise the microalgal communities that sustain suspension feeders.

The aerosol-driven radiative effects on marine low-level cloud represent a large uncertainty in climate simulations, in particular over the Southern Ocean, which is also an important region for sea spray aerosol production. Observations of sea spray aerosol organic enrichment and the resulting impact on water uptake over the remote Southern Hemisphere are scarce, and therefore the region is under-represented in existing parameterisations. The Surface Ocean Aerosol Production (SOAP) voyage was a 23 d voyage which sampled three phytoplankton blooms in the highly productive water of the Chatham Rise, east of New Zealand. In this study we examined the enrichment of organics to nascent sea spray aerosol and the modifications to sea spray aerosol water uptake using in situ chamber measurements of seawater samples taken during the SOAP voyage. Primary marine organics contributed up to 23 % of the sea spray mass for particles with diameter less than approximately 1 µm and up to 79 % of the particle volume for 50 nm diameter sea spray. The composition of the submicron organic fraction was consistent throughout the voyage and was largely composed of a polysaccharide-like component, characterised by very low alkane-to-hydroxyl-concentration ratios of approximately 0.1–0.2. The enrichment of organics was compared to the output from the chlorophyll-a-based sea spray aerosol parameterisation suggested by Gantt et al. (2011) and the OCEANFILMS (Organic Compounds from Ecosystems to Aerosols: Natural Films and Interfaces via Langmuir Molecular Surfactants) models. OCEANFILMS improved on the representation of the organic fraction predicted using chlorophyll a, in particular when the co-adsorption of polysaccharides was included; however, the model still under-predicted the proportion of polysaccharides by an average of 33 %. Nascent 50 nm diameter sea spray aerosol hygroscopic growth factors measured at 90 % relative humidity averaged 1.93±0.08 and did not decrease with increasing sea spray aerosol organic fractions. The observed hygroscopicity was greater than expected from the assumption of full solubility, particularly during the most productive phytoplankton bloom (B1), during which organic fractions were greater than approximately 0.4. The water uptake behaviour observed in this study is consistent with that observed for other measurements of phytoplankton blooms and can be partially attributed to the presence of sea salt hydrates, which lowers the sea spray aerosol hygroscopicity when the organic enrichment is low. The inclusion of surface tension effects only marginally improved the modelled hygroscopicity, and a significant discrepancy between the observed and modelled hygroscopicity at high organic volume fractions remained. The findings from the SOAP voyage highlight the influence of biologically sourced organics on sea spray aerosol composition; these data improve the capacity to parameterise sea spray aerosol organic enrichment and water uptake.